of an approximation of a derivative of the Roman foot in metric*

The Roman foot was between approximately 0.96 and 1.1 international feet (most commonly about 0.97 ft, except in modern Belgium where it was 1.091 ft/13.1 in, the size of Nero Claudius Drusus' foot). After that, the foot in Britain was based off the North German foot (~13.2 in), but in the late 13th century it became more like 12 in (so around the same as the modern foot). Later the English foot was between 11.7 and 12.01 in, and the US foot was based on the English foot until the 19th century when they made the US Customary Units and defined the foot as exactly 1200/3937 meters. The British made the British Imperial system and a bit later defined the foot as 1200/3937.0113 meters. They didn't switch to metric because they saw "French Revolutionary units" (metric) as "atheistic". Later, we advanced our understanding of physics, and the British adopted a foot of 304.8 mm in 1930, and the Americans followed them in 1933, based on the new "industrial inch" from the now-unused 1927 light wave definition of the meter (which used the International Prototype, made of a standard bar). The modern foot is defined as exactly 0.3048 meters, by international agreement in 1959 between some English-speaking countries, after the newer Kypton standard definition of the meter (which is also now not used).

Now it's based on the modern meter definition (distance travelled by light in a vacuum in 1/299792458 of a second, which is defined based on the uperturbed ground-state hyperfine transition frequency of a caesium-133 atom being 9192631770 Hz)