Two systems, one convention neither fully avoided
Every country in the world has signed the treaty that created the modern metric system, including the United States, which is usually cited as the exception. The paradox resolves once you look at the actual legislative history: the US never rejected the metre and kilogram. It legalised them in 1866, tied its own customary units to them in 1893, and has used metric definitions as the legal basis for the inch, foot and pound for well over a century. What the US never did is make metric mandatory for everyday commerce — a political choice, not a technical one.
1875: the Metre Convention
On 20 May 1875, representatives of seventeen nations signed the Convention du Mètre in Paris, establishing the Bureau International des Poids et Mesures (BIPM) near Paris and committing signatories to a common system of physical prototype standards for the metre and the kilogram. The United States was one of the original seventeen signatories, alongside France, Germany, Russia and the Austro-Hungarian Empire — it joined the treaty nine years after Congress had already legalised metric use domestically, and it received its own numbered copies of the metre and kilogram prototypes in 1890.
1866: legal, not mandatory
The US Metric Act of 1866, signed by Andrew Johnson on 28 July 1866, made it lawful to use the metric system in the United States for weights, measures and contracts — until then, using metric units in official business had no clear legal standing. The act also fixed a legal equivalence for conversion purposes, one metre equal to 39.37 inches exactly, a ratio that would itself later be superseded by more precise definitions. What the act did not do is require metric use anywhere. That distinction, legal versus mandatory, is why the US could adopt metric definitions internally while continuing to print highway signs in miles.
1893: the order most people have never heard of
This is the fact worth remembering: since 5 April 1893, the inch, foot, yard and pound have not been independently defined units. The Mendenhall Order, issued by Thomas Corwin Mendenhall, then superintendent of the US Coast and Geodetic Survey, declared the national prototype metre and kilogram — the physical standards the US had received under the 1875 convention — the fundamental standards of length and mass for the United States. The yard and the pound became derived units, defined as fixed fractions of the metre and kilogram, rather than standards in their own right. American customary units have been legally metric ever since. Nobody rewrote the road signs; the definitions underneath them simply changed.
1959: closing the gap between three countries' inches
The 1893 order fixed a ratio, 1 yard = 3600/3937 metre ≈ 0.914401829 m, that produced a yard fractionally different from the yard already in use in Britain and the Commonwealth. By the mid-twentieth century this had become a real metrological nuisance: an American-made part and a British-made part built to nominally identical inch dimensions differed by about two parts per million. On 1 July 1959, the national standards laboratories of the United States, United Kingdom, Canada, Australia, New Zealand and South Africa agreed a single international yard of exactly 0.9144 metres and an international avoirdupois pound of exactly 0.45359237 kilograms. Every inch, foot, yard, ounce and pound conversion this site displays for customary units traces back to those two numbers, not to any older, slightly different value.
The tiny discrepancy did not fully disappear, though. The pre-1959 "US survey foot" (0.3048006096… m) stayed in official use for land surveying and geodetic data in the US for another six decades, because millions of recorded property boundaries had been measured against it. NIST and NOAA's National Geodetic Survey jointly retired the survey foot at the end of 2022, moving all US geodetic work onto the 1959 international foot.
What actually differs between the two systems
| Property | Metric (SI) | US customary / Imperial |
|---|---|---|
| Base reference | Seven base units, each independently defined; all others are exact products of these | Yard and pound, each independently defined as an exact fraction of a metric base unit since 1959 |
| Scaling between related units | Always a power of 10 (mm → cm → m → km) | Arbitrary historical ratios (12 in/ft, 3 ft/yd, 1,760 yd/mile) |
| Who still uses it for general measurement | Almost every country | United States; Liberia and Myanmar retain customary units alongside metric |
| Where it survives inside metric countries | — | UK road distances (miles), draught beer (pints), aviation altitude (feet) worldwide |
Why the ratios matter more than the labels
The practical cost of imperial units is not that inches and feet are "less scientific" — a foot is exactly 0.3048 m, no less precise than a metre. The cost is compounding: converting 3.5 miles to feet means multiplying by 5,280, then converting that result to inches means multiplying by 12, and neither factor has any relationship to the other or to base-10 arithmetic. Converting 3.5 km to metres means moving a decimal point. Every extra multiplication is one more place for a transcription or calculator error to hide, and American pharmaceutical dosing works primarily in metric internally (milligrams and millilitres, not grains and fluid ounces) for exactly that reason, regardless of what's on a US road sign.
Where imperial units still win
Not every argument favours metric. Base-12 fractions of a foot divide evenly in more ways than base-10 fractions of a metre divide, which is one reason carpentry and some engineering trades resisted conversion even in fully metric countries: a third of a metre is an ugly 333.33 mm, while a third of a foot is a clean 4 inches. That is a real, defensible advantage in specific manual trades, not a nostalgia argument, and it is the same underlying reason ancient measurement systems tended toward 12 and 60 rather than 10 in the first place.
The takeaway
"Metric vs imperial" implies two systems that never touched. They have been legally welded together since 1893: every inch on an American tape measure is a metric quantity wearing an old name. What separates the US from the rest of the world is not the definition of its units — it is a political decision, repeated every decade since the Metric Conversion Act of 1975 made metrication voluntary rather than forcing the changeover through.