I am used to the metric system where everything is simple and consistent. I can do quick conversions between metric and imperial in my head, but especially when it comes to volumetric measures, I cannot be sure if British or US Customary units are used. Here I refer to units such as gallons, pints, etc. not cubic-linear units.

Questions arising from this confusion is how to know which unit is being used.

In technical publications, will the author or publisher note the applicable units somewhere?

For everyday use, is there a convention that says U.S. gallons or otherwise must be specified, or is it up to the whims of the manufacturer? If not, how can I reduce my chances of making an error if I cannot ask or measure?

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    $\begingroup$ commodities sold in the US invariably have the SI equivalent and/or explicitly state 1 QT US. For example. I don't know the details but I'm pretty sure that's required by law. $\endgroup$ – agentp Jan 8 '18 at 22:43
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    $\begingroup$ the real burning question i have, if i ask for a pint of guinness in an american bar, should that be an us customary or imperial pint? $\endgroup$ – agentp Jan 9 '18 at 12:42

According to a quick scan of Wikipedia, no country actually officially uses imperial units any more. There are plenty of casual uses of imperial (for example, I live in the UK and measure my height in feet and inches, my weight in stone and pounds, and my milk and beer in pints) but no technical uses that I know of.

In technical publications, will the author or publisher note the applicable units somewhere?

In technical publications from the US, US Customary units will be used. In technical publications from the UK, metric units will be used.

So this should work for the vast majority of cases. i.e. pints, gallons etc. are US ones. The main caveat is the publication date: the further back in time you go, the more likely it is that imperial units could have been used (see the Wikipedia article for further detail).

  • $\begingroup$ Aren't BTU still used in the US : eia.gov/energyexplained/index.cfm?page=about_btu $\endgroup$ – Solar Mike Jan 9 '18 at 10:50
  • $\begingroup$ @SolarMike - Sure. What's the relevance to my answer? BTU are part of US Customary. Current use of BTU in the UK would be relevant, and Wikipedia states that the derived unit of therm is used in UK wholesale natural gas. Seems a bit of a corner case to me. $\endgroup$ – AndyT Jan 9 '18 at 11:26
  • $\begingroup$ BTUs are certainly still used. The BTU is however exactly the same in us customary and imperial systems, so beside the point of this question. It is really only the gallon (and fractions of) that pose a potential ambiguity. . $\endgroup$ – agentp Jan 9 '18 at 12:39
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    $\begingroup$ So what I gather from all the answers and comments is that there is no standard defined to distinguish between the two. You'll only really know if the person feels like specifying it or is required. Luckily for most official purposes, most of the world is already converted to metric. $\endgroup$ – ChP Jan 9 '18 at 12:49
  • $\begingroup$ Your conclusion is unclear to me. There certainly are standards. Were you expecting there was some sort of "international law" requiring a farmer to say "US Gallon" every time? I guarantee any sort of refereed technical publication would be rejected if they were unclear about it. $\endgroup$ – agentp Jan 9 '18 at 15:39

It should be clear which is being used from the environment :

If you are in the UK on a farm in Kent for example and you are told "20 gallons" then that will be 20 imperial gallons, but if you are in the USA on a farm in Texas and they say 20 gallons then that will be 20 US gallons (which by the way are smaller).

If you are being sent a product from abroad then usually the units are specified clearly ie US gallons, Imperial gallons (Imp. Gallons).

  • $\begingroup$ Does remind me of the joke about specifying computer disc access rates in "Furlongs per fortnight"... $\endgroup$ – Solar Mike Jan 8 '18 at 13:50
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    $\begingroup$ en.wikipedia.org/wiki/FFF_system $\endgroup$ – Mark Jan 8 '18 at 18:19
  • $\begingroup$ I thought as much that it would be regionally specific. I suppose that the best then to do is find reputable suppliers and do thorough homework. $\endgroup$ – ChP Jan 9 '18 at 7:02
  • $\begingroup$ @Mark, thanks for the link, I found it quite entertaining. I think I've heard of it somewhere but did not really know what it's all about $\endgroup$ – ChP Jan 9 '18 at 7:06
  • $\begingroup$ So a Texan's ten gallon hat is really only about 8 gallons. $\endgroup$ – Brian Drummond Jan 10 '18 at 15:27

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