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This answer was down voted because of the choice of units.

The answer was giving a calculation in imperial units, rather than SI units. Can we get a consensus on the use of units of measurement, please?

Is the down vote of an answer due to its use of non-metric units OK? Is it desirable? Should such an answer be edited? Or should we, on the other hand, prefer imperial units or some other system?

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Units should preferably be SI, being an international standard. The metric system as a fallback is acceptable, and it is preferable in situations where it’s more common (e.g. minutes, hours, days, months or years instead of seconds for long durations).

The rationale is simple: imperial units (or any other non-metric system) are not used outside the US and virtually not understood outside the US, the UK and perhaps Down Under. While the language on these boards is exclusively English, the audience is still international and imperial units should be considered too localised. For instance, I have no idea, not even a ballpark estimate, of how much a gallon is, and imagine that most people outside the US have the same problem.

SI, on the other hand, is an international standard and the standard for scientific communication (just like English). It should be universally understood, even in the US.

I propose the following guidelines:

  • When quoting from elsewhere, preserve the original units, but supply a translation.
  • For everything else, use SI or the metric system.
  • Edit existing answers to supply SI or metric units, preserving the original author’s notation where necessary (see above).
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    " I have no idea, not even a ballpark estimate, of how much a gallon is" That entirely depends on whether you're asking about an Imperial, a US liquid or a US dry gallon. Just another reason to stick with SI and metric.
    – John Lyon
    Commented Feb 22, 2012 at 6:02
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    There is another reason why SI/metric makes sense: Many of the sources one might want to cite will come from scientific publications, and virtually all of them will use SI units.
    – Lagerbaer
    Commented Mar 8, 2012 at 16:10
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    "It should be universally understood, even in the US." That's a bad assumption. Most people in the U.S. are about as familiar with the kilometer as you are with the gallon. Editing answers to add metric units makes sense, but replacing the existing units is counterproductive.
    – reirab
    Commented May 2, 2016 at 3:44
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    " I have no idea, not even a ballpark estimate, of how much a gallon is" bing.com/search?q=how+many+ml+in+a+gallon+of+liquid
    – Andy
    Commented Jul 23, 2018 at 21:35
  • @Andy gee, thanks, that's real helpful. 🙄
    – Konrad Rudolph
    Commented Jul 23, 2018 at 21:49
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Note, that metric system and SI system is not exactly the same. There are many measures used typically in metric system, that are not official SI. I don't think you should translate everything to official SI. In fact in most cases it would be counterproductive.

  • time: SI unit it s (seconds); minutes, hours, days, years etc. are not SI. Which affects units below
  • speed: SI unit is m/s; typical metric unit is km/h
  • energy: SI unit is J (joule) = W·s (watt second); typically used metric unit is Wh (watt hour) or kWh (kilo watt hour). Later is very typical when talking about electricity consumption.

Other units used typically within metric system, not being officially part of it:

  • temperature: °C (Celsius), SI unit is K (Kelvin)
  • angle: ° (degree), SI unit is rad (radian)
  • astronomical distances: light year, astronomical unit, SI unit is m (meter)
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We should use the units more familiar to an international audience. Wikipedia claims "SI is the world's most widely used system of measurement"

However, units in quotations from other sources should be left alone to preserve their integrity. Conversions to metric units can be provided outside the quotation.

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  • I made a large change to this to make it fit the new context; kindly check I haven't accidentally distorted your point of view.
    – Oddthinking Mod
    Commented Oct 15, 2012 at 2:11
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IMO, if this is treated as "policy", then it should be modified to (at the very least) exclude situations where the unit and/or quantity in question is actually irrelevant. A recent question happens to have mentioned "4 oz." of water, but the amount in question was utterly irrelevant to anything -- if the amount had been 4 liters, or 4 drops the answer would have remained the same (and this fact was immediately obvious).

If, for example, the claim in question was about the quantity of a material necessary for some particular result, it would be perfectly reasonable to ask that the quantity in question be expressed in terms recognizable/understandable to all. When the answer, however, is "no, that would violate the law of conservation of energy" (or, the second law of thermodynamics, conservation mass, etc.), regardless of the quantity, it is foolish to demand that people provide a translation of an utterly irrelevant quantity just because an assertion being mentioned happens to mention such a thing.

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    This is, I believe a case of “use your head”. If the quantity is irrelevant, leave it be (but better leave it out in the first place).
    – Konrad Rudolph
    Commented Feb 20, 2012 at 9:20
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    @KonradRudolph: I've come across situations where the numbers are relevant just to establish a ratio: the numbers in gallons/liters/blue-whale-mouthfuls is not particularly meaningful but the ratio between two of them (sourced separately) is.
    – Jivlain
    Commented Oct 11, 2012 at 9:01
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    @Jivlain Even that can be a problem though because Imperial units don’t have a common base: I have no idea how many inches there are to a foot, so if somebody uses two values with such mixed units I’m at a loss how to relate them. For quantities in a single unit there’s of course no such problem.
    – Konrad Rudolph
    Commented Oct 11, 2012 at 10:36
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I'd say there should be one exception to the use of metric units: when the question or answer is inherently tied to a non-metric system of measurement. For example, someone asking about the US recommendation of "8 8-fluid-ounce glasses of water a day" would not need to translate it into "8 235 ml glasses of water" before asking, and an answer to a question about differing automotive fuel efficiency between the United States and the UK could freely reference the difference in the size of the gallon.

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    I disagree with both: one is a medical recommendation, which, while somewhat arbitrary, is (hopefully) based on evidence that was published in journals which use SI units. And it, like a fuel efficiency question comparing US and UK cars in galleons, would be completely opaque to the rest of the (English-speaking) world: We are neither a US nor a UK site, and even native/fluent English speakers who didn’t grow up in those two countries do not generally know Imperial units.
    – Konrad Rudolph
    Commented Apr 20, 2015 at 23:25
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Using the metric system is fine, but please don't outright remove existing English units from questions/answers, significant portions of the sites traffic is from the U.S. Those units are gaining traction, but they aren't that well known at every scale. ie 100 meters is fairly universal most everyone knows how long that is, but 2,000 KM may as well be in cubits.

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    My sympathy is quite limited, I have to say. There isn’t really a good excuse for not knowing metric units. Especially on a site where the majority of sources ultimately draws on scientific publications: all relevant scientific publications use metric units.
    – Konrad Rudolph
    Commented Oct 11, 2012 at 10:41
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    @KonradRudolph isn't that position going against the whole don't be a dick rule? I'm not asking for everyone to provide measurements in real units, I'm just asking you not to declare war on anything you don't understand and delete it. Also this isn't a scientific journal its a U.S. based website.
    – Ryathal
    Commented Oct 11, 2012 at 12:20
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    I’m not declaring war on “anything you don’t understand” – I’m declaring war on an inferior, outdated, provincial system of measurement that has been supplanted by a superior system that is universally accepted except in one small corner of the world: I’m declaring war on special pleading. And no, this has got nothing to do with the “don’t be a dick rule”. For the same reason you could argue that we should accept answers without references, in order not to be dicks.
    – Konrad Rudolph
    Commented Oct 11, 2012 at 12:24
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    @KonradRudolph: I agree with your opinion, but doubt your particular statement that "all relevant scientific publications use metric units". Isn't it possible, if not inevitable, that old scientific papers, particularly those published in the US, but also those published in Canada, the UK, and other countries before converting to metric/SI, are still relevant?
    – Flimzy
    Commented Feb 14, 2014 at 0:22
  • @Flimzy Hence my use of the word “outdated”. Of course you’re right.
    – Konrad Rudolph
    Commented Feb 14, 2014 at 9:52
  • @KonradRudolph: Fair enough. :)
    – Flimzy
    Commented Feb 14, 2014 at 12:21
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    @KonradRudolph You're sounding like a dick to me. You argue "don't use imperial units because people won't understand them", then when faced with the fact that millions of people and arguably the bulk of SE users don't understand metric, you suddenly have a dearth of sympathy.
    – user11643
    Commented Aug 20, 2019 at 16:21
  • @fredsbend All a matter of perspective. To non-US citizens, I can assure you, the opposite attitude sounds even more dickish. What I oppose is special treatment for a single country based on no good reason except special pleading. It’s incredibly entitled and ignorant. If opposing that makes me a dick in your eyes I can live with that.
    – Konrad Rudolph
    Commented Aug 20, 2019 at 16:25
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    @KonradRudolph "special pleading". You keep saying that. I don't think you know what it means. 1) It's a u.s. based website. 2) The bulk of its users are u.s. based. Are those arguments of special pleading? The dickish part of your comments is that you ignore and dismiss this, feel justified in your own special pleading, and apparently don't care if the majority of the site's users can't immediately understand the units used here. And then you exhibit further dickery by calling it ignorance and arrogance when one points this out to you, while ironically not having a clue how large a gallon is.
    – user11643
    Commented Aug 20, 2019 at 16:50
  • @fredsbend I’m not ignoring that the majority of the users are US based. I am saying that this fact has no bearing on the intended target audience. The target audience is international, not US-only. And, what’s more, the subject matter is scientific, thus suggesting using scientific conventions. You want to make a special rule for a subset of the users. That’s by definition special pleading. Claiming that “the majority of the site’s users can’t immediately understand the units”, if true, is an indictment of the US education system but otherwise irrelevant here.
    – Konrad Rudolph
    Commented Aug 21, 2019 at 10:23

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