Why does the US use miles instead of km?

The common explanation — “America never got around to going metric” — isn’t quite right. The US legally defines its own customary units, including the mile, in exact metric terms, and federal law has called metric the “preferred system” since 1975. What never happened is a law forcing everyday, consumer-facing use — road signs, speed limits, and grocery-store units — to switch. That gap between legal preference and legal requirement is the entire story.

The mile has been a metric unit since 1893

In 1893, the US Superintendent of Weights and Measures signed the Mendenhall Order, which made the meter and kilogram — not the yard and pound — the fundamental standards the US customary units are legally defined against. From that point on, an American mile wasn’t an independent measurement; it was a fixed multiple of the meter. The 1959 international yard-and-pound agreement later pinned the number exactly: 1 mile = 1,609.344 meters, no rounding. In a real legal sense, the US has run on metric definitions for over 130 years — it just kept customary names and customary everyday use.

1975: metric becomes “preferred,” not required

The closest the US came to a national switch was the Metric Conversion Act of 1975, which declared the metric system the country’s “preferred system of weights and measures” and set up a board to coordinate a voluntary transition. The operative word is voluntary — the law created no deadline, no mandate, and no penalty for continuing to use customary units. Industries that traded internationally converted on their own initiative (metric fasteners in manufacturing, metric dosing in medicine); industries whose output was purely domestic — road construction, retail grocery, residential real estate — had no commercial reason to.

1988: federal agencies go metric, consumers don’t

The Omnibus Trade and Competitiveness Act of 1988 amended the 1975 law to require federal agencies to use metric units in their own procurement, grants, and business activities “to the extent economically feasible.” This is why NASA missions, military logistics, and federal construction specs run largely in metric today. But the law was aimed at government operations, not at Congress rewriting speed-limit signage or forcing supermarkets to sell milk in liters — so the visible, everyday units Americans encounter stayed untouched.

Why the switch never reached the street

Road signage is the clearest case of the cost-versus-mandate problem. State transportation departments own their signs, and converting a state’s entire mile-marker and speed-limit system to kilometers is a multi-decade, multi-billion-dollar undertaking with no federal funding attached and no federal requirement forcing it. A handful of pilot conversions were tried in the 1970s and largely reversed under public pushback. With no legal deadline and a real cost, the path of least resistance — keep the miles — won by default, the same way most holdout customary units elsewhere in daily American life survived.

The practical takeaway

If you’re converting between systems — reading a European road sign as an American driver, or the reverse — the number itself is not in dispute: it’s a fixed, legally exact ratio, not an estimate one country got wrong. A miles-to-km converter applies the same 1.609344 factor NIST and the international agreement define, regardless of which country’s road signs you’re reading.