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# Draft:Metric system rewrite

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 Poetry of realityScience We must know. We will know. A view from theshoulders of giants. v - t - e
 Part of aconvergent series onMathematics 1+1=11 v - t - e
 “”In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy to heat up by one degree centigrade — which is 1 percent of the difference between its freezing point and its boiling point. An amount of hydrogen weighing the same amount has exactly one mole of atoms in it. Whereas in the American system, the answer to "How much energy does it take to boil a room-temperature gallon of water?" is "Go fuck yourself", because you can't directly relate any of those quantities. —Wild Thing[1]

The metric system is a pinko commie plot system of units for measurement that are all interrelated in a mathematically convenient way,[2] and are used in a decimal fashion for smaller and larger measurements, using a system of prefixes to indicate orders of magnitude. This makes the system incredibly easy to work in. It has been almost universally adopted for use around the world, and is a quite common system of measurement normally used in scientific work[3] (but see the problems caused by using incompatible systems of units below). Nowadays, teaching the metric system in grade school is standard.

## Where does the SI come from?

### The metric system in the old days

The metric system had been discussed (well... not under that name but the main idea was there) as early as the 16th and 17th centuries. The 18th came up with some advancement in science, urging the need for a less sloppy system of units to facilitate education of pupils. Its first implementation came in between 1789 and 1799 during the French Revolution as starving laypeople rejected en masse everything that could refer to the old regime -including the measurement units- but still require something to work with. The outcome was a system of the following units:

• the mètre for length
• the are for surface
• the stère for volume (of firewood, if it makes sense)
• the litre for volume (of water)
• the gramme for mass (not the beloved kilogram)
• the franc for currency
• the français for language

At this time, it must have been sooo much fun to go shopping for 2kg of some delightful frogs when you had to count 2000 thousand grams to put on the scale before the deal is concluded. It should be noted that the time (as a dimension to be measured) has been omitted from this metric system, that's because it deserved much more than a mere definition. Ladies and gentlemen, please welcome the infamous:

### Metric time

When the French revolutionaries brought in the new metric system, they also brought in the French Republican Calendar, which decimalized time with uniformly thirty-day months (five or six days being added at the end of each year to keep the seasons in step), ten-day weeks, ten-hour days,[4] 100-minute hours and 100-second minutes. And catchy names like Brumaire and Thermidor, which we can only hope made some sense in French (not that much, in fact). This did not prove terribly popular and was abolished less than 15 years after it was brought in.

Nearly two centuries later, Saturday Night Live parodied it in their "Metric day" sketch.

Using a decimal system for the "arbitrary" increments of the day (as noted above), the "new second" would be within 10% of the "old second." So counting off would translate to "one nine hundred, two nine hundred, three nine hundred..." See how easy this would be? It's effectively just a way of stating as a decimal fraction the proportion of the day that has elapsed.

There are apps such as LogiClock which can display decimal time, here is another one for you to bright in society.

Strictly speaking, SI time is already a decimal standard: like any other SI unit, the second can be used with decimal prefixes. Whereas this is common with the reducing prefixes (milli-, micro-, nano-, etc), the magnifying prefixes (kilo-, mega-, giga-, etc) are not widely used in this way, because they don't align neatly with days, weeks, months and years.[5] Given that the way humans typically experience time is cyclical (unlike any other physical dimension, such as length, mass or temperature), time is a rather unusual dimension. Moreover, any system of timekeeping that was decimalized on Earth would make no practical sense to humans living in deep-space habitats, or on colonies on other planets, which would have very different time cycles. So there is relatively little reason to adjust the SI – all units derived from the second, such as the joule, newton and pascal – such that there are 100 000 seconds in a day.

Other attempts of reforming have been performed across the globe, but without more success.

### The metric system in the less old days

 “”The SI is not static but evolves to match the world's increasingly demanding requirements for measurement. Currently much work is under way related to the intended future revision of the SI. —Bureau International des Poids et Mesures, in Introduction to the SI[6]

Time goes by as the young metric system is argued, enhanced, completed, cancelled, implemented again, and so on. And someone must has thought it was good, but not enough.

An obscure Scottish dude (some other goons sticking around) came up and proposed to clean up the mess with the idea that one should consider base units different from derived units. Base units being defined as a unit (duh!) that cannot be formulated as a combination of other units of the system. In addition, they proposed a system of units that is coherent (namely the CGS system, CGS standing for Centimetre-Gram-Second), meaning that its units have getaways between each others. The modern metric system is getting close.

The modern version of the metric system, first announced in 1960, is the Système International (International System), abbreviated "SI", which defines the specific units used in science. The SI follows strict mathematical principles such as coherency: each physical dimension (such as length or mass) has a single unit associated to it, and these units are interrelated by dimensional analysis, always using a factor of one. This means that the mathematical relationship between any two physical dimensions is identical to the relationship between the units that measure them; in modern physics and engineering, equations are almost always written in SI units for this reason. Nowadays, the metric system refers to a subset of the SI. It's still a common usage to speak about the "metric system" when people are unused to the SI, but the official name is SI. The SI actually contains the following seven base units:

• the metre (symbol m) for length
• the kilogram (symbol kg) for mass
• the second (symbol s) for time
• the ampere (symbol A) for electric current
• the kelvin (symbol K) for thermodynamic temperature
• the mole (symbol mol) for quantity (of elementary entities)
• the candela (symbol cd) for luminous intensity

These few units are base units. There are two important consequences:

1. each unit must be defined by a dimensional unit, i.e. "metre is to be used to measure a length, but how long is a metre?" For that purpose, the SI provides definitions based upon physical phenomena, so any nerdy can create its very own international unit standard, provided it has access to decent equipment. At this date (July 2018) only the mass unit is defined by an artifact with the issue that we know.
2. for practical purpose, the base units of the SI can be combined to deal with multi dimensional physical phenomena. For instance the measurement of pressure, usually rendered in pascal (symbol Pa, named after the guy), 1 Pa equals 1.6810 m 1 kg.m-1.s-2.

### Past is ok, what about today?

The SI has two main usages, in scientific literature and in metrology. In science, to be short: it's quite useful to standardize the literature (at least for the scripture of the algebra calculation and result interpretation). As stated above, the SI has been designed to be coherent, making exploration of new real sciences far more easier (by facilitating the peer review) than it would have be with an empirical system of units (typically, a new phenomenon would end with a new unit). In metrology, as the SI defines base units with physical phenomena (except for the mass), the duplication of standards is facilitated. Therefore, every organization applying the SI is able to produce its own working standards recognized internationally, provided it can justify the SI has been followed.

To do so, the BIPM defines the following groups of units (it appears the NIST only recognizes the 2 first):

• the base units, the ones that are standardized against physical phenomena and cannot be resulted from a combination of other units.
• the derived units, the ones that use base units by combination. Since they are widely used in science & engineer, they have been granted a specific symbol (like the Pascal or the Newton) but remain coherent with the base units group.
• the non-SI accepted for use units, the ones that are quite useful for specific fields of application (e.g. quantum electrodynamics), or for historical and cultural reason (e.g. calendar. This group has no coherency with the SI.
• the non-SI not recommended for use units, the ones that have lost any utility other than fun (briefly summarized).

The reader is free to guess in which group of units the imperial / US customary system falls takes place .

It should be noted that the SI is only focused upon defining its base units and providing practical guidance to realize standards for these units. Whether and how an organization decides or not to use the SI, is out of scope of the SI. If the fore mentioned organization goes for the SI, we'll talk about metrication. In the USA, the NIST refers to metrication as "Metric program", a wording less subject to put underwear on fire, studies say.

## Countries that have not completely adopted the system

In spite of the obvious and well-established practical and mathematical advantages of the metric system, the following few countries remain outside of the pack.

### In the United States

Despite the utter sense that the SI makes, the ease of use, and the regulations adopting it, in the United States there has been tremendous resistance to its full adoption for five reasons:

1. It was (apparently) invented by the French, and all good American citizens know they're not to be trusted;
2. Resistance to consider change;
3. American Exceptionalism ("We don't have to change to match the rest of the world!");
4. The lack of ability to spell such complicated words as "metre" instead of "meter". Do not mind that everyone feel comfortable with "metric" (the adjective derived from the "metric" noun should have been "meteric"!) and others uncommon words like "theatre" ; and
5. All those awful complicated headachy conversion thingies. In spite of this, everyone in the U.S. is quite comfortable with two-litre bottles of carbonated sweetened beverages, 9 mm bullets, and nutritional information and medicine measured in grams and milligrams (example: a fat-free product saying it has 0 g of fat).[7] And every alcoholic knows perfectly well that 750 mL is close enough to a "fifth" (1/5 US gal) that it doesn't really matter.)

Extreme crackpot right-wing responses to the SI are not unheard of. For example, Dean Krakel, Director of the National Cowboy Hall of Fame:

Metric is definitely communist. One monetary system, one language, one weight and measurement system, one world — all communist! We know the West was won by the inch, foot, yard, and mile.[8]

Never mind that that was before communism was really a thing, and that time the commies were beaten, it was done by the kilometre.

And Dave Barry (who - it should be noted - is a humorist):

"[…] my belief, as an American, is that if I have to start understanding the metric system, then the terrorists have won."[9]

Americans have an overall poor grasp of measurement, the subject in which they perform worst compared to the rest of the world, with the greatest gap between whites and minorities found there, likely at least partly from not having a coherent measuring system[10]. The metric system has, however, found wide use among entrepreneurs in America's inner cities,[11] among American scientists, and hospitals, the last of which tend to have scales that measure in kilograms for the hospital's information on the patient and can switch to pounds for laypeople who aren't completely familiar with the system (and they assume every patient is not).

### In the United Kingdom

In the UK, by comparison, there is less outright hostility (well, with the exception of Daily Mail readers, who probably think centimetres cause cancer) to the system and more tedious bloody-mindedness to just stick with the traditional system.[12] Despite EU regulations that say food products must be sold in metric kilograms (a form of standardization across Europe), some traders insist quite forcefully in using old, Imperial units. Because this is illegal and against Trading Standards laws, it leads to the so-called "Metric Martyr" cases. Originally just a tabloid euphemism for traders prosecuted for dealing in Imperial units, the term is now used for those fighting against the original prosecutions, and expanded to outright opposing Europe in general.[13] It's not yet known whether Brexit will enable the UK to finally go back to furlongs and firkins.

A couple of extremely marginal crackpot organizations, namely the British Weights and Measures Association and Active Resistance to Metrication, serve as rallying points for charlatans and luddites of every persuasion to unite in their opposition to modern, scientific units of measure. The latter group of anti-metric extremists delights in their obnoxious and antisocial habits of vandalism, altering metric distances on signs (even privately owned ones) to – often less precise – values in obsolete units, and even posting pictures of their crimes to social media. Notably, their leader has a standing conviction for criminal damage. Their former website was a treasure-trove of lunatic advocacy for far-right causes, such as protesting the "Islamification" of Britain and the "redefinition of marriage", on which the organization's zealot members went by bizarre pseudonyms such as "Wun Tun" and "Hundred Weight".

And, as everyone knows from watching Top Gear, British automobiles still measure their speeds in miles per hour, and their fuel efficiency in miles per (imperial) gallon. However, the metrication of transport in the UK is underway, even if it is plodding along at a top speed of 1 mph. The bizarre hybrid of measurements used in the UK is such that buses measure their speeds in miles per hour, whereas trams use kilometres per hour. Although road speeds are typically measured in miles per hour, speed limiters are commonly set to speeds in kilometres per hour, thus you can see bizarre signs on vans and trucks such as "Limited to 68 mph" (i.e. 110 km/h). Trains are transitioning slowly from the Ye Olden miles and chains to kilometres.

They're miles way ahead of the US in using the SI.

## Arguments against metrication

The logical, simple and obviously superior English units compared to the Satanic Metric units
 “”The metric system is the tool of the devil! My car gets forty rods to the hogshead, and that's the way I likes it! —Fuel efficiency lover Abraham Simpson[14]
Divisions into tenths are awkward to work with on a practical basis!
The division of a foot into 12 equal pieces, for example, allows you to easily divide lengths in feet by 3 or 4, which apparently crops up all the time in construction or baking or some such. In practice, this is not a problem when working with SI, since there is nothing to prevent you from using 300 mm intervals divided into twelfths of 25 mm. Indeed, the case could be made that the small size of the millimetre makes it a far more practical unit than the inch, since it is rarely necessary to work with fractions (17/64", anyone?), and it is easy to work in intervals that can be subdivided in many ways (e.g. 360 mm) if so desired. In practice, the users of the obsolete units often nod to the greater utility of base-ten arithmetic by decimalizing inches etc. Of course, this is not done consistently, so there is often need to consult a fraction-decimal conversion chart.[15] Anyway, what is a third of 11 inches? What is a third of a pint? The fact that the olden units don't use a consistent base at all makes this argument void.
Fahrenheit degrees are smaller, so they offer greater precision than Celsius! Aha!
If we are to accept this reasoning, we also have to look at measures of distance and volume. Kilometres and centimetres offer greater precision than miles and inches, and litres offer more precision than gallons. Also the SI prefixes allow to scale down any unit to its 10-24 order of magnitude (that's very tiny). In practice, using a decimal point where appropriate solves all these apparent problems, so accuracy is really a non-issue.
Fahrenheit is more practical for everyday use than Celsius! 100 °F gives a nice cutoff for really hot weather, and 0 °F for very cold weather!
Celsius is not part of the SI, but that's not much of an argument for avoiding an entire system of weights and measures. One gets used to thinking in Celsius, just as one did for Fahrenheit. The Celsius scale also integrates neatly with the kelvin (which is part of the SI), as 1°C equals 1+273.15 K. The equivalent to the Kelvin scale for Fahrenheit (the Rankine scale) is so obscure you probably just heard of it right now and hardly anybody in science or media uses it for anything ever.
It's too hard (for old people) to learn a new system!
The IS isn't new, and it's been widely used in the English-speaking world since the 1970s. This is really just an argument from laziness and ignorance. Some Luddites don't like change, modern things, science or education, and will refuse to learn. The world will leave them behind and be none the poorer, as has always been the case with Luddism. Moreover, this argument is needlessly patronizing towards those of older generations, who are in general quite capable of learning and using SI if they choose to. It is a completely unproven claim that old people couldn't cope with the switch to a simpler system, and besides, since there will perpetually be a demographic of old people in the population of society, this claim seems to imply that progress in society will have to be postponed infinitely (won't someone PLEASE think of the seniors?!). In addition, remember that parts of the EU went in 1999 into a change of the current currencies in favor of the euro. Old timers went well through it too as they had previously learn basic arithmetic.
What about kitchen measurements? Replacing cups and spoonful with, say, 75 millilitres? That doesn't have the right feel to it at all!
It depends upon where you put it Is that really a serious argument to avoid standardizing with the rest of the world? In reality, it is entirely practical to measure everything in the kitchen in grams and millilitres as appropriate. Even in metric countries, spoonful are used for small quantities. One teaspoonful equals 5 millilitres, making conversion easy. One tablespoonful equals 15 ml. A cup equals 0.2 litres, which means one litre makes five cups. Plus, why are you measuring butter in cups anyhow? As a further aside, the First Amendment totally allows you to write a cooking book with cups and furlongs and eggshells as measurements all you like. But don't try to force pound force on NASA. Incidentally, how many times have you cursed your measuring cups when you needed to halve a recipe that called for a tablespoon or 1/3 cup of something? But if you really are into molecular gastronomy, one drop equals quite exactly 0.05 millilitres (or cubic centimetres), making exact measurements even easier - even in the kitchen!
Changing everything in the country would be too expensive!
In practice, make EU pay for it! this wasn't a serious problem in countries like Australia, which just got on with it and changed everything over to the new system in a few years. Metrication is a one-off cost, whereas unit conversions and the associated confusions, problems and incompatibilities are recurring. Speaking of "too expensive", see our below non-comprehensive list of costs incurred solely by the failure of the US to adopt the metric system like the rest of the world.
Important to know.
Drugs are measured in metric, especially cocaine and other "hard drugs"!
Besides the fact that this claim is untrue (as ounces are a typical measurement for drugs as well), it's simple poisoning the well. If anything, it's probably a sign of both the precision and the ease-of-use for any measurement system that gets picked up by street hustlers and drug cartels that know violence may erupt in response to measurement error.
SI is better for scientific use but English units are better "calibrated" for common use! For example, the gallon has the size is does is because that is a good size for laypeople to measure volumes with, and a foot has roughly the size of a human foot. A meter is defined to be some fraction of the distance traveled by light in a second, which may be great for some Communist scientist in a laboratory, but not for a farmer!
This argument is common in the USA, yet everywhere else where metric units are used, laypeople have absolutely no problem using metric units in everyday situations. In fact, they generally become lost when told to use the supposedly "better calibrated" imperial units. Imperial units have a huge number of distinctly named, arbitrarily sized units and because of the unsystematic definition of the units, conversion between units is nearly impossible without memorizing them all and the chaotic plethora of conversion factors between them. Everyone who regularly uses metric agrees that metric units are much easier for common usage, including farmers all over the world. Additionally, even with the metric system, the expression "foot" is still used as an everyday layperson measurement. This is because people in countries that use the metric system still have feet, and thus, still have a general idea of what size range you mean when you compare something to "the size/length/volume" of a human foot. The loss of the ability to compare things and measurements to human feet within the metric system is, in other words, an unfounded fear. The Quarterpounder is still sold outside the US and Dutch people all know that an ons is exactly 100 grams.
Metric Units are a communist atheist plot cooked up by some proto-communist during the French Revolution, just like the Guillotine and decimal time. We prefer American units
Let's forget the French Revolution occurred in 1789 and Karl Marx was born in 1818. While it is true that the metric system originates in the French Revolution, so does the declaration of the rights of man and the citizen, so this guilt by association is a bit stupid. Also, the imperial measurements are the same King George III and his goons left behind when you threw them out. Furthermore, the guillotine was in use well before the French Revolution and the people involved in it (including its most radical leaders like Robespierre) were not atheists. On another note, is that how you thank the Marquis de La Fayette for his service?
The SI can't provide a proper definition of a year! This is a mark of the devil lack of common sense!
That's because the definition of a year -as a time duration- is defined by astronomy (with great precision). The same goes for months, weeks, days, hours, and minutes time division units and formats. The SI has regards for only the seconds. This complaint is not really supposed to be. In addition, it is because of the SI defined second that the actual calendar stay on track!
Metricated people do not always use the SI, this is sign of rejection!!!
Indeed, there are few units of the SI that are of less utility for laypeople in their day to day life. For instance:
• Vehicles speedometer are graduated in km/h. A strict application of the SI would require the usage of seconds instead of hours (and metres instead of kilometres, to avoid having too many decimal on the road sign). But it would be forgetting that cars and speedometers popped out before the SI -and before road signs too, by the way. Still, the km/h remains quite useful as the driver could guesstimate the travel duration, using m.s-1 instead would require much more focus on the calculation than on the driving... kind of dangerous, one may say.
• Distances at sea and air are still measured as nautical miles (= 1852 m exactly) and speeds as knots (nautical miles per hour). At inland waterways the CEVNI rules use kilometres and km/h, though. For conversion purposes, 1 m/s is very nearly 2 kn. These are examples of historical units the SI deals with without big deal. Needless to say that sea and air navigations are much more subject to adverse weather conditions than terrestrial locomotion, meaning having an accurate speedometer in m/s tilting every millisecond is quite useless. The usage of "wider" and ad hoc units allow the pilot to 1) read the current value and 2) adjust the maneuver to stay on the expected trajectory.
• There is a fairly common use of "metric horsepower" or pferdestärke (instead of kilowatts) for measuring power of automotive engines. As for the speedometers, the SI came after engines started to be manufactured, and engineers needed a few metrics to characterize their product. Nowadays, some local laws in European Union are still based upon the "metric horsepower" to calculate the tax bill to allow the vehicle to take the road. Recent regulations in EU are now based upon the kilowatts and merely only the old timers remember what a horsepower is (probably thanks to this popular-in-its-time car).
• It happens that dudes in pub order beer by shouting "bring one more pint of this [Alcohol|delicious beverage] here!". Remarks, if any, are about the dude impolite behavior rather than the usage of a non metric unit of volume. To be noticed that in some cantons of Switzerland, waiters take order of beer/wine only if rendered in decilitres(!). Well, no one really care about that, provided the beverage has been diligently delivered.
• Angles are likewise measured in degrees, while the absolute unit of angle, radian, has very few practical applications outside of mathematics. Probably because calculations in degrees are more intuitive and easier without calculator than with radian (the latter implies π). The laypeople can get over it by considering that a radian is almost equals sixty degrees, but it's kind of non sense. The degree is a measure of the perceived angle from the measurer point of view, there are [Relativity|some hints it's not the correct way to go]. On the other hand, the radian is a measure of the travelled length independent of the measurer point of view. Guess which one is more reliable.
Clothe sizes are fucked up despite using the SI, this joke has longed enough!
As for the year duration stuff, the SI is about defining a unit for length. How it's used afterward is purely out of scope of the SI. Clothe manufacturers go by a reciprocal I fart at your general direction mindset, deal with it as a customer and contact your local customer service.
The kilogram is designed for mass mess! It do not respect the SI recommendations and therefore must be burnt!
Historical reasons put a SI prefix (the kilo) in the name of the base unit. Counterintuitive effect is that the gram is 1/1000 of a kilogram base unit, and that a milligram has an order of magnitude against the base unit of 10-6 instead of the 10-3 as suggested by the prefix milli. In addition, it's really uncommon to see in literature the magnifying prefixes of the SI applied with the kilogram, for instance, the mass of the Earth is given as 5.972×1024 kg instead of the expected 5.972 Yg (yottagrams) despite the simpler scripture. The ton (also know as 1000 kg) is still of a common usage with the addition of the magnifying prefixes, so Earth can weight either 5.972×1021 tons or 5.972 zettatons. The reader is welcome to notice how nicely the Earth's mass has been converted into four units without calculations, meaning that "flaws" upon the kilogram design are quite trivial in the end.

## Problems caused by having two systems

 “”If kilometers are shorter than miles, could I save gas by taking my next road trip in kilometers? —shittyaskscience[16]

It is often claimed that, while the metric system offers important advantages in science, there is no real harm to retaining the old units and using them in parallel with SI. However, there are many ways in which this "dual-units" approach can cause problems which are just as severe as those caused by the lack of metrication, leading some metric advocates to adopt a position of "don't duel with dual"[17]:

• The Gimli Glider incident of 1983 was the result of improper conversions that left a plane with less than half of its needed fuel for a trip. Incredible piloting prevented any deaths.
• In 1999, NASA's Mars Climate Orbiter was lost due to unit mismatch in software: the orbiter interpreted some instructions to be in newtons, while the command software on Earth sent them in pounds-force. A pound-force is more than 4 N, so the probe's thrusters fired far too hard, decelerating it too much and causing it to enter the Martian atmosphere extremely fast, burning up on entry.
• There are frequent problems when doses of medicine are measured in non-metric units such as drams or teaspoons (the latter of which is readily confused with the tablespoon, a factor of three or four larger). Such confusions have repeatedly led to fatal overdoses, meaning that medicine has increasingly standardized on metric units, requiring all devices for measuring liquid medicines to display millilitres only, and asking parents to dispense medicines to their children using millilitres only.[18][19][20]
• Dose errors can also be caused by confusion regarding patient weight, since medicines are commonly prescribed in a quantity which is proportional to the body mass of the patient (e.g. milligrams of drug per kilogram of patient body mass); notably, no medication is commonly prescribed or described in terms of imperial units. In the US, confusion between pounds and kilograms could cause a dose to be wrong by a factor of 2.2, and in the UK, incorrect dosage due to imperial weighing in the past (e.g. a patient weight recorded as 13 stone 7 pounds and misread as 137 kg, which is about 60 % more) has forced standardization on metric measures for weighing patients, with imperial measurements being explicitly forbidden.[21] This practice also allows ready calculation of the patient body mass index, which is the patient's body mass (kg) divided by the square of their height (m2).
• In the UK, the confusing situation of fuel being sold exclusively by the litre (which is a legal requirement) but driving distances typically still being measured in miles (due to inaction by the Department for Transport), there is no truly useful measurement of fuel economy. Neither miles per imperial gallon (since fuel has not been sold by gallons for decades) nor litres per 100 kilometres (since road signs and associated measurements are in miles and yards) is compatible with these conflicting units for fuel volume and road distance.
• Incompatibility between US Customary and metric units might have contributed to the failure of the US company Target's attempt to expand into Canada.[22] It has also been suggested that similar incompatibilities have caused problems for Boeing when outsourcing work to other countries; their competitors (such as Airbus) have not faced such problems.[23] This point relates to the main reason for the push for metrication in the 1970s: the economic benefit to everyone of the world sharing a single unambiguous language of measurement.
• The education of children is hampered by their having to use two incompatible sets of units in their studies (as in the USA) or the conflict they observe between the units they use exclusively in school, and the older units that are still retained in the outside world (as in the UK). One of the main motivations for metrication was to prevent children from having to spend the enormous amounts of classroom time required to learn the extensive conversion factors necessary to use imperial and US customary units correctly, but until metrication is complete this cannot truly happen, nor can children be free from the need to learn the clumsy conversion factors between imperial and metric units.
• Even the most trivial calculations are made unnecessarily difficult by imperial units, impeding people who use imperial from gaining an intuitive understanding of many measurements, e.g. how they scale up. The smooth and intuitive scaling of metric units does not pose such problems: if one knows what a drive down a 500-metre section of highway feels like, then one can easily reason that a 60-kilometre drive comprises 120 such sections; the same comparison between 500 yards and 60 miles is not so easily made. Likewise with innumerable scalings and comparisons of everyday measurements – if a steak is weighed in ounces, a bicycle in pounds, a man in stones, and a car in long tons and hundredweights, how does one get an intuitive feel of the ratios of their weights? To speak in an aphorism, one might say that the essence of stupidity is to make trivial things difficult, while the essence of intellect is to make difficult things trivial.
• Refusing to convert to the metric system makes travel more difficult for Americans (also for anyone coming to the US, but we're talking about self-interest here). Americans traveling literally anywhere else in the world have to convert all distances, weights, measures, and temperatures to their system to get a feel for what the local figures mean. So no, you don't have to change to what everyone else has settled on, but refusing to change does come with a cost.