Britain uses two systems side by side. Most trade, science, medicine and education are metric, yet road signs are in miles, people give their height in feet and inches, and beer comes in pints. This guide explains where each system is used, how the units are defined and how to convert between them quickly and accurately.
How the units are defined today
Imperial units are no longer independent standards. Since the 1959 international yard and pound agreement, the yard and the pound have been defined in terms of the metre and the kilogram, and the UK's Weights and Measures Act 1985 sets out the same definitions. Every imperial length, mass and volume therefore has an exact metric equivalent. The table below lists the main ones.
| Unit | Metric equivalent | Status |
|---|---|---|
| Inch | 25.4 mm | exact |
| Foot (12 in) | 0.3048 m | exact |
| Yard (3 ft) | 0.9144 m | exact |
| Mile (1,760 yd) | 1,609.344 m | exact |
| Nautical mile | 1,852 m | exact |
| Ounce (avoirdupois) | 28.349523125 g | exact |
| Pound (16 oz) | 0.45359237 kg | exact |
| Stone (14 lb) | 6.35029318 kg | exact |
| Long ton, UK (2,240 lb) | 1,016.0469088 kg | exact |
| Short ton, US (2,000 lb) | 907.18474 kg | exact |
| Tonne (metric) | 1,000 kg | by definition |
| UK fluid ounce | 28.4130625 ml | exact |
| US fluid ounce | 29.5735295625 ml | exact |
| UK pint (20 UK fl oz) | 568.26125 ml | exact |
| US pint (16 US fl oz) | 473.176473 ml | exact |
| UK gallon (8 UK pt) | 4.54609 L | exact |
| US gallon (8 US pt) | 3.785411784 L | exact |
| Acre (4,840 sq yd) | 4,046.8564224 m² | exact |
| Square foot | 0.09290304 m² | exact |
Where imperial units are still used in the UK
UK law requires goods sold by weight or measure to be sold in metric units, but some imperial units remain the primary legal unit for particular uses: the mile, yard, foot and inch for road signs and distances, the pint for draught beer and cider and for milk in returnable containers, and the troy ounce for precious metals. Shops may also show imperial equivalents alongside metric ones. In everyday speech, stones and pounds for body weight and feet and inches for height remain common.
UK imperial and US customary units
The UK and US share the inch, foot, yard, mile, ounce and pound, which are identical. Volumes are different. In 1824 Britain created the imperial gallon, while the US kept an older gallon of 231 cubic inches. As a result, a US gallon, quart, pint and fluid ounce are all smaller than their UK namesakes, except that a US fluid ounce is slightly larger than a UK one because the US pint is split into 16 fluid ounces instead of 20. Tons differ too: a UK long ton is 2,240 lb and a US short ton is 2,000 lb, and neither is the same as a metric tonne.
Shortcuts for mental conversion
| To convert | Rough method | Example (exact answer) |
|---|---|---|
| Kilometres to miles | × 5 ÷ 8 | 80 km → 50 mi (49.71 mi) |
| Miles to kilometres | × 8 ÷ 5 | 30 mi → 48 km (48.28 km) |
| Kilograms to pounds | × 2, then add 10% | 30 kg → 66 lb (66.14 lb) |
| Pounds to kilograms | ÷ 2, then subtract 10% | 150 lb → 67.5 kg (68.04 kg) |
| Metres to feet | × 3, then add 10% | 10 m → 33 ft (32.81 ft) |
| Centimetres to inches | × 0.4 | 50 cm → 20 in (19.69 in) |
| Litres to UK pints | × 1.75 | 2 L → 3.5 pt (3.52 pt) |
| °C to °F | × 2, then add 30 | 15 °C → 60 °F (59 °F) |
| Square metres to square feet | × 10.76 | 20 m² → 215 ft² (215.28 ft²) |
Rounding honestly
Convert first and round last, and do not report more precision than the original measurement had. A room measured as "about 4 metres" is not 13.1234 feet; "about 13 feet" is the honest conversion. Where a definition is exact, such as 1 inch = 2.54 cm, the conversion adds no error of its own. Where the result is a recurring decimal, such as metres to feet, our converters show "≈" to make clear the figure has been rounded.
A short history
The metric system was introduced in France in the 1790s, built on decimal multiples and on units derived from the metre. Britain authorised metric units for trade in 1864 and began a large-scale move to metric in 1965, supported by industry. The process was gradual: decimal currency arrived in 1971, petrol moved to litres in the 1980s, and metric units became mandatory for most packaged and loose goods around the turn of the century. Today the International System of Units (SI) defines all its base units from fixed constants of nature, and imperial units are defined from those.
Frequently asked questions
Is the UK officially metric?
Largely, yes. Metric units are the main legal units for trade, but specific imperial units are retained for road signs, draught beer and cider, returnable milk bottles and precious metals.
Are UK and US miles the same?
Yes. Both are 1,760 yards, exactly 1,609.344 m.
Why are US gallons smaller?
The US kept an older English wine gallon of 231 cubic inches when Britain introduced the larger imperial gallon in 1824.