US customary units are the everyday measures of the United States: the inch, the foot, the mile, the ounce, the pound, the cup and the gallon. They descend from the English units the American colonies used before independence. They are not the Imperial system, although the two share most of their names, and the differences between them are large enough to matter.
The distinction is easy to state. Britain reformed its units in 1824 and created the Imperial system. The United States had already been independent for forty years and did not follow. It kept the older English standards, including the wine gallon that Britain abandoned. Every difference between the two systems today traces back to that fork.
English measurement before 1824 was not one system but several overlapping ones. There were at least three gallons in common use: the wine gallon of 231 cubic inches, the ale gallon, and the Winchester corn gallon for dry goods. Which one applied depended on what was being sold. When the United States began standardising its own measures after independence, it selected from this set. Congress adopted the wine gallon for liquids and the Winchester bushel for dry goods.
Britain took a different path. The Weights and Measures Act of 1824 swept away all the older gallons and defined a single new one based on the volume of ten pounds of water. That gallon was about 20 per cent larger than the wine gallon the Americans had just adopted. Neither country changed afterwards, so the gap has persisted for two centuries. Length and mass were not affected, because both countries kept the same yard and the same avoirdupois pound.
US customary units have not been defined by American physical standards since the nineteenth century. The Mendenhall Order of 1893 declared that the national prototypes of the metre and the kilogram, received under the Metre Convention, were the fundamental standards, and that the yard and the pound would be derived from them. From that point the United States has defined its customary units metrically.
The international yard and pound agreement of 1959 fixed the values now in use. The yard became exactly 0.9144 metres and the avoirdupois pound exactly 0.45359237 kilograms, identical to the Imperial definitions. This is why conversions of length and mass between American, British and metric units are exact rather than approximate.
One legacy unit outlived the agreement. The US survey foot, very slightly longer than the international foot, remained in use for land surveying and mapping because changing it would have shifted established coordinates. The difference is about two parts per million, which is negligible over a room and significant over a state. The United States retired the survey foot at the end of 2022, leaving a single definition of the foot for all purposes.
For length and mass, US customary and Imperial units agree exactly. An inch is 25.4 millimetres in both. A pound is 0.45359237 kilograms in both. A mile is 1609.344 metres in both. If a figure in feet, inches, miles, ounces or pounds is being converted, the system it came from does not matter.
The differences in this area are of usage rather than size. The stone, fourteen pounds, is standard in Britain for body weight and effectively unused in the United States, where weights are given in pounds alone. The hundredweight differs: the American short hundredweight is 100 pounds while the Imperial long hundredweight is 112. The ton follows the same split, with the US short ton at 2000 pounds against the Imperial long ton at 2240. A metric tonne is different again at 1000 kilograms.
| Unit | US customary | Imperial | Same? |
|---|---|---|---|
| Inch | 25.4 mm | 25.4 mm | Yes |
| Foot | 0.3048 m | 0.3048 m | Yes |
| Mile | 1609.344 m | 1609.344 m | Yes |
| Pound | 453.59237 g | 453.59237 g | Yes |
| Ton | 2000 lb | 2240 lb | No |
| Gallon | 3.785 L | 4.546 L | No |
| Pint | 473 mL | 568 mL | No |
| Fluid ounce | 29.57 mL | 28.41 mL | No |
Volume is the source of nearly every practical error. The US liquid gallon is exactly 3.785411784 litres. The Imperial gallon is exactly 4.54609 litres. The Imperial gallon is therefore about 20 per cent larger, and the same ratio applies to the quart and the pint that divide it.
The fluid ounce reverses the pattern, which catches people out. A US fluid ounce is 29.5735 millilitres and an Imperial fluid ounce is 28.4131 millilitres, so the American unit is the larger of the two. The reason is arithmetic: an Imperial pint contains 20 fluid ounces while a US liquid pint contains 16. The pint is bigger in Britain, but it is divided into more parts, so each part is smaller.
Fuel economy is where this matters most in daily life. A car rated at 40 miles per gallon in the United Kingdom achieves about 33 miles per US gallon, without any change in the car. Recipes are the other common trap, particularly older British ones written in Imperial fluid ounces.
The United States keeps two separate sets of volume units, one for liquids and one for dry goods such as grain, fruit and vegetables. They are not interchangeable. The US dry pint is 550.6 millilitres while the US liquid pint is 473.2 millilitres, a difference of about 16 per cent.
The dry measures descend from the Winchester bushel, an English standard for corn. The liquid measures descend from the wine gallon. Because both were inherited separately and neither was reformed, the United States ended up with two parallel volume ladders. In practice the dry units survive mainly in agriculture and in the sale of produce, where berries and shellfish are still sold by the dry pint or dry quart. This site treats them as distinct units for that reason.
| Unit | Symbol | Metric value | Convert |
|---|---|---|---|
| US liquid pint | pt | 473.176 mL | US liquid pints to liters |
| US dry pint | pt | 550.610 mL | US dry pints to liters |
| US gallon | gal US | 3.785411784 L | US gallons to liters |
American recipes measure by volume where most of the world measures by weight. The cup, the tablespoon and the teaspoon are the working units of an American kitchen, and none of them is standardised internationally. A US customary cup is 236.588 millilitres, defined as half a liquid pint. A US legal cup, used on nutrition labels, is rounded to 240 millilitres. A metric cup, used in Australia and New Zealand, is 250 millilitres. An older British recipe cup was 284 millilitres, half an Imperial pint.
The tablespoon is worse. It is 14.787 millilitres in the United States, 15 millilitres in most metric countries, and 20 millilitres in Australia. A recipe calling for four tablespoons therefore varies by more than a fifth depending on where it was written. For dry ingredients the volume-to-weight relationship adds further error, because a cup of flour can vary by twenty per cent depending on whether it was scooped or spooned. This is why baking recipes intended to travel are written in grams.
Volumetric flow in American practice is usually expressed as gallons per unit of time. Gallons per minute is standard for pumps, plumbing and irrigation, while gallons per hour appears in fuel consumption and larger water systems. Because the gallon differs between the two systems, a flow rate quoted in gallons per minute means different things in Britain and the United States, and equipment specifications should state which gallon applies.
Pressure in the United States is normally given in pounds per square inch. The atmosphere is a separate unit again, defined as exactly 101,325 pascals, and is used as a reference pressure rather than a working one. It is convenient because it approximates air pressure at sea level, which makes it a natural comparison for diving, altitude and vacuum work. One atmosphere is about 14.7 pounds per square inch.
| Unit | Symbol | Convert |
|---|---|---|
| US gallon per second | gal US/s | to liters per second |
| US gallon per minute | gal US/min | to liters per minute |
| US gallon per hour | gal US/h | to liters per hour |
| Atmosphere | atm | atmospheres to pascals |
The United States has been legally metric for longer than most people assume. The Metric Act of 1866 made metric units lawful for trade. The Metric Conversion Act of 1975 declared metrication the national policy and created a board to coordinate it. The board was abolished in 1982 and conversion was left voluntary, which is why the policy exists on paper without having changed daily life.
Where American practice is metric, it is thoroughly metric. Science, medicine and pharmaceuticals use SI exclusively. The military uses metric for maps and ammunition. Food packaging carries both systems by law. Wine and spirits are sold in millilitres. Cameras, engines and electronics are specified in metric units because they are made for a global market.
What remains customary is the visible layer: road signs, weather forecasts, body weight, recipes, construction and retail. The result is a country where a nurse records a patient in kilograms while the patient thinks in pounds. Converting between the two is routine rather than exceptional, which is what makes accurate conversion worth getting right.
You can browse every unit on this site from the unit dictionary, or compare these units against the Imperial system and the SI system.