When this question arises over and over again, I point out that the US did go officially metric in 1959, including a re-definition of the US customary unit the inch to exactly 2.54 cm. One might say that they didn't tell anyone about it, however. Because of this re-definition, another customary unit, the US Survey Foot, was added following the old definition (39.37 inch = 1 meter, slightly different) so that we did not have to rewrite every single land tenure document in the country. Perhaps the last holdout for customary units will be the screw-fastener industry.