One of the strangest things that comes up in discussions of metrification, is that US traditional units (“feet”, “inches”, etc.) are referred to as Imperial units.
They’re not the same.
Correctly speaking Imperial units began with the British Weights and Measures Act of 1824 which essentially rolled up all previous legislation, repealed it, and set up a full set of standards for weights and measures. This was obviously after the US declared independence so had no effect over there.
The US units were based on traditional English units which were chiefly defined by the Exchequer Standards or the Winchester Standards (technically there were several standards that could be called “Winchester” dating back to Alfred the Great). That is an oversimplification – various “laws” (the earliest ones were simple pronouncements of the monarch) covering weights and measures are within every single century from the 10th century onwards.
The differences between US traditional units and Imperial units are subtle, but significant in the area of volume – Imperial units of volume don’t distinguish between “wet” and “dry”. A US wet pint is 473mL, a US dry pint is 550mL, and an Imperial pint is 563mL.
So the old saying “a pint’s a pound the world around,” is complete nonsense.
To throw more petrol (or gas) on the fire, both US traditional units and Imperial units are defined by legislation in terms of metric units, so as defined today, neither are proper units of measurement.