Is there any official unit system for answer on world building?*
*When OP didn't use any unit system in his question.

Because every heavy Math based answer in imperial units is for me a foreign language answer.

In the case of OP asking "The weight in lb of a 10 foot spider?"
I get that we should answer in imperial units, but it still feels like a heavily localized answer.

  • $\begingroup$ Is this unit system a new tag ? $\endgroup$ – Drag and Drop Jan 12 '17 at 13:19
  • $\begingroup$ Yes, I added [systems-of-units] because I feel pretty certain that if we don't already have a handful of questions on our Meta about such issues, we're sure to end up with them eventually. Also, having just [discussion] (or any of the other darker-gray, required tags) on a question doesn't really help much with categorization. $\endgroup$ – a CVn Jan 12 '17 at 13:37
  • $\begingroup$ I'm so pround to be the Father of SOU. Note that's it's /su/ like Sue. $\endgroup$ – Drag and Drop Jan 12 '17 at 13:42
  • $\begingroup$ FYI, here's a complete guide to the proper use of SU units: physics.nist.gov/Pubs/SP330/sp330.pdf -- use it in good health! $\endgroup$ – Catalyst Jan 24 '17 at 10:03
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    $\begingroup$ Despite being proudly 'Merican, I look down my nose with disdain at those who use Imperial. If you take a long look around the site, Pierre, you will see that all the legit Hard Science tagged questions are answered in metric, as is right and proper. For less math intensive answers, I don't think that unit conversion is too much effort. Overall, I don't think this is really an issue for this site. $\endgroup$ – kingledion Jan 27 '17 at 3:10

As far as I know...

...there is no official rule regarding what unit system to use

Aside from that it is usually silently agreed upon, as you also say, to use the unit system the OP uses to describe the problem. If they use meters, use meters; if they use feet, do the conversion (which isn't that hard to be fair).

Regarding the * When Op didn't use any unit system in his question:

If they used numbers without annotating them with units (aka meaning) it's best to point it out to them via commentary and/or propose an edit that adds units to all numbers (this might or might not be met with goodwill by the OP, but is a legit practice as you merely strive to improve quality; you do not have a right to force them to accept said edit though..)

If they do not make use of any numbers/units in their question but it is clearly their intent to get numbers as a result, just use what you prefer. You cannot know if they understand metric/imperial before they say so themselves.

How I tend to see it:

The Stackexchange network and its users are all over the world; the Network uses UTC instead of anything localized and thus you should also strive for using SI units whenever you can.

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    $\begingroup$ I agree. We should encourage people/default towards SI units, but if a question specifies something else there's no harm in using it. $\endgroup$ – Tim B Jan 12 '17 at 13:08
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    $\begingroup$ Not Worldbuilding, but I went as far as to use megagrams (Mg) in a recent answer of mine over on Space Exploration. (I could have used kilograms, but it would have implied a much too high precision.) Where authoritative data used imperial units (in that case, feet and feet per second; note that second is a SI unit!), I tried to use the authoritative numbers and offered conversions to SI for ease of reading for those not in the US. Personally I feel that's a reasonable compromise for questions where authoritative data exists. $\endgroup$ – a CVn Jan 12 '17 at 13:14
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    $\begingroup$ I agree! We should burn people not using SI... that will encourage the best practice! $\endgroup$ – Drag and Drop Jan 12 '17 at 13:16
  • $\begingroup$ We could also just use both unit systems (conversions, not mixed). That way everyone can understand the question/answer. $\endgroup$ – Frostfyre Jan 12 '17 at 13:24
  • $\begingroup$ @MichaelKjörling, I get that kilograms is "too precise" in many case but thats why there is Tons, that as a nice conversion rate to megagrams $\endgroup$ – Drag and Drop Jan 12 '17 at 13:28
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    $\begingroup$ @PierreLebon The problem with "ton" is that there are three kinds of them: short ton, long ton, and metric ton, each similar but slightly different from the others. You are thinking of metric ton, which is equal to 1 Mg (but is not an official SI unit). Using megagrams instead of ton avoids the risk of that ambiguity, which in the case of that answer can be significant (where the figures involved are on the order of three or four digit numbers of tons, or hundreds of thousands to millions of kilograms). $\endgroup$ – a CVn Jan 12 '17 at 13:33
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    $\begingroup$ @Frostfyre Then you better make sure the conversions are exact, at which point you introduce uncalled-for significant digits. To say that the temperature is 40 degrees Farenheit is not the same thing as saying that the temperature is 4.44 degrees Celsius (and even that is an approximation). You might not even know if the poster intended 40 +/- 0.5 degrees F, or 40 +/- 5 degrees F, which makes a huge difference when converting. $\endgroup$ – a CVn Jan 12 '17 at 13:35
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    $\begingroup$ For whatever it's worth, I love it when people use unusual prefixes with well-known units. Megameters (or gigameters), femtograms, whatever. It adds flavor. $\endgroup$ – a CVn Jan 12 '17 at 13:49
  • $\begingroup$ @MichaelKjörling True, but I would consider significant digits to only really matter in hard-science questions, where I would also advise using only SI units. The average user doesn't care if it's 4.44 C or 4.444 C or 4.44444 C. $\endgroup$ – Frostfyre Jan 12 '17 at 13:50
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    $\begingroup$ @Frostfyre the average user is unlikely to post a question on WBse $\endgroup$ – dot_Sp0T Jan 12 '17 at 13:53
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    $\begingroup$ @dot_Sp0T "thus you should also strive for using SI units whenever you can." It also helps with educating us Americans. Seriously... I can't think in those units because they just don't occur much in daily life, so having to interact with SI units here is actually helpful, giving me story context in which they are used helps me visualize the unit scale. Over time, maybe the rest of the world can bring USA around. SI really is a better system, it's just not common for us. $\endgroup$ – SRM Jan 21 '17 at 15:43
  • $\begingroup$ @Michael and Pierre: one can use tonne to unambiguously indicate a metric ton. $\endgroup$ – Catalyst Jan 23 '17 at 3:29

My vote would be to execute the formula in your own system and any key results along the way, simply put in parenthesis the alternative figure.

For example, I would say something was 10m (32.8ft) and do the inputs and math in metric until the end result is, say 7m/s2 (15mph).

All the stuff in the middle, for me, would remain metric. It takes very little effort to place an alternative number in parenthesis for the purpose of an answer.

  • $\begingroup$ Yeah use two system. And at the end for the fun I will use an other system the Spanish Inquisition System to rate this idea. No joke Consistency is the key! $\endgroup$ – Drag and Drop Jan 25 '17 at 7:23

Use the system you work with, it will reduce chances for your error, and will make a noticing error an easier task for you. And no matter wich system OP uses. If he uses any and your results are interesting for him, he will be capable of converting, if he is not capable he had no need in your numbers in first place.

  • $\begingroup$ Except many scientific equations require S.I. Units to product meaningful answers. $\endgroup$ – Ash Aug 1 '18 at 15:00
  • $\begingroup$ @Ash you familiar with SI forms of those equations, and it isn't the same as equations do require SI units. Besides imperial units, there are other unit systems which are used in certain fields or were used/popular in the modern era. $\endgroup$ – MolbOrg Aug 1 '18 at 18:45
  • $\begingroup$ No there are equations that state categorically that X must be in Newtons, or Pascals or Coulombs, you can't get a meaningful answer from that equation using Footpounds or PSI, there may be other equations that you can use that include circuitous workarounds to get a result in those units but they're not the same equations as the ones I'm thinking of. $\endgroup$ – Ash Aug 1 '18 at 18:59
  • $\begingroup$ @Ash interesting, I'm open to see such equation, feel free to post/link one $\endgroup$ – MolbOrg Aug 2 '18 at 5:14
  • $\begingroup$ One example I don't even have to look up are the ideal gas laws, Charle's Law for example is only true for temperatures measured in Kelvin. $\endgroup$ – Ash Aug 5 '18 at 10:59
  • $\begingroup$ @Ash here is how they handle that $\endgroup$ – MolbOrg Aug 5 '18 at 19:47
  • $\begingroup$ Yup like I said, circuitous workarounds. $\endgroup$ – Ash Aug 6 '18 at 10:07
  • $\begingroup$ @Ash oO? how is it a workaround? Build their absolute scale the same way as Kelvin scale is built around Celsius? Where exactly is there a workaround? $\endgroup$ – MolbOrg Aug 6 '18 at 21:52
  • $\begingroup$ There's two, R is known as the universal gas constant and is defined by a separate equation, Avogadro's Law, it has to be expressed in moles, although other units may be used for other parts of the unit definition, so you have to change your mass units to metric and defining R means you're no longer using the Clarke's Law equation which means you are using a work around, by definition. $\endgroup$ – Ash Aug 7 '18 at 11:42
  • $\begingroup$ @Ash Avogadro constant pay attention to "Value of NA in various units". It can be defined in any system which has mass $\endgroup$ – MolbOrg Aug 9 '18 at 15:59
  • $\begingroup$ But it must be expressed in units based on the SI Mole. $\endgroup$ – Ash Aug 12 '18 at 10:51
  • $\begingroup$ @Ash what SI in a mole, it just number of particles in a unit of weight. Unit of weight is a system specific, but the concept of it not. idk, we can go on and on about all that - but maybe it will be easier for you to think about all that in a different angle. All SI units and constant and formulas are defined by few base units. Those units are arbitrarily defined - we just did said let it be those definitions. We could choose them differently, by selecting different stuff to be what we agree to be base units. constants would have different numerical value, but the rest would be the same. $\endgroup$ – MolbOrg Aug 13 '18 at 22:25
  • $\begingroup$ The standard Mole is defined as the number of atoms in a 12g sample of pure Carbon-12, it used to be 1g of Hydrogen-1 but that's a harder thing to have good purity control over, but either way it's a metrically defined unit. $\endgroup$ – Ash Aug 14 '18 at 10:57
  • $\begingroup$ As I said, the number of particles in a unit of mass. It is unitless constant. And it changes its value according to the definition of the unit of mass/volume. "In 1811, Avogadro states that equal volumes of different gases at the same temperature contain equal numbers of molecules." - that's basically definition, it is independant from units, and (wiki) "The change in name to Avogadro constant (NA) came with the introduction of the mole as a base unit in the International System of Units (SI) in 1971, which regarded amount of substance as an independent dimension of measurement. .." $\endgroup$ – MolbOrg Aug 15 '18 at 7:24
  • $\begingroup$ @Ash I'm not exactly accurate with my last comment, but comment length limitations are the issue there. $\endgroup$ – MolbOrg Aug 15 '18 at 7:28

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