An Early Decision That Stuck
The US adopted its measurement system from British colonial practice in the 18th century. When the metric system spread through Europe in the 1800s, the US was already industrializing with customary units — machinery, railroads, and land records were all built around inches, feet, and acres. Switching later would have meant retooling an enormous industrial base.
Metric Is Already Here
Despite everyday use of customary units, the US is officially metric: a 19th-century law made metric units legal for trade, and a 1988 law made metric the preferred system for federal procurement. Medicine is dosed in milligrams, science uses SI, the military uses metric for many operations, and car speedometers show both mph and km/h.
Why Full Conversion Stalls
Converting every road sign, product label, building code, and school textbook would cost billions and require broad public support that has never materialized. So the US lives with a practical dual system — customary for daily life, metric for science and international trade.