Metric vs imperial: why the US still uses inches, pounds and Fahrenheit
America's inches and pounds have been defined by the meter and kilogram since 1893. Here is why the switch was never finished, how US and UK units differ, and how to convert with confidence.
Key takeaways
- Metric has been legal in US trade since 1866, but no law ever made it mandatory for everyday use.
- Since the 1893 Mendenhall Order, US yards and pounds have been derived from the meter and kilogram; since 1959 an inch is exactly 2.54 cm and a pound exactly 0.45359237 kg.
- A US gallon is 3.785411784 L and an imperial gallon is 4.54609 L, about 20.1% larger, while an imperial fluid ounce is about 3.9% smaller than a US one.
- The 1999 Mars Climate Orbiter was lost because one team used pound-force seconds and another expected newton-seconds, a factor of about 4.45.
- The UK and Canada are officially metric but still use miles, pints, pounds, and feet and inches in daily life.
The United States still uses inches, pounds and Fahrenheit because it never made the metric system mandatory. Metric has been legal in US trade since 1866, US customary units have been defined in metric terms since 1893, and federal law has called metric the preferred system since 1988, but every step was voluntary, and everyday life never had to change.
So the US is not really a non-metric country. It runs on metric underneath and shows customary units on the surface. Here is how that happened, how American units differ from British ones, and how to move between the two systems without mistakes.
How the metric system began in 1790s France
Before the French Revolution, France had hundreds of local units that changed from town to town. In 1791 the French Academy of Sciences proposed a new unit of length, the meter, defined as one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. Two astronomers, Jean-Baptiste Delambre and Pierre Méchain, spent most of the 1790s surveying the arc from Dunkirk to Barcelona to pin that length down. France adopted the metric system by law in 1795, and in 1799 platinum standards of the meter and the kilogram were deposited in the national archives.
The big ideas were decimal multiples (no more 12 inches to a foot or 16 ounces to a pound) and units linked to each other: a liter is a cubic decimeter, and a liter of water has a mass of about one kilogram. The United States was paying attention. Thomas Jefferson proposed a decimal system of weights and measures for the new nation in 1790, and the US did adopt decimal currency, but Congress never acted on his measurement plan.
The American path, from 1866 to the Mars Climate Orbiter
The US story is a long series of half-steps. Each one moved the country closer to metric without forcing anyone to change the units they used at work or in the kitchen.
1866 and 1875: legal, and a founding member
The Metric Act of 1866 made it lawful to use metric units in US contracts and trade. In 1875 the US was one of the 17 original signatories of the Treaty of the Meter, which created the International Bureau of Weights and Measures (BIPM) near Paris. The US received its own national prototype meter and kilogram in 1890.
1893: the Mendenhall Order
In 1893 Thomas Mendenhall, Superintendent of the US Coast and Geodetic Survey, ruled that the yard and the pound would from then on be derived from the international meter and kilogram instead of from British standards. This is the quiet turning point. Since 1893, every American inch and pound has been a metric unit in disguise. Under the order the yard was 3600/3937 of a meter, or about 0.9144018 m.
1959: the International Yard and Pound Agreement
The US, the UK and other Commonwealth countries each had slightly different yards and pounds. In 1959 the US, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed on common values: the yard is exactly 0.9144 m and the pound is exactly 0.45359237 kg. That makes the inch exactly 2.54 cm. The new yard was about 2 parts per million shorter than the Mendenhall yard, a gap that mattered only to surveyors. The older value lived on as the US survey foot, which NIST retired at the end of 2022.
1975: the Metric Conversion Act
President Gerald Ford signed the Metric Conversion Act in December 1975. It declared a national policy of coordinating the switch to metric and set up the US Metric Board, but conversion was voluntary. Without a deadline, most businesses and the public saw no reason to change, and the board was abolished in 1982. The Omnibus Trade and Competitiveness Act of 1988 later amended the law to name metric the preferred system for US trade and commerce and to require federal agencies to use it in their business where practical.
1999: the Mars Climate Orbiter
The cost of running two systems became famous in September 1999, when NASA lost the Mars Climate Orbiter as it arrived at Mars. Ground software supplied by the contractor Lockheed Martin reported thruster impulse in pound-force seconds, while the navigation team at NASA's Jet Propulsion Laboratory expected newton-seconds. One pound-force is 4.4482 newtons, so each small thruster firing was underestimated by a factor of about 4.45. The errors built up over months, the spacecraft passed far too low through the Martian atmosphere, and it was lost.
US customary vs British imperial: same names, different sizes
Many people assume American and British units are the same thing. For length and weight, since 1959, they mostly are: an inch, a foot, a mile, an ounce and a pound are identical on both sides of the Atlantic. Volume is where they split. The US kept the old English wine gallon of 231 cubic inches, while Britain introduced the larger imperial gallon in 1824. Today the imperial gallon is defined as exactly 4.54609 L. The table below is computed from those exact definitions.
| Unit | US customary | British imperial | Imperial vs US |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | +20.1% |
| Quart (1/4 gal) | 946.35 mL | 1,136.52 mL | +20.1% |
| Pint (1/8 gal) | 473.18 mL | 568.26 mL | +20.1% |
| Fluid ounce | 29.57 mL | 28.41 mL | −3.9% |
| Fluid ounces per pint | 16 | 20 | +25.0% |
| Ton | 907.18 kg (short ton, 2,000 lb) | 1,016.05 kg (long ton, 2,240 lb) | +12.0% |
| Hundredweight | 45.36 kg (100 lb) | 50.80 kg (112 lb) | +12.0% |
Three things stand out. First, every imperial unit of liquid volume from the gallon down to the pint is about 20% larger than its US namesake, so a British pint of beer is noticeably bigger than an American one. Second, the fluid ounce flips the other way: an imperial fluid ounce is about 4% smaller, because the imperial pint is split into 20 fluid ounces instead of 16. Third, a "ton" can mean three different things. The US short ton is 2,000 lb, the British long ton is 2,240 lb, and the metric tonne is 1,000 kg, which sits between them. When a figure says "tons", check which one.
The same trap appears in fuel economy. A car rated in UK miles per imperial gallon always shows a bigger number than the same car rated in US mpg. For everyday volume conversions, the liters to gallons converter shows both gallons side by side.
Where the US already uses metric
Metric is everywhere in American life once you look for it. It tends to appear where precision, safety or international trade matters.
- Science and engineering. US research labs, universities and most engineering fields work in SI units, and NASA now runs its missions in metric.
- Medicine. Doses are written in milligrams and milliliters, and hospitals commonly record patient weight in kilograms for drug calculations, even if the scale at a checkup shows pounds.
- The military. US forces measure distance in kilometers (soldiers call them "klicks"), and ammunition is sized in millimeters, such as 5.56 mm and 9 mm, in line with NATO allies.
- Drinks. Soda comes in 2-liter bottles, and wine and spirits are sold in metric sizes, with 750 mL the standard wine bottle.
- Food labels. Nutrition Facts panels list fat, carbohydrate and protein in grams, and most packaged goods show a metric quantity next to the US customary one.
- Cars. Engine sizes are given in liters, and modern cars are largely built with metric fasteners.
An American can drink a 2-liter soda, take a 500 mg tablet and run a 5K on the same Saturday without thinking of any of it as "metric".
The UK and Canada: officially metric, mixed in practice
The US is often described as the last holdout, but its closest English-speaking neighbors are not fully metric either.
United Kingdom
Britain began its move to metric in 1965. Shops now sell most goods by the gram and the liter, and schools teach metric first. Yet road signs still show distances in miles and speed limits in miles per hour, draught beer and cider are served in pints, and many people give their body weight in stone and pounds (a stone is 14 lb, or 6.35 kg). Heights are often quoted in feet and inches, while weather forecasts use Celsius.
Canada
Canada began converting in the 1970s, and its road signs switched to kilometers per hour in 1977. Official measures, fuel pumps and weather reports are metric. Everyday speech is still mixed: many Canadians give their height in feet and inches and their weight in pounds, lumber is sold as 2x4s, and grocery flyers often advertise prices per pound. The mix has caused real trouble. In 1983 an Air Canada Boeing 767, later nicknamed the Gimli Glider, ran out of fuel in flight after its fuel load was worked out in pounds when kilograms were needed. The crew glided it to a safe landing at a former air base in Gimli, Manitoba.
Why the US has not fully switched
The reasons are practical more than stubborn. The voluntary approach of 1975 gave nobody a deadline. Changing road signs, building codes, tools, property records and school curricula costs money up front, while the benefits are spread thin. Many US industries that sell abroad went metric on their own, which removed the strongest economic pressure for everyone else. And customary units are woven into culture: a football field is 100 yards, a quarter-pounder is a quarter pound, and a comfortable room is about 70 °F.
The result is a stable two-layer system. Official definitions, science and exports use metric, while consumers see inches, pounds and Fahrenheit. That is why converting well is a useful everyday skill for anyone who works, cooks or travels across the two.
Practical tips for switching between systems
- Memorize the exact anchors. 1 in = 2.54 cm, 1 lb = 0.45359237 kg, 1 mile = 1.609344 km and 1 US gallon = 3.785411784 L. Everything else follows from these.
- Use rough multipliers for quick estimates. A kilogram is about 2.2 lb, a meter is a little over 3 ft 3 in, a kilometer is about 0.62 mile and a liter is a little more than a US quart.
- Learn temperature landmarks instead of the formula. 0 °C is 32 °F, 10 °C is 50 °F, 20 °C is 68 °F, 30 °C is 86 °F, 37 °C is 98.6 °F and 100 °C is 212 °F. Every 10 °C step is 18 °F. For exact values, use the Fahrenheit to Celsius converter.
- Check which gallon, pint or ton. A British recipe that calls for a pint means 568 mL, not 473 mL. A shipping quote in tons needs to say short, long or metric.
- Keep the unit with the number. The Mars Climate Orbiter was lost because numbers crossed between systems without anyone catching the unit. Write "12 lbf·s" or "12 N·s", never just "12".
- Convert once, at the end. Do a calculation entirely in one system and convert the final answer, rather than converting and rounding at every step.
- Round to match the job. A body height needs the nearest centimeter or half inch; a machined part needs far more precision. The cm to inches converter shows the nearest 1/16 inch for tape-measure work.
For travel, the km to miles converter turns road signs and race distances into familiar numbers, and the length converter covers every metric and customary length unit in one place.
