It's perfectly acceptable to use the metric system in the United States -- Congress originally authorized it in 1866 and has repeated those sentiments in the years since -- but tradition tells a whole other tale. Although the government now requires metric use in some public sectors and strongly encourages it in many private industries, the American public never really took to the system and largely dismissed it, making the United States the only industrialized nation where that's the case.
In an effort to move the matter along, Congress even passed a Metric Conversion Act of 1975 and set up a U.S. Metric Board to take care of all the planning for the desired transition, but they apparently didn't empower the board with enough authority, and the American people essentially said, "meh" to adopting metric and continued on with their miles, pounds, ounces and all the rest. Similarly lackluster efforts since then have done little to get Americans to change their ways.
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