Convert values between common metric and imperial length units with a live visual breakdown.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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Unit conversion is one of the most practical mathematical skills in everyday life — whether you are following a recipe from an American website while cooking in London, buying timber from a US lumber yard in feet while your plans show millimetres, or understanding a UK weather forecast in Celsius when you are used to Fahrenheit. This guide covers all the major unit categories, the key differences between US customary, British Imperial, and metric (SI) systems, common conversion mistakes, and why the US and UK ended up with different measurement systems despite sharing a common language.
The International System of Units (SI) is the global scientific standard, used as the primary or official measurement system in almost every country in the world including the UK. The metric system is decimal-based — every unit is a power of 10 times a base unit. The seven base SI units include metre (length), kilogram (mass), second (time), Kelvin (temperature), ampere (electric current), mole (amount of substance), and candela (luminous intensity).
The United States uses the US customary system for most everyday measurements. This is derived from the old English system but diverged from the British Imperial system at several points. Notable differences: the US gallon (3.785 L) is smaller than the UK gallon (4.546 L), and the US fluid ounce is slightly larger than the UK fluid ounce.
The UK officially uses metric for most purposes but retains Imperial units in specific areas: road distances (miles), speed (mph), draught beer pints, and informal height measurement (feet and inches). The Imperial system shares many unit names with US customary but has different values for volume measures.
| From | To | Multiply by | Example |
|---|---|---|---|
| Inches | Centimetres | 2.54 | 12" × 2.54 = 30.48 cm |
| Feet | Metres | 0.3048 | 10 ft × 0.3048 = 3.048 m |
| Yards | Metres | 0.9144 | 100 yd × 0.9144 = 91.44 m |
| Miles | Kilometres | 1.60934 | 60 mph × 1.609 = 96.5 km/h |
| Kilometres | Miles | 0.62137 | 100 km × 0.621 = 62.1 miles |
| Metres | Feet | 3.28084 | 1.8 m × 3.281 = 5.91 ft |
| From | To | Multiply by | Notes |
|---|---|---|---|
| Pounds (lb) | Kilograms | 0.45359 | 150 lb × 0.454 = 68.1 kg |
| Kilograms | Pounds | 2.20462 | 70 kg × 2.205 = 154.3 lb |
| Stone (UK) | Kilograms | 6.35029 | 11 st × 6.35 = 69.85 kg |
| Stone (UK) | Pounds | 14 | 1 stone = 14 pounds exactly |
| US short ton | Metric tonne | 0.90718 | 2,000 lb = 907.2 kg |
| UK metric tonne | US short tons | 1.10231 | 1,000 kg = 1.102 US short tons |
Temperature conversion between Fahrenheit (US) and Celsius (UK/metric) is the most commonly needed daily conversion for anyone comparing weather between the two countries.
| Description | Celsius (UK) | Fahrenheit (US) |
|---|---|---|
| Water freezes | 0°C | 32°F |
| Cold UK winter day | −5°C | 23°F |
| Cool spring day | 10°C | 50°F |
| Warm UK summer | 22°C | 71.6°F |
| Hot US summer day | 35°C | 95°F |
| Body temperature | 37°C | 98.6°F |
| Water boils | 100°C | 212°F |
| Oven moderate (UK Gas 4) | 180°C | 356°F |
One of the most dangerous unit conversion mistakes is assuming US and UK gallons are the same. They are not:
This difference matters significantly for fuel economy: a US car rated at 30 mpg achieves fewer miles per equivalent UK gallon. To convert US mpg to UK mpg (Imperial): multiply US mpg by 1.201. A US car getting 30 mpg is equivalent to approximately 36 mpg in UK Imperial terms.
| Volume Unit | Litres | Notes |
|---|---|---|
| 1 US fluid ounce | 0.02957 L | Slightly larger than UK fl oz |
| 1 UK fluid ounce | 0.02841 L | Slightly smaller than US fl oz |
| 1 US cup (cooking) | 0.2366 L (236.6 mL) | 8 US fl oz; no exact UK equivalent |
| 1 US pint | 0.4732 L | 16 US fl oz |
| 1 UK pint | 0.5683 L | 20 UK fl oz — 20% larger than US pint |
| 1 US gallon | 3.7854 L | 128 US fl oz |
| 1 UK gallon | 4.5461 L | 160 UK fl oz |
The United States is one of only three countries that has not officially adopted the metric system as its primary measurement system (the others are Liberia and Myanmar). The US has never passed legislation requiring metric adoption in everyday commerce, though the metric system has been legal in the US since 1866 and the US officially defines its customary units in terms of metric units since 1893.
The US Metric Conversion Act of 1975 established a voluntary conversion program but lacked enforcement mechanisms. The Omnibus Trade and Competitiveness Act of 1988 designated metric as the preferred system for federal government use, but exempted consumer goods. The result is a country where science, medicine, the military, and international trade use metric, while everyday consumer life uses customary units.
The UK began formally adopting the metric system in the 1960s following the Hodgson Report (1951) and a government decision in 1965. Most industrial and commercial measurements converted to metric by the 1980s. The UK legally requires metric pricing for retail goods sold by weight or volume since 2000. Road distances and speed limits remain in miles and mph — an exception maintained by law. Pub beer is still served in pints by law. Height in everyday conversation remains in feet and inches despite medical records using centimetres.
Multiply miles by 1.60934. So 60 miles = 60 × 1.609 = 96.56 km. A quick approximation: multiply by 1.6. To convert km to miles, multiply by 0.621. Both the US and UK use miles for road distances, but continental Europe uses kilometres, making this conversion essential for European travel.
Multiply pounds by 0.45359. So 180 lb × 0.454 = 81.6 kg. To convert kg to pounds, multiply by 2.20462. In the UK, body weight is often expressed in stone and pounds: 1 stone = 14 pounds = 6.35 kg. So 12 stone = 168 lb = 76.2 kg.
°C = (°F − 32) × 5/9. So 72°F: (72 − 32) × 5/9 = 40 × 5/9 = 22.2°C. A quick mental approximation: subtract 30 from Fahrenheit and halve the result. So 72°F: (72 − 30) ÷ 2 = 21°C (slightly off but useful for quick estimates).
No. The UK Imperial gallon = 4.546 litres. The US gallon = 3.785 litres. The UK gallon is about 20% larger. This means fuel efficiency quoted in US mpg is not directly comparable to UK mpg. To convert US mpg to UK mpg, multiply by 1.201.
Exactly 2.54 centimetres per inch. This is a defined exact value (not an approximation) since 1959 when the international yard and pound agreement standardised the inch as exactly 2.54 cm. So 1 foot = 12 inches = 30.48 cm exactly.
The main difference is in volume measurements. Length and weight units with the same names (inch, foot, pound) are identical between US customary and Imperial. But volume units differ: the US gallon is 3.785 L while the UK gallon is 4.546 L. The US pint is 473 mL while the UK pint is 568 mL. This divergence occurred in 1824 when the UK redefined the gallon.
Multiply square feet by 0.0929. So 1,200 sq ft × 0.0929 = 111.5 m². To convert m² to sq ft, multiply by 10.7639. These conversions are essential when comparing property sizes listed in different units — UK estate agents use square feet and square metres interchangeably.
Both countries retained miles for road distances due to the enormous cost and disruption of changing all road signs, regulations, and driver training, and because of public resistance to the change. The UK's metrication programme specifically exempted road distances and speed limits. In the US, there has never been a serious political effort to change road signage to kilometres. Both countries now use the same mile, defined as exactly 1,609.344 metres.
Disclaimer: Conversion factors are provided for informational and practical use. Scientific and legal conversions should use the latest official definition values from NIST (US) or BSI/NPL (UK). Some historical units may have slightly different values in older documents.