Metric and imperial at a glance
Most of the world uses the metric system, built on powers of ten: 100 centimetres in a metre, 1,000 grams in a kilogram. The United States mainly uses US customary units (inches, pounds, gallons). The UK uses a mix: road distances in miles, body weight often in stones, but most other things in metric.
Since 1959, imperial and US units have been officially defined in terms of metric ones. An inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg. That's why conversions between them are exact.
Most-used conversions
| From | To | Multiply by |
|---|---|---|
| Inches | Centimetres | 2.54 |
| Feet | Metres | 0.3048 |
| Miles | Kilometres | 1.609344 |
| Pounds | Kilograms | 0.45359237 |
| Ounces | Grams | 28.3495 |
| US gallons | Litres | 3.78541 |
| US cups | Millilitres | 236.588 |
| Square feet | Square metres | 0.092903 |
| Acres | Hectares | 0.404686 |
| Miles per hour | Kilometres per hour | 1.609344 |
To go the other way, divide by the same number. For example, centimetres to inches is ÷ 2.54.
Converting temperature
Temperature is different because the scales don't start at the same zero, so you can't just multiply.
°F = °C × 9 ÷ 5 + 32
°C = (°F − 32) × 5 ÷ 9
K = °C + 273.15
Example: 25 °C is 25 × 9 ÷ 5 + 32 = 77 °F.
Quick estimate: double the Celsius and add 30. That gives 80 °F for 25 °C, close enough for the weather.
| Celsius | Fahrenheit | Feels like |
|---|---|---|
| −18 °C | 0 °F | Very cold winter day |
| 0 °C | 32 °F | Water freezes |
| 20 °C | 68 °F | Room temperature |
| 37 °C | 98.6 °F | Body temperature |
| 100 °C | 212 °F | Water boils (at sea level) |
| 180 °C | 356 °F | Moderate oven |
Watch out: US vs. UK gallons
A UK (imperial) gallon is about 4.546 litres. A US gallon is 3.785 litres, about 20% smaller. US and UK fluid ounces and pints differ too. When a recipe or fuel figure comes from the other side of the Atlantic, check which one it means. The converter lists both gallons.
Cooking conversions
- 1 US cup = 16 tablespoons = 48 teaspoons ≈ 237 ml
- 1 tablespoon = 3 teaspoons ≈ 14.8 ml
- Many metric recipes use a 250 ml "metric cup," a little larger than a US cup.
- Cups measure volume, not weight. A cup of flour weighs much less than a cup of sugar, so for baking, grams are more reliable.
Frequently asked questions
How many centimetres is 5 feet 6 inches?
5 ft 6 in is 66 inches. 66 × 2.54 = 167.64 cm.
How many pounds is a stone?
One stone is exactly 14 pounds, about 6.35 kg. It's mainly used in the UK and Ireland for body weight.
Is a tonne the same as a ton?
No. A metric tonne is 1,000 kg. A US "short" ton is 2,000 lb (907 kg) and a UK "long" ton is 2,240 lb (1,016 kg). The converter uses the metric tonne.
How precise are the results?
The calculator uses the exact official conversion factors and shows up to 6 decimal places, which is far more precise than everyday use needs.
Last reviewed November 2026.