SI, Imperial and US Customary: What Actually Differs
These three systems overlap more than people expect and diverge in exactly the places that cause trouble. Length and mass agree. Volume and temperature don't.
Volume, where it falls apart
| Measure | Imperial (UK) | US customary | Difference |
|---|---|---|---|
| Gallon | 4.54609 L | 3.785411784 L | US is 17% smaller |
| Quart | 1.1365 L | 0.9464 L | US is 17% smaller |
| Pint | 568.3 mL | 473.2 mL | US is 17% smaller |
| Fluid ounce | 28.41 mL | 29.57 mL | US is 4% larger |
| Cup | 284.1 mL (rare) | 236.6 mL | US is 17% smaller |
This is the real fault line. An imperial gallon is about 17 percent larger than a US gallon, and the difference runs through every subdivision. Fuel economy figures inherit it: a UK car quoted at 50 mpg is doing roughly 42 US mpg, purely because the gallon is bigger. The fuel economy converter handles that, and the liters to gallons converter shows both gallons side by side.
The fluid ounce reverses the pattern, which catches people out. The imperial fluid ounce is smaller than the US one, because an imperial gallon is divided into 160 fluid ounces while a US gallon is divided into 128.
Temperature
The UK and effectively everywhere else uses Celsius. The US uses Fahrenheit in daily life. Kelvin is the SI base unit and is used in science regardless of country.
This is the one difference with no shared ground at all, which is why oven temperature conversion is such a persistent problem for anyone cooking from a recipe written abroad. The oven temperature chart covers the practical case.
Why the US units are metric underneath
The Mendenhall Order of 1893 defined US customary units in terms of the meter and the kilogram, and the 1959 agreement tightened those definitions to exact values. So the US customary system has been defined by metric standards for over a century.
What this means practically: every conversion between the systems is exact for length and mass, because it is a definition rather than a measurement. The exact versus rounded guide explains why that matters for the precision of your answers.
Who uses what
Almost every country uses SI for official, scientific and commercial purposes. The United States is the only large economy where customary units dominate everyday life, though its science, medicine and military use metric.
The UK is a mixed case: officially metric, with imperial units retained for road distances and speeds, draught beer and milk in returnable containers, and widely used informally for body weight in stones and height in feet.
What this means in practice
Three rules cover most of the trouble. First, length and mass conversions are safe and exact. Second, any volume figure needs its origin checked before conversion, because gallon, pint and cup all mean two different things. Third, a recipe or spec sheet that doesn't state its system is ambiguous, and weight is usually the safer route than volume.
The common mistakes guide lists the specific pairs that produce wrong answers.
Common questions
What is the difference between imperial and US customary?
They share most length and mass units but not volume. The imperial gallon is 4.546 liters and the US gallon is 3.785, about 17 percent apart, and the split runs through quarts, pints and cups. Temperature also differs, since the UK uses Celsius and the US uses Fahrenheit.
Is the US on the metric system?
Legally, yes in part. US customary units have been defined in terms of metric units since 1893, and the 1959 agreement fixed the inch at exactly 2.54 cm and the pound at exactly 0.45359237 kg. In daily use the customary units remain dominant.
Which countries don't use the metric system?
The United States is the only large economy where customary units dominate everyday life. Liberia and Myanmar are usually named alongside it, though both use metric units in much official and commercial contexts. The UK is officially metric with imperial units surviving in specific uses such as road signs and draught beer.
Why is the imperial fluid ounce smaller when the imperial gallon is bigger?
Because the two systems divide the gallon differently. An imperial gallon holds 160 imperial fluid ounces; a US gallon holds 128 US fluid ounces. The bigger gallon is cut into more, smaller pieces.
Related guides
Sources
- NIST Guide to the SI, Appendix B: Conversion Factors. National Institute of Standards and Technology. Source for every conversion factor on this page, and for which factors are exact rather than rounded.