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