Metric vs Imperial: What Is the Difference?

Two systems, two design philosophies

The metric system and the imperial system both let people describe length, mass and volume, but they are built on different principles. The metric system was designed on purpose. It began in France in the 1790s, during a period of reform that wanted measures based on a rational scheme instead of local custom, and it was launched there in 1799. The imperial system grew out of centuries of English and British practice and was tidied up by law much later.

The practical result is that metric units are linked by powers of ten, while imperial units are linked by assorted factors that reflect how people once measured things: twelve inches to a foot, three feet to a yard, 1,760 yards to a mile. Neither design is wrong, but they behave very differently when you calculate with them.

How the metric system is organised

In the metric system, each quantity has one base unit, and larger or smaller units are made by adding a prefix. A kilometre is 1,000 metres, a centimetre is one hundredth of a metre and a millimetre is one thousandth. The same prefixes work for grams, litres and many other units, so once you know the pattern you know it for everything.

That regularity makes conversion within the system a matter of moving a decimal point. To change 3.5 kilometres into metres, multiply by 1,000 to get 3,500 metres. To change 450 millimetres into metres, divide by 1,000 to get 0.45 metres. Volume and mass are tied to length as well: a cube 10 centimetres on each side holds exactly one litre, and there are 1,000 litres in a cubic metre.

How the imperial system is organised

Imperial units form a family of related but irregular steps. For length, there are 12 inches in a foot, 3 feet in a yard and 5,280 feet in a mile. For volume, 20 imperial fluid ounces make an imperial pint and 8 pints make a gallon, so a gallon holds 160 imperial fluid ounces. For mass, 16 ounces make a pound and 14 pounds make a stone.

Working inside the system means switching between different multipliers. Converting 2.5 miles into feet requires 2.5 × 5,280, which is 13,200 feet. Converting that to yards means dividing by 3, giving 4,400 yards. None of this is difficult with a calculator, but each step uses a different number that has to be remembered or looked up, and the chance of choosing the wrong one is higher than when every step is a power of ten.

A bit of history

The imperial system was first defined as a single standard by the Weights and Measures Act 1824, which took effect on 1 January 1826. The act defined the imperial gallon by the volume of ten pounds of distilled water under specified conditions, replacing older standards. The metric system, by contrast, started with the idea that the metre should be a fixed fraction of the Earth’s size, specifically one ten-millionth of the distance from the equator to the North Pole.

The international community later standardised metric measurement. The General Conference on Weights and Measures introduced the modern International System of Units, or SI, in 1960. In May 2019 the SI was redefined so that all its base units rest on fixed natural constants, which is a long way from the physical objects and body-based measures behind many older units.

They are less separate than they seem

One surprising fact is that imperial units are now defined in terms of metric ones. Under an international agreement of 1 July 1959 between Australia, Canada, New Zealand, South Africa, the United Kingdom and the United States, the yard was fixed at exactly 0.9144 metres and the avoirdupois pound at exactly 0.45359237 kilograms. From these follow the inch at exactly 25.4 millimetres and the mile at exactly 1.609344 kilometres.

This means that a conversion between metric and imperial is not a measurement of two competing realities. It is an exact multiplication by a defined factor. For example, 10 pounds is 10 × 0.45359237 = 4.5359237 kilograms. The only rounding comes from how many digits you choose to show.

Where each system is used today

Most of the world uses the metric system for trade, science and daily life. Imperial and related units survive in particular places and habits. In the United Kingdom, road distances are given in miles and beer is commonly sold by the pint, even though much else is metric. The United States uses a close relative, US customary units, which share many names with imperial units but differ in some of them, particularly for volume and for larger weights. The US has officially encouraged metric use for trade, but customary units remain dominant in everyday life.

Science and engineering use SI almost everywhere. That is why people who work across borders, such as cooks, travellers, students and engineers, so often need to move between the two systems.

Choosing and converting sensibly

If you are free to choose, metric is generally easier for calculation, because scaling needs only a shift of the decimal point. If you must follow local convention, such as a recipe in cups or a sign in miles, the sensible approach is to convert once, carefully, and keep working in the unit you understand best.

A unit converter such as this one handles the exact factors for you and shows the full equation, so you can see exactly what was converted. For anything where an error matters, such as medicine or engineering, check the original specification and its stated units rather than relying on a quick conversion alone.