Imperial & US Customary Units
The imperial system and the US customary system descend from older English units. They share many names — inch, foot, pound — but differ in some volume measures, which can cause confusion.
Common units and metric equivalents
These are the everyday imperial/US customary units and their exact equivalents in metric (SI) units.
| Unit | Symbol | Metric equivalent |
|---|---|---|
| Length | ||
| inch | in | 0.0254 m |
| foot | ft | 0.3048 m |
| yard | yd | 0.9144 m |
| mile | mi | 1609.344 m |
| Mass | ||
| ounce | oz | 0.028349523125 kg |
| pound | lb | 0.45359237 kg |
| Volume | ||
| pint | pt | 0.473176473 L |
| gallon | gal | 3.785411784 L |
UK imperial vs US customary
The most important difference is volume. The UK imperial gallon is 4.546 L, while the US gallon is only 3.785 L — about 20% smaller. Pints and fluid ounces differ too, so recipes and fuel economy figures do not transfer directly between the two systems.
| Unit | US customary | UK imperial |
|---|---|---|
| fluid ounce | 0.0295735295625 L | 0.0284130625 L |
| pint | 0.473176473 L | 0.56826125 L |
| gallon | 3.785411784 L | 4.54609 L |
History and modern use
The imperial system was standardised in the UK in 1824; the US kept its own customary units after independence. Today the United States, Liberia and Myanmar are the only countries that have not officially adopted the metric system, though the UK still uses miles and pints in daily life.
Frequently asked questions
What is the difference between imperial and US customary units?
They share length and mass units, but differ in volume: a UK imperial gallon (4.546 L) is larger than a US gallon (3.785 L), and their pints and fluid ounces also differ.
How many millilitres are in a US fluid ounce?
A US fluid ounce is about 29.57 mL, while a UK imperial fluid ounce is about 28.41 mL.
Which countries still use imperial units?
The United States uses US customary units; the UK uses a mix of metric and imperial (miles, pints). Most other countries use the metric system officially.