Metric vs imperial: what is the difference?

Three names, two important distinctions

The metric system is the decimal measurement family used by most of the world. Its modern international form is the International System of Units, or SI. A metre is the base unit for length, a kilogram for mass, and prefixes such as centi- and kilo- scale units by powers of ten.

Imperial units developed in Britain. US customary units share many names and historical roots with them, including inch, foot, yard, mile, ounce and pound. They are not completely interchangeable systems: the clearest everyday difference is liquid volume.

Why metric conversion is usually simpler

Moving between metric units generally means shifting the decimal point. One kilometre is 1,000 metres, one metre is 100 centimetres, and one kilogram is 1,000 grams.

• Large to small metric unit: multiply by the appropriate power of ten.
• Small to large metric unit: divide by the appropriate power of ten.
• For area and volume, apply the scale factor twice or three times because the unit is squared or cubed.

Common metric and imperial relationships

Some cross-system relationships are defined exactly. One international inch equals exactly 25.4 millimetres, so one foot equals exactly 0.3048 metres. One avoirdupois pound equals exactly 0.45359237 kilograms.

• 1 inch = 2.54 centimetres exactly
• 1 foot = 0.3048 metres exactly
• 1 mile = 1.609344 kilometres exactly
• 1 pound = 0.45359237 kilograms exactly

Which system should you use?

Use the units required by your audience, document, recipe, drawing, or regulation. Science and international engineering normally use SI. Everyday US measurements frequently use US customary units. British road distances still use miles, while most other UK measurements are metric. When accuracy matters, label every value and avoid silently mixing systems.