Metric vs Imperial System
Metric vs imperial system is really a question about history: the metric system was designed in revolutionary France to be rational and decimal, while the imperial system grew out of centuries of English custom — feet, stones, gallons — and was never redesigned. Both measure the same world; they just disagree about the size of the steps, and that disagreement still causes real confusion at borders, in kitchens and on factory floors.
Two systems, side by side
| Quantity | Metric | Imperial / US customary |
|---|
| Length | millimetre, centimetre, metre, kilometre | inch, foot, yard, mile |
| Mass | gram, kilogram, tonne | ounce, pound, stone, ton |
| Volume | millilitre, litre | fluid ounce, pint, quart, gallon |
| Temperature | degrees Celsius | degrees Fahrenheit |
The metric column converts by tens: 1 metre = 100 centimetres = 1,000 millimetres, and 1 litre of water weighs 1 kilogram by design. The imperial column converts by twelves, threes, sixteens and other historical accidents — and it has traps the metric system avoids entirely, like the US gallon (3.785 litres) versus the UK gallon (4.546 litres), two units with the same name and different sizes.
Why the split survives
France adopted the metric system in 1799 and most of the world followed over the next two centuries, often through trade and colonisation. Britain metricated trade and industry from the 1960s but kept miles on road signs, pints in pubs and stones for body weight, so the UK now lives in both systems at once. The US went furthest in the other direction: the 1975 Metric Conversion Act encouraged metric use but made it voluntary, and without a legal deadline, road signs, recipes, sports and construction stayed imperial. Science, medicine and the US military use metric throughout — so the US is not purely imperial either, just split by context.
The conversions you actually need
One inch is exactly 25.4 millimetres, one pound is exactly 0.45359237 kilograms, and one mile is exactly 1,609.344 metres — these are defined conversions, not approximations, fixed by international agreement in 1959. For everyday use, 1 kg ≈ 2.2 lb, 1 litre ≈ 0.22 UK gallons, and 20 °C ≈ 68 °F will get you close. The full tables live in our converters: start with the feet to metres converter for length, and the stone to kg converter for the most British conversion of all.
Frequently asked questions
What is the difference between metric and imperial?
Metric units are decimal: 1,000 millimetres make a metre, 1,000 grams make a kilogram, and every conversion is a power of ten. Imperial units are historical: 12 inches make a foot, 3 feet make a yard, 1,760 yards make a mile — each conversion factor has to be memorised separately.
Which countries still use the imperial system?
The United States is the only large economy that still uses imperial units in everyday life. Liberia and Myanmar also retain non-metric usage, while the UK is mixed: metric in trade and science, but miles, pints and stones in everyday speech.
Why did the US never switch to metric?
The US Metric Conversion Act of 1975 made metrication voluntary rather than mandatory, so industry and the public never had a deadline. The cost of converting road signs, machinery and everyday habits, plus simple familiarity, kept imperial units in daily use.
Is the metric system more accurate than imperial?
No — both can be equally precise. The metric advantage is convenience: decimal conversions are easier to do mentally and to scale, and one coherent system covers length, mass and volume instead of separate historical units for each.
What is a metric mile?
There is no official metric mile. The nearest round figure is 1,600 metres (a common race distance), against the imperial mile of 1,609.344 metres — close enough that the two are often confused, but not the same distance.