Metric vs imperial: which units the UK still uses

The UK is a mixed-up place for measurement. Fuel is sold by the litre but car economy is quoted in miles per gallon. Road signs show miles while the speedometer may show both. People weigh themselves in stones and pounds but buy fruit in kilograms. If you are working across the two systems, or you are just trying to understand why your recipe, your tape measure and your road atlas do not agree, this guide sets out where each system is used, the exact conversion factors, and a few worked examples that show how to move between them without guessing.

By Valtrinity. Updated 18 October 2026.

How the UK ended up with both systems

Britain's traditional units were gradually standardised into the imperial system in the nineteenth century. Metric units arrived later. The UK began a planned move towards metric in the 1960s and 1970s, and most trade, science, medicine, building materials and school teaching have been metric ever since. The shift was never completed for everyday life, which is why many people use both systems without noticing.

Where the law requires metric, it usually does so for goods sold by weight or measure in shops, and for packaging. Where the law allows imperial, it tends to be for a short list of specific cases. Most of the rest is habit: a person's height in feet and inches, a baby's birth weight in pounds and ounces, a pint at the pub. Habit is powerful, and it explains why older and younger people often describe the same thing in different units.

Rules on labelling and what shops may use have been reviewed more than once, so if you sell goods and need to be sure, check the current guidance on GOV.UK rather than relying on a general article.

Where imperial units are still used in the UK

The clearest case is the road. Distances on road signs are in miles, yards and feet, and speed limits are in miles per hour. That is set in law, and it is not expected to change soon. Fuel economy is almost always quoted as miles per gallon, even though the fuel itself is sold in litres.

Draught beer and cider are sold in pints and halves, and milk in returnable bottles can be sold in pints. Many people still buy milk in pints even from supermarkets, although cartons are normally labelled in litres or millilitres as well. Precious metals can still be sold in troy ounces. Land is often described in acres, and property listings routinely mention square feet alongside square metres.

People are measured in imperial more than anywhere else in daily life. Height is given in feet and inches, adult body weight in stones and pounds, and newborn weight in pounds and ounces. Health services record in metric, but many patients quote themselves in the older units, so a nurse or a doctor may switch between the two within a single conversation.

Where metric is the rule

Science, engineering, medicine and almost all schools use metric. Packaged food and drink show weight and volume in grams, kilograms, millilitres and litres. Building materials are sold in millimetres and metres, and plans are drawn in metric. Weather forecasts give temperatures in degrees Celsius, although people often still talk about a hot day as being in the 80s, which is Fahrenheit.

Petrol and diesel are sold per litre, and fuel prices are shown per litre on forecourt signs. Paint, flooring and most DIY products are labelled in square metres and litres, although the old habit of thinking in feet and inches for room sizes lives on in some estate agents' listings.

For anyone moving between the two, the practical rule is to pick one system for each job. Use metric for anything you are going to calculate with, order or cut, and use the traditional unit for conversation. Then convert once, at the end, using the exact factors below.

The exact conversion factors

Length: 1 inch is exactly 2.54 cm. 1 foot is 30.48 cm, or 0.3048 m. 1 yard is 0.9144 m. 1 mile is 1.609344 km, which is why 1 km is about 0.6214 miles.

Weight: 1 pound is 0.45359237 kg, or about 453.6 g. 1 ounce is about 28.35 g. 1 stone is 14 pounds, which is about 6.35 kg. One kilogram is about 2.2046 pounds.

Volume: 1 UK pint is 568.26 ml, which is slightly more than half a litre. 1 UK gallon, the imperial gallon, is 4.54609 litres and holds 8 pints. Be careful with the US versions: a US pint is 473.18 ml and a US gallon is 3.78541 litres, so the same word means a different amount on the other side of the Atlantic.

Temperature: Fahrenheit is converted to Celsius by subtracting 32 and multiplying by 5/9. Celsius is converted to Fahrenheit by multiplying by 9/5 and adding 32. So 20 degrees Celsius is 68 degrees Fahrenheit, and 100 degrees Fahrenheit is 37.8 degrees Celsius.

Worked examples you can check

Speed and distance: a 30 mph limit is 30 times 1.609344, which is 48.28 km/h. A 70 mph limit is 112.65 km/h. A 5 mile run is 8.05 km, and a 10 km race is 6.21 miles.

Body weight: 11 stone 4 pounds is 11 times 14 plus 4, which is 158 pounds. Multiply by 0.45359237 and you get 71.67 kg. Going the other way, 80 kg is 80 times 2.2046, which is 176.4 pounds, or 12 stone 8.4 pounds.

Drinks: a UK pint is 568 ml and the metric alternative served in many countries is 500 ml, so a UK pint holds about 14 per cent more. If a bar quotes a price per half litre, multiply by 1.136 to compare it with a pint. A litre of milk is about 1.76 UK pints.

Cooking: 4 ounces of flour is 113 g, which recipe writers often round to 110 g or 115 g. A cup is not a fixed UK unit, so always check which cup size a recipe means before converting.

Fuel economy: miles per gallon and litres per 100 km

A car that does 40 miles per gallon travels 40 miles on 4.54609 litres, which is 64.37 km. That means it uses 4.54609 divided by 64.37 times 100, which is 7.06 litres per 100 km. The formula is 282.48 divided by the mpg figure, if you use the UK gallon.

This is a place where mixing systems causes errors. A car quoted at 40 US mpg uses a smaller gallon, so it is actually more economical than a car at 40 UK mpg: it uses 5.88 litres per 100 km rather than 7.06. Always check whether a figure is a UK or US gallon before you compare two cars.

When you work out running costs, convert everything into pence per mile. At 40 mpg and a fuel price of 140 pence per litre, one gallon costs 4.54609 times 140, which is 636.5 pence, so each mile costs 15.9 pence.

Tips for switching between systems

Convert once, at the end. Keep all your working in one system, then change units on the final answer. This keeps rounding errors small.

Use the exact factors when it matters, and rounded ones when you are estimating. For a quick check, a kilometre is about five eighths of a mile, a kilogram is a bit over two pounds, and a litre is a bit under two pints.

Write the unit next to every figure. Many errors come from a number that has lost its unit, such as a measurement copied from a plan or a recipe that does not say whether it means grams or ounces.

Tools mentioned in this guide

Frequently asked questions