Why the United States Never Fully Went Metric

The question goes back to the founding of the republic. In 1790 Thomas Jefferson, then Secretary of State, sent Congress a plan for a decimal system of weights and measures. The French were developing the metre at the same time, and the US briefly considered joining them. Congress debated the matter but adopted neither Jefferson’s plan nor the French system. The country did, however, adopt decimal currency — dollars and cents — which is why American money is metric-style even though its measures are not.

The Metric Act of 1866 made it lawful to use the metric system in contracts and trade throughout the United States. It did not require anyone to use it, but it settled that metric measures were legally valid.

In 1875 the US was one of the 17 original signatories of the Treaty of the Metre (the Convention du Mètre) in Paris, which created the International Bureau of Weights and Measures (BIPM). In 1890 the US received its own national copies of the new international prototype metre and kilogram.

In 1893 Thomas Mendenhall, Superintendent of the US Coast and Geodetic Survey, issued an order declaring that the fundamental standards of length and mass in the US would be the international metre and kilogram. From then on the yard and the pound were defined as fractions of metric standards rather than by their own physical artefacts.

In 1959 the US, UK, Canada, Australia, New Zealand and South Africa agreed on the international yard and pound, fixing them exactly: 1 yard = 0.9144 metres and 1 pound = 0.45359237 kilograms. That is why an inch is exactly 2.54 centimetres. Strictly speaking, every customary measurement in America has been a metric measurement in disguise ever since.

By the late 1960s most of the English-speaking world was converting. The UK began its conversion in 1965, and Canada, Australia and New Zealand followed in the 1970s. Congress passed the Metric Conversion Act in 1975, declaring a national policy of coordinating voluntary conversion and creating the US Metric Board to oversee it.

The key word was voluntary. There were no deadlines and no requirement for businesses or the public to switch. Some changes did happen — a number of road signs went up in kilometres, and the two-litre soda bottle arrived and stayed — but public reaction was often hostile, and many people saw the change as costly and pointless. The Metric Board was abolished in 1982.

Several factors made conversion harder in the US than elsewhere: an enormous domestic market that did not depend on exports, a huge stock of existing infrastructure and tooling in inches, decentralised road and building codes set at state and local level, and no single government deadline forcing the issue.

Conversion did not stop; it simply went out of sight. The Omnibus Trade and Competitiveness Act of 1988 designated the metric system as the preferred system of weights and measures for US trade and commerce and required federal agencies to use it in their business activities where practical. In 1992 Congress amended the Fair Packaging and Labeling Act so that most consumer products must carry both customary and metric quantities, which is why nearly every package in an American supermarket shows grams or millilitres.

At the end of 2022 the US also retired the “US survey foot”, a slightly different foot used for land surveying, leaving the international foot as the single standard.

The cost of running two systems side by side became famous in 1999, when NASA lost the Mars Climate Orbiter. Software supplied by a contractor produced thruster data in pound-force seconds, while the navigation team expected newton-seconds. The small, repeated error put the spacecraft on a trajectory far too close to Mars, and it was lost as it arrived. The investigation board identified the unit mismatch as the root cause.

A similar mix-up happened in Canada in 1983, shortly after the country switched to metric. An Air Canada Boeing 767 — later nicknamed the “Gimli Glider” — ran out of fuel in mid-air because the fuel load had been calculated in pounds instead of kilograms. The pilots glided the aircraft to a safe landing at a former air force base in Gimli, Manitoba.

Look closely and metric units are everywhere in the US:

There is no active plan to force the issue. The more likely future is the one already under way: a slow drift toward metric in industry, trade and science, while customary units remain in daily conversation, on road signs and in kitchens. For most Americans the practical skill is not choosing one system but moving comfortably between both — which is exactly what converters and quick reference tables are for.