How to use the Unit Converter
- Pick the quantity type — length, mass, volume, area, speed or temperature.
- Choose the unit you have and the unit you want.
- Enter the value and read the converted figure.
- For volume, check whether you need US or imperial gallons; they differ by about 20%.
How the calculation works
Most conversions are a single multiplication by a defined factor. Since 1959 the international inch has been exactly 25.4 mm and the pound exactly 0.45359237 kg, so length and mass conversions are exact rather than approximate. Area and volume factors are those linear factors squared and cubed, which is why an error in a length conversion becomes much larger in a volume one.
Temperature is the exception because the scales have different zero points as well as different step sizes, so conversion needs an offset as well as a factor. Volume also carries a trap: a US gallon is 3.785 litres while an imperial gallon is 4.546, and US fluid ounces differ from imperial ones — recipes and fuel figures crossing the Atlantic are wrong by roughly a fifth if the wrong one is used.
1 in = 25.4 mm exactly ; 1 lb = 0.45359237 kg exactly ; °F = °C × 9/5 + 32 ; 1 US gal = 3.785411784 LSource: International yard and pound agreement (1959) and SI definitions maintained by the BIPM.
Worked example
Converting a 6 ft 2 in height and a 187 lb weight into metric for a medical form.
- 6 ft 2 in = 74 in; 74 × 25.4 = 1,879.6 mm.
- That is 1.8796 m, commonly written 188 cm.
- 187 × 0.45359237 = 84.82 kg.
About 188 cm and 84.8 kg — figures that would give a BMI of roughly 24.0.
Frequently asked questions
Is a US gallon the same as a UK gallon?+
No. A US gallon is 3.785 litres and an imperial gallon is 4.546, a difference of about 20%. Fuel economy figures and recipe volumes are meaningfully wrong if the two are mixed.
Why is 1 kg not exactly 2.2 lb?+
The exact factor is 2.20462262. The rounded 2.2 is fine for a rough mental conversion but drifts by about a kilogram over 50, which matters for freight, dosing and shipping limits.
How do I convert areas and volumes?+
Square or cube the linear factor. A square metre is 10.7639 square feet because 3.2808² ≈ 10.764, and a cubic metre is 35.31 cubic feet.
What about kelvin?+
Kelvin shares the Celsius step size with a zero at absolute zero, so K = °C + 273.15. It is the SI base unit for temperature and never takes a degree symbol.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.