Metric vs Imperial: Which Countries Really Use Which?

You will often read that only three countries do not use the metric system: the United States, Liberia and Myanmar. That line is half true and mostly misleading. It comes from a narrow legal point (which countries have no law making metric units mandatory) and is then repeated as a claim about daily life, where it is wrong in both directions. This guide separates the law from everyday practice, explains why the UK and Canada sit in between, and shows how US customary units and British imperial units differ even when they share a name.

Where the "three countries" claim comes from

The list traces back to older editions of the CIA World Factbook, which named Burma (Myanmar), Liberia and the United States as the countries that had not adopted the metric system. NIST puts it more carefully: only three countries do not have mandatory metric laws, namely Liberia, Myanmar and the United States. That is a statement about law, not about what people use at the shop or on the road. It also leaves out what has happened since:

• Myanmar. Its Ministry of Commerce began discussing measurement reform in 2011, and in October 2013 a deputy minister announced that the country was preparing to adopt the metric system. Speed limits are already set in km/h, weather forecasts use Celsius and fuel is priced per liter. Traditional Burmese units such as the viss (1.63293 kg, about 3.6 lb) remain in use in local trade.
• Liberia. The government announced in 2018 that the country should adopt the metric system. A 2022 law created the Liberia Standards Authority, which began operating in 2024.
• The United States. Metric units have been legal for trade since the Metric Act of 1866, and the country was a founding signatory of the Metre Convention in 1875. Since the Mendenhall Order of 1893, US length and mass units have been defined from metric standards. The 1988 amendment to the Metric Conversion Act calls the metric system "the preferred system of weights and measures for United States trade and commerce". Conversion has always been voluntary, though.

So the accurate version is: three countries have no law making metric mandatory, two of them have announced metrication, and the United States is the one large country where non-metric units still dominate daily life. As of 2011, about 95% of the world's population lived in countries where the metric system was the only legal system of measurement.

Law versus daily life, country by country

A 2011 study cited on Wikipedia adds three small Pacific states with close ties to the US (Micronesia, the Marshall Islands and Palau) where metric use was not mandatory in practice.

The United Kingdom: metric by law, imperial by habit

Britain began a voluntary move to metric in 1965. Packaged goods had to be sold in metric units from 1995, and goods sold loose by weight from 2000. The law still allows a short list of imperial exceptions as the main unit:

• Road signs. Distances and speed limits are shown in imperial units (miles and mph) only.
• Draught beer and cider are sold by the pint (and half and two-thirds of a pint).
• Milk in returnable containers may be sold by the pint, though supermarket milk is usually labeled in both pints and liters.
• Precious metals are traded by the troy ounce.

Since September 2024 still and sparkling wine may also be sold in a 568 mL bottle, which is one imperial pint. A 2023 government consultation drew more than 100,000 responses, and the government decided to keep metric as the primary unit for trade. Socially, imperial survives too: in a 2022 YouGov survey most British adults said they would give a person's height and weight in imperial units.

Canada: a metric country with imperial habits

Canada created a Metric Commission in 1971, switched every road speed sign from mph to km/h in 1977, and now uses metric for most official purposes, including weather reports. The Commission was wound up in March 1985, before the job was finished, and several areas stayed imperial:

• Construction. Lumber and drywall are still sold in imperial sizes.
• Grocery prices. Produce and meat are often advertised per pound, while the package is priced and weighed per kilogram.
• Personal measures. Many people give height in feet and inches and weight in pounds, even though driver's licenses use centimeters.
• Kitchens. Ovens imported from the US often show Fahrenheit, and Canadian recipes use a 250 mL metric cup.

Mixed systems have a cost. In July 1983, Air Canada Flight 143, a Boeing 767, ran out of fuel in flight because a fuel density in pounds per liter (1.77) was used as if it were kilograms per liter. The plane carried 22,300 lb of fuel instead of the 22,300 kg it needed, and glided to a landing at a former air force base in Gimli, Manitoba.

US customary and British imperial units are not the same

Both systems grew from older English units, but they split in 1824, when Britain's Weights and Measures Act replaced its several gallons with one imperial gallon, originally the volume of 10 lb of water. The United States kept the older 231 cubic inch wine gallon. Lengths and masses were later unified by the 1959 international yard and pound (1 inch = 25.4 mm, 1 lb = 0.45359237 kg), but volumes and large masses still differ. The ratio column is the imperial unit divided by the US one.

The US ton is called the short ton (907.18 kg) and the imperial ton the long ton (1,016.05 kg). The metric tonne (1,000 kg) is a third ton, 10.2% heavier than a short ton. Inches, feet, miles, ounces and pounds are identical in both systems, which is why most people never notice the split until they compare fuel economy or beer.

Worked example: the same "50 mpg" in two countries

A car review in Britain says a hatchback does 50 mpg. A US review of the same car may quote a much lower figure, and the gallon is the reason. Converting both to liters per 100 km:

1. One mile is 1.609344 km. A US gallon is 3.785412 L; an imperial gallon is 4.54609 L.
2. For miles per US gallon: L/100 km = 100 × 3.785412 ÷ 1.609344 ÷ mpg = 235.215 ÷ mpg.
3. For miles per imperial gallon: L/100 km = 100 × 4.54609 ÷ 1.609344 ÷ mpg = 282.481 ÷ mpg.
4. 50 mpg (imperial) = 282.481 ÷ 50 = 5.65 L/100 km.
5. 50 mpg (US) = 235.215 ÷ 50 = 4.70 L/100 km, a car that uses about 17% less fuel.

The SamemaS fuel economy converter has separate fields for mpg (US) and mpg (UK), so you can enter either and read the other along with L/100 km. Our guide to mpg and L/100 km covers the conversion in more depth.

Converting between the systems

Every SamemaS converter shows metric and non-metric units side by side and updates all of them as you type, with the calculation running in your browser. The cooking converter lists US and UK fluid ounces, pints and gallons separately, and the length converter uses the exact international inch, foot and mile. For mass, see our kilograms to pounds guide.

The short version

• Only the US, Liberia and Myanmar lack a mandatory metric law (NIST), but Liberia and Myanmar have both announced metrication.
• The United States is the main country where customary units dominate daily life, though metric is legal and widely used there.
• The UK and Canada are officially metric with imperial habits: miles and pints in Britain, lumber, pounds and feet in Canada.
• US and imperial pints, quarts and gallons differ by about 20%; tons and hundredweights by 12%.
• Always check which gallon a fuel economy figure uses before comparing cars.

Frequently asked questions

Sources

1. Metrication — Wikipedia
2. Metrication law — National Institute of Standards and Technology
3. Choice on units of measurement: guidance on markings and sales — GOV.UK
4. Liberia joins the club — Metric Views
5. Myanmar units of measurement — Wikipedia
6. Comparison of the imperial and US customary measurement systems — Wikipedia