Technically US customary units were re-defined as specific values of their equivalent metric units (for instance, 1 inch === 25.4 mm), so you are using the metric system regardless, but with weird conversion factors.

The majority of U.S. customary units were redefined in terms of the meter and kilogram with the Mendenhall Order of 1893 and, in practice, for many years before. These definitions were refined by the international yard and pound agreement of 1959.

https://en.wikipedia.org/wiki/United_States_customary_units