Metric vs Imperial vs US Customary: Why There Are Two Systems
Most of the world uses metric, but miles, pounds and gallons persist. The two families are now tied together by exact definitions.
By Editorial Team · Updated September 19, 2026 · 2 min read
Nearly every country uses the metric system for science, trade and most daily life. Yet miles, pounds and gallons remain part of everyday speech in some places, which is why conversions are still needed.
Where each system comes from
The metric system was created in France in the 1790s, built on tens: 10 millimetres in a centimetre, 100 centimetres in a metre, 1,000 metres in a kilometre. Its modern form is the International System of Units (SI).
The imperial units developed in Britain from older traditional measures, and the US customary units come from the English units in use at the time of American independence. The two share names such as pound, foot and gallon, but not every size matches.
Exact links between the systems
Since an international agreement in 1959, the yard and the pound have been defined in terms of metric units.
| Unit | Exact metric value |
|---|---|
| 1 inch | 25.4 millimetres |
| 1 foot | 0.3048 metres |
| 1 yard | 0.9144 metres |
| 1 mile | 1,609.344 metres |
| 1 pound | 0.45359237 kilograms |
Because of these definitions, converting between the systems involves no measurement error, only the rounding you choose.
Where the systems differ in size
Volume is where the confusion lies. The US gallon is defined as 231 cubic inches, which is 3.785411784 litres. The imperial gallon is 4.54609 litres, about 20% larger.
Fluid ounces and pints differ too. A US pint is 16 US fluid ounces, or about 473 millilitres, while an imperial pint is 20 imperial fluid ounces, or about 568 millilitres. A recipe or fuel figure from one country may be quoted in the other's unit without saying so.
Who uses what today
The United States is the main country where customary units remain standard in daily life, though science, medicine and much of industry there use metric. The United Kingdom is a mixture: road distances in miles and beer in pints, alongside metric for most packaged goods.
Tips for accurate conversion
- Check which gallon, pint or ton is meant. There are short and long tons as well as the metric tonne.
- Use the exact factors above, and round only at the end.
- Convert temperature with its own formula, since the scales have different zero points.
Try the length converter, weight converter and volume converter, which use these exact definitions.
Frequently asked questions
- Is a US gallon the same as a UK gallon?
- No. A US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres.
- Why are miles still used?
- Because of tradition and the cost of changing signs, laws and habits. Some countries use them on roads even though they use metric elsewhere.
- Are the conversion factors approximations?
- The inch, foot, yard, mile and pound factors are exact by definition.
About the author
Editorial Team. The Editorial Team writes, checks and maintains every tool and guide on this site. The calculation logic behind the calculators is covered by automated tests, examples are worked out and re-checked before publishing, and pages are updated when we find a mistake or a rule changes. Corrections are welcome through the contact page.
Tools mentioned in this article
- Length ConverterConvert between millimeters, centimeters, meters, kilometers, inches, feet, yards and miles.
- Weight ConverterConvert kilograms, grams, pounds, ounces, stone and tonnes.
- Volume ConverterConvert liters, milliliters, US gallons, imperial gallons, quarts, pints, cups and cubic units.
- Temperature ConverterConvert between Celsius, Fahrenheit and Kelvin.
- Cooking Unit ConverterConvert teaspoons, tablespoons, cups, fluid ounces, pints, quarts and milliliters.
- Area ConverterConvert square meters, square feet, acres, hectares, square miles and more.