To convert metric to imperial, use three exact definitions: 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, and °F = °C × 9/5 + 32. So divide centimetres by 2.54 to get inches, divide kilograms by 0.45359237 (or multiply by about 2.2046) to get pounds, and multiply Celsius by 1.8 and add 32 to get Fahrenheit. This guide gives the formulas, quick mental shortcuts, worked examples and ready-made tables for the conversions people search for most.
- Since 1959 the inch, foot, yard and pound have been defined exactly in metric units, so these conversions involve no guesswork.
- cm → inches: ÷ 2.54. Inches → cm: × 2.54. A foot is exactly 30.48 cm.
- kg → lb: ÷ 0.45359237 (about × 2.2). A stone is 14 lb, about 6.35 kg.
- °C → °F: × 1.8 + 32. °F → °C: − 32, then × 5/9. The scales meet at −40.
- US and UK gallons, pints and fluid ounces are different sizes; check which one a recipe or label means.
Why the conversions are exact
Imperial and US customary units were once defined by physical standards kept in London and Washington, and they differed slightly. In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa signed the international yard and pound agreement, which fixed the yard at exactly 0.9144 metres and the pound at exactly 0.45359237 kilograms. Everything else follows: the inch (a 36th of a yard) is exactly 2.54 cm, the foot is 30.48 cm and the mile is 1.609344 km.
That means a converter that uses these numbers is not approximating: the only rounding is in how many decimals you choose to show. Temperatures are different in kind, because the Celsius and Fahrenheit scales have different zero points as well as different degree sizes, but the formula linking them is also exact.
Length: cm to inches, feet and inches
- inches = cm ÷ 2.54 and cm = inches × 2.54
- feet = inches ÷ 12; 1 ft = 30.48 cm; 1 m = 39.37 in = 3 ft 3.37 in
- 1 yard = 91.44 cm; 1 mile = 1.609 km, and 1 km = 0.621 miles
Worked example (a height): 172 cm ÷ 2.54 = 67.72 in. 67.72 ÷ 12 = 5 feet with 7.72 inches left over, so 172 cm is 5 ft 7.7 in, or about 5′8″. Back the other way, 6 ft 1 in = 73 in × 2.54 = 185.4 cm.
Mental shortcut: multiply centimetres by 4 and divide by 10 (40 cm → 16 in; exact 15.7 in). For inches to centimetres, multiply by 2.5 (12 in → 30 cm; exact 30.48 cm).
| Centimetres | Inches | Feet and inches |
|---|---|---|
| 1 cm | 0.394 in | – |
| 10 cm | 3.937 in | – |
| 30 cm | 11.811 in | 0 ft 11.8 in |
| 100 cm | 39.370 in | 3 ft 3.4 in |
| 150 cm | 59.055 in | 4 ft 11.1 in |
| 160 cm | 62.992 in | 5 ft 3.0 in |
| 170 cm | 66.929 in | 5 ft 6.9 in |
| 180 cm | 70.866 in | 5 ft 10.9 in |
| 190 cm | 74.803 in | 6 ft 2.8 in |
For any other value, the cm to inches converter works both ways and includes full charts.
Weight: kg to pounds and stones
- pounds = kg ÷ 0.45359237 (the same as kg × 2.20462…) and kg = pounds × 0.45359237
- 1 stone = 14 lb = 6.35029318 kg; 1 lb = 16 oz; 1 oz = 28.349523125 g
- Tons: a metric tonne is 1,000 kg; a US (short) ton is 2,000 lb (907.2 kg); a UK (long) ton is 2,240 lb (1,016 kg)
Worked example (body weight in stones): 68 kg ÷ 0.45359237 = 149.91 lb. 149.91 ÷ 14 = 10 stones (140 lb) with 9.91 lb left, so 68 kg is about 10 st 10 lb. Back again: 10 st 10 lb = 150 lb × 0.45359237 = 68.04 kg.
Mental shortcut: for kg to lb, double the number and add a tenth of the result (60 kg → 120 + 12 = 132 lb; exact 132.3 lb). For lb to kg, halve it and take away a tenth of the result (180 lb → 90 − 9 = 81 kg; exact 81.6 kg).
| Kilograms | Pounds | Stones and pounds |
|---|---|---|
| 1 kg | 2.205 lb | 0 st 2.2 lb |
| 5 kg | 11.023 lb | 0 st 11.0 lb |
| 10 kg | 22.046 lb | 1 st 8.0 lb |
| 50 kg | 110.231 lb | 7 st 12.2 lb |
| 60 kg | 132.277 lb | 9 st 6.3 lb |
| 70 kg | 154.324 lb | 11 st 0.3 lb |
| 80 kg | 176.370 lb | 12 st 8.4 lb |
| 90 kg | 198.416 lb | 14 st 2.4 lb |
| 100 kg | 220.462 lb | 15 st 10.5 lb |
Use the kg to lbs converter for exact values, stones and a pounds-to-kilograms chart.
Temperature: Celsius to Fahrenheit
- °F = °C × 9/5 + 32 (9/5 = 1.8)
- °C = (°F − 32) × 5/9
- K = °C + 273.15; absolute zero is 0 K = −273.15 °C = −459.67 °F
Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level, so 100 Celsius degrees cover the same span as 180 Fahrenheit degrees. That is where the 9/5 comes from, and the + 32 lines up the two freezing points.
Worked examples: 25 °C × 1.8 = 45, + 32 = 77 °F. An oven at 400 °F: 400 − 32 = 368, × 5/9 = 204.4 °C, which recipes round to 200 °C. And −40 °C equals −40 °F, the one point where the scales agree.
Mental shortcuts: double the Celsius figure and add 30 for a rough answer (good for weather; 10 °C → 50 °F is exact). For an exact answer, double, subtract a tenth of that, then add 32 (30 °C → 60 − 6 + 32 = 86 °F). Going the other way, subtract 30 and halve (70 °F → 20 °C; exact 21.1 °C).
| Celsius | Fahrenheit | Typical meaning |
|---|---|---|
| −18 °C | −0.4 °F | Domestic freezer |
| 0 °C | 32 °F | Water freezes |
| 4 °C | 39.2 °F | Fridge |
| 20 °C | 68 °F | Room temperature |
| 30 °C | 86 °F | Hot day |
| 37 °C | 98.6 °F | Typical body temperature |
| 38 °C | 100.4 °F | Fever threshold used by the NHS |
| 100 °C | 212 °F | Water boils at sea level |
| 180 °C | 356 °F | Moderate oven (recipes say 350 °F, gas mark 4) |
The Celsius to Fahrenheit converter also converts kelvin and has an oven and gas mark table.
Volume: watch out for US and UK units
Length and weight units are the same in the US and the UK, but liquid measures are not, which catches out many cooks:
| Unit | US | UK (imperial) |
|---|---|---|
| Gallon | 3.785 litres | 4.546 litres |
| Pint | 473 ml (16 US fl oz) | 568 ml (20 UK fl oz) |
| Fluid ounce | 29.57 ml | 28.41 ml |
A UK pint of milk is about 20% bigger than a US pint, and a US recipe's "cup" is about 237 ml. When a figure matters, convert through millilitres and check which system the source uses.
Tips for accurate conversions
- Convert once, round at the end. Rounding in the middle of a calculation (for example using 2.2 for kg to lb, then rounding again) adds up errors.
- Keep sensible precision. A person's height to the nearest centimetre or half inch is plenty; engineering drawings may need three decimals.
- Watch compound units. 5 ft 7 in is not 5.7 feet: it is 5 + 7/12 = 5.583 ft. The same applies to stones and pounds (10 st 7 lb is 10.5 stones).
- Label everything. In 1999 NASA lost the Mars Climate Orbiter because one team's software produced results in pound-force seconds while another expected newton-seconds. Writing the unit next to every number avoids smaller versions of the same mistake.
Who uses which system?
Almost every country uses the metric system officially. The United States uses US customary units for most everyday purposes, from road signs to recipes, and Fahrenheit for weather. The UK is mixed: road distances and speeds are in miles, beer and milk come in pints, many people give their height in feet and inches and their weight in stones, while food labels, science and most shops use metric. That is why searches like "cm to inches", "kg to lbs" and "celsius to fahrenheit" are among the most common conversion questions online.
Converting for a health check? Our BMI calculator and calorie calculator accept both metric and imperial units, so you do not need to convert first. More quick helpers are in everyday tools.
Frequently asked questions
How many centimetres are in an inch?
Exactly 2.54 cm. The value has been fixed by international agreement since 1959.
How do I convert kg to pounds quickly?
Double the kilograms and add a tenth of the result: 50 kg → 100 + 10 = 110 lb (exact 110.2 lb). For an exact figure, divide by 0.45359237.
What is the formula for Celsius to Fahrenheit?
Multiply by 1.8 and add 32. To go from Fahrenheit to Celsius, subtract 32 and multiply by 5/9.
How many pounds are in a stone?
14 pounds, which is 6.35 kg. Stones are mainly used for body weight in the UK and Ireland.
Is a UK pint the same as a US pint?
No. A UK (imperial) pint is about 568 ml and a US pint about 473 ml, so the UK pint is roughly 20% larger.
Why does the US still use imperial units?
Strictly, the US uses US customary units, which grew from older English units. Metric is legal and used in science, medicine and much of industry, but everyday habits, signage and the cost of changing have kept customary units in daily life.