Why does the US still use pounds instead of kilograms?

Ask why the United States still weighs groceries, luggage, and people in pounds, and the usual answer is "American stubbornness." The real story is more specific and more interesting: the US pound is, by federal law, defined as a fraction of a kilogram — and the country tried, twice, to switch to metric for everyday use and backed off both times for reasons that had more to do with politics than measurement.

The pound is legally defined by the kilogram

Since the Mendenhall Order of 1893, US customary units haven't been independent standards — they've been metric units in disguise. That order made the meter and kilogram the US's fundamental reference standards, and defined the yard and the pound as fixed fractions of them. The modern figure, codified in NIST Handbook 44, is exact:

1 pound (avoirdupois) = 0.45359237 kilograms exactly

So when a US bathroom scale reads "150 lb," that number is legally traceable back to a kilogram-based definition — the US never actually maintained an independent physical pound standard the way it maintains, say, its own currency. The customary system riding on top of a metric foundation is the first surprise in this story.

Two failed pushes to go fully metric

The US had real opportunities to switch. The Metric Conversion Act of 1975 declared metric the "preferred system" for US trade and commerce and established a federal Metric Board to coordinate a voluntary transition. But the law contained no mandate and no deadline — conversion was optional, industry by industry. Highway signs, supported by a Department of Transportation push, briefly experimented with dual km/mile signage in the late 1970s, and some manufacturers dual-labeled products. Public and congressional resistance mounted quickly: metrication was framed as costly, un-American, and unnecessary for a domestic market that mostly traded with itself. Funding for the Metric Board was cut, and it was formally dissolved in 1982. A second, smaller push came with the Omnibus Trade and Competitiveness Act of 1988, which required federal agencies to use metric in their own procurement and business — a mandate that changed government contracting but left consumer life untouched.

Why the switch never reached the supermarket

By the time either law existed, customary units were already embedded in tooling, road infrastructure, packaging, and generations of intuition — a pound of coffee, a mile to school, a foot as a rough forearm-length estimate. Converting consumer-facing measurement is a coordination problem: it only works smoothly if everyone switches roughly together, and without a mandate, no single retailer or manufacturer had reason to go first. Countries that did fully metricate — most of the world, over the 19th and 20th centuries — nearly all did it with a legal deadline and enforcement behind it. The US never applied that pressure to consumer use, so the voluntary system voluntarily stayed put. Science, medicine, the military, and export manufacturing in the US use metric routinely; it's specifically the household and retail layer that never converted. For more on how other countries handled (or partially handled) the same transition, see our guide on why metric won everywhere except the US.

Converting between the two today

Because the pound is an exact metric derivative, converting kilograms to pounds has zero uncertainty — only rounding choices. Multiply kilograms by 2.2046226218 to get pounds, or divide pounds by that same figure to get kilograms; most everyday use rounds to 2.2046 or even 2.2. See the kilograms vs pounds comparison for the mental-math shortcuts and where they break down.