Why isn't a gallon the same size in the U.S. and UK?

Inches are inches, feet are feet.
But a U.S. gallon is 3.785 liters and a UK gallon is 4.546 liters.
Why the discrepancy?

holy crap… i didnt feel like reading this whole thing but it answers your question somewhere in all that

I would quote the exact text but I don’t know how to do that properly and not get the moderators angry at me for plagarism or something weird like that, so specifically scroll down to “Volume by Volume”

According to this page, the divergence came in 1824 when the UK adopted the Imperial measure, while the US standardised a wine measure used previously.

This has already been answered, but here’s an earlier thread with some good information (assuming the links inside still work):


Thanks guys. Let this thread sink to oblivion.


And feet and inches also not identical. According to a “Documenta Geigy” I have that’s about 30 years old, until 1960, the US inch (and all US linear measures) was slightly larger than the Imperial inch (and all Imperial linear measures). This difference was roughly 0.0003cm, if I can remember correctly.

Then the USA redefined the inch to equal exactly 2.54cm (it had equalled something like 2.540001cm or something like that, according to the measurements of the day).