Exact conversion factors
Many unit conversions are defined exactly by international agreement. Since 1959 the international inch has been exactly 2.54 centimeters, which makes a foot exactly 0.3048 meters, a yard 0.9144 meters and a mile 1.609344 kilometers. The international avoirdupois pound is exactly 0.45359237 kilograms, so an ounce is 28.349523125 grams. The US liquid gallon is defined as 231 cubic inches, or exactly 3.785411784 liters. This converter uses these exact definitions rather than rounded approximations, so results are accurate to many decimal places.
Choose a conversion from the list, type a value, and the answer appears instantly along with the conversion factor for one unit, which is handy if you need to convert many values in a spreadsheet.
Temperature is different
Most conversions are a simple multiplication, but temperature scales have different zero points, so they need an offset as well. To convert Celsius to Fahrenheit, multiply by 9/5 and add 32: 20 °C is 20 × 1.8 + 32 = 68 °F. To go back, subtract 32 and multiply by 5/9: 98.6 °F is 37 °C. Kelvin uses the same size degree as Celsius but starts at absolute zero, so K = °C + 273.15. Water freezes at 0 °C, 32 °F and 273.15 K, and the two common scales meet at −40, where −40 °C equals −40 °F.
US customary versus imperial
The United States and the United Kingdom share many unit names but not always the same sizes. Lengths and weights match, but volumes do not. A US gallon is 3.785 liters while a UK imperial gallon is 4.546 liters, about 20% larger. A US pint is 16 US fluid ounces; an imperial pint is 20 imperial fluid ounces. This converter uses US measures for volume, which is what American recipes and fuel prices use. Similarly, the US short ton is 2,000 pounds, the UK long ton 2,240 pounds and the metric tonne 1,000 kilograms, about 2,204.6 pounds. In the UK and Ireland body weight is often given in stone, where one stone is 14 pounds.
Quick reference
A few approximations are worth memorizing. One kilometer is about 0.62 miles, so a 5K run is 3.1 miles and a marathon of 26.2 miles is 42.2 km. One meter is about 3.28 feet. One kilogram is about 2.2 pounds. One liter is a little more than a US quart. One hectare, a square 100 meters on each side, is about 2.47 acres. For speeds, 100 km/h is about 62 mph. Note also that computer storage uses two systems: a gigabyte (GB) is 1,000,000,000 bytes, while a gibibyte (GiB) is 1,073,741,824 bytes, which is why a new drive appears smaller in your operating system than on the box.
Frequently asked questions
How many kilometers are in a mile?
Exactly 1.609344 kilometers. One kilometer is about 0.621 miles.
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 (1.8) and add 32. For example, 25 °C is 77 °F.
How many pounds are in a kilogram?
About 2.20462 pounds. One pound is exactly 0.45359237 kilograms.
Is a US gallon the same as a UK gallon?
No. A US gallon is about 3.785 liters and a UK imperial gallon is about 4.546 liters.