Man könnte ja genauso gut argumentieren, dass das metrische System auch in den USA offiziell gilt, denn schließlich gehörten die USA 1875 zu den 17 Erstunterzeichnern der Meterkonvention.
The practical result of that is that all units of measurement in the non-metric US system of measurement are defined on the basis of the international metric standards.
Back in the '70s, the US was moving to adopt the metric system completely. The non-digital speedometers on cars showed the speed in both English and metric. States were erecting mileage signs on interstate highways that gave certain distances using both systems so that people could get a feel for what a km was -- I remember one that said CHICAGO 100 miles, 161 km, and then later CHICAGO 100 km, 62 miles (or whatever the exact conversion is). Outdoor digital clocks (often on bank buildings, for some reason) that also displayed the temperature would flash the temp between °F and °C. Food packaging began to include both the English weight/volume and the metric equivalent, and various containers of pop were sold in metric-based sizes -- typically 500 ml, 1 liter, and 2 liters.
Then came Ronald Reagan, who basically put an end to the whole conversion. (Except that pop is still sold in those sizes, but cans remain at 12 oz.)
Apparently, the "Metric Conversion Act of 1975," amended by another act in 1988, still requires that the US switch to the metric system, at least as far as the federal government goes.
I think we started learning the fundamentals of the metric system in junior high, and all of my science classes in high school and college used the metric system exclusively. (A physics teacher in high school did give us one lecture on the units of measure in the English system. He took great delight in pointing out that English system's unit of mass (not weight) was called the "slug." It was all so inane that I took nothing else out of that lecture. I'm sure I didn't even take any notes.)