Use the Metric vs Imperial Units What Is The Difference
Metric units are based mainly on powers of 10, using units such as metres, kilograms and litres. Imperial units use older units such as inches, feet, miles, ounces, pounds, stone, pints and gallons.
In the UK, metric units are common in shops, packaging, science, medicine and official measurements. Imperial units still appear in everyday speech, road speeds, body height, body weight, pints and some property measurements. Related Calculatorz pages include Rounding converted measurements without losing accuracy, a conversion factor and a kilogram.
Fast examples
- Metric: 180 cm, 75 kg, 2 litres, 20°C.
- Imperial: 5 ft 11 in, 11 st 11 lb, 3 pints, 68°F.
- Mixed UK use: buy milk in litres, order beer in pints, drive at mph, measure a room in metres or feet.
What are metric units?
The metric system is a decimal measurement system. That means many conversions move by powers of 10, which makes them easier to scale up or down.
| Measurement | Common metric units | Everyday example |
|---|---|---|
| Length | millimetres, centimetres, metres, kilometres | 30 cm ruler, 5 km run |
| Weight / mass | grams, kilograms | 500 g pasta, 75 kg body weight |
| Volume | millilitres, litres | 330 ml drink, 2 litre bottle |
| Temperature | Celsius, Kelvin | 20°C room temperature |
The SI system is the international authority base for metric measurement. Its base units include the metre, kilogram, second, ampere, kelvin, mole and candela.
What are imperial units?
Imperial units are older measurement units still widely recognised in the UK and some other countries. They are not usually based on powers of 10, so conversions often need a fixed conversion factor.
| Measurement | Common imperial units | Everyday example |
|---|---|---|
| Length | inch, foot, yard, mile | 6 ft height, 10 mile journey |
| Weight | ounce, pound, stone | 12 st 4 lb body weight |
| Volume | fluid ounce, pint, gallon | pint of beer |
| Speed | miles per hour | 70 mph motorway speed limit |
Imperial units are often useful because they match how people talk in everyday life, but they can be harder to calculate mentally because the step sizes vary.
Why does the UK use both metric and imperial?
The UK uses a mixed system in practice. Metric units are used in many official, commercial and product-labelling contexts, while imperial units remain common in road travel, body measurements, drinks and everyday conversation.
UK context: When selling packaged or loose goods in England, Scotland or Wales, metric measurements such as grams, kilograms, millilitres and litres are generally required. Imperial can still appear in specific cases or alongside metric where allowed.
This is why a UK unit converter should handle both systems clearly rather than assuming everyone uses one system all the time.
Metric vs imperial units table
| Feature | Metric | Imperial |
|---|---|---|
| Base style | Decimal, often powers of 10 | Older mixed unit steps |
| Length examples | mm, cm, m, km | inch, foot, yard, mile |
| Weight examples | g, kg | oz, lb, stone |
| Volume examples | ml, litre | fl oz, pint, gallon |
| Temperature examples | Celsius, Kelvin | Fahrenheit |
| Easy mental scaling? | Usually easier | Depends on the unit |
| UK everyday use? | Retail, packaging, medicine, education, science | Road speeds, height, body weight, pints, informal use |
Common metric to imperial conversions
These are the conversions people often need in daily life. Use the relevant converter if you need a precise decimal result.
| Conversion | Approximate result | Useful for |
|---|---|---|
| 1 inch to cm | 2.54 cm | screen sizes, small lengths |
| 1 foot to cm | 30.48 cm | height, room dimensions |
| 1 mile to km | 1.609 km | travel distance |
| 1 kg to lb | 2.205 lb | body weight, luggage, parcels |
| 1 stone to kg | 6.35 kg | UK body weight |
| 1 litre to UK pints | about 1.76 UK pints | drinks and volume |
| 20°C to °F | 68°F | weather and room temperature |
| 70 mph to km/h | about 113 km/h | driving abroad |
Need the exact number?
Use the main unit converter for length, weight, temperature, volume and speed conversions.
Which system should you use?
Use the system that matches the job. For official, retail, scientific or health-related measurements, metric is often the clearest choice. For UK road speeds, body weight, informal height and pints, imperial may be more familiar.
Use metric when…
- You need easy scaling by 10, 100 or 1,000.
- You are reading packaging or product labels.
- You are doing science, medicine or school work.
- You want a more internationally understood result.
Use imperial when…
- You are talking about UK road speeds in mph.
- You are using height in feet and inches.
- You are using body weight in stone and pounds.
- You are dealing with pints in familiar UK contexts.
Common metric vs imperial mistakes
- Mixing UK and US pints: a UK pint is not the same as a US liquid pint.
- Rounding too early: convert first, then round the final result.
- Assuming cups equal grams: cups are volume, grams are weight, so food conversions depend on ingredient density.
- Confusing pound weight with pound money: lb is a weight unit, £ is currency.
- Using Fahrenheit like Celsius: temperature conversions need an offset formula, not just a multiplier.
FAQs
What is the difference between metric and imperial units?
Metric units are part of a decimal measurement system using units such as metres, kilograms and litres. Imperial units include inches, feet, miles, ounces, pounds, stone, pints and gallons.
Does the UK use metric or imperial?
The UK uses both in everyday life. Metric is common in retail, packaging, education, science and medicine, while imperial remains common for road speeds, body height, body weight, pints and some informal measurements.
Is metric easier than imperial?
Metric is often easier for calculation because it usually moves in powers of 10. Imperial can feel more familiar for some UK everyday uses, but the conversions are less consistent.
What is 1 inch in centimetres?
1 inch is exactly 2.54 centimetres.
What is 1 mile in kilometres?
1 mile is 1.609344 kilometres. For rough mental conversion, multiply miles by 1.6.
Sources and notes
This guide uses standard unit conversion factors, GOV.UK weights and measures guidance for UK context, and BIPM information on the International System of Units.