Unit conversion is the one piece of arithmetic where being approximately right is not good enough — a recipe, a piece of luggage, or a machined part does not care that you were close. Every factor used here is the exact defined value, not a rounded one: an inch is exactly 2.54 cm by international agreement, and a pound is exactly 0.45359237 kg.
Which one do you want?
| If you are… | Use |
|---|---|
| One-off conversion, any units | Unit Converter — length, weight, temperature, volume, area and speed in one box. |
| Looking up a factor, or checking one | Conversion Reference — every factor at full precision, with the exact definitions marked as exact. |
Temperature is the exception that catches people out: it converts with a formula, not a factor, because the scales do not share a zero point. °F = °C × 9/5 + 32, never °C × 1.8 alone.
Where conversions go wrong
- US and imperial gallons are different units. A US gallon is 3.785 L; an imperial gallon is 4.546 L, about 20% larger. Any figure quoted per gallon — fuel economy above all — is meaningless until you know which gallon is meant.
- There are two kinds of ounce, and two kinds of mile. A weight ounce is 28.35 g and a fluid ounce is 29.57 mL; a troy ounce, used for precious metals, is 31.10 g. A statute mile is 1,609 m and a nautical mile is 1,852 m.
- Exact factors and derived ones are not the same thing. An inch is defined as exactly 2.54 cm, so that conversion carries no error at all. Going the other way gives 1 m = 3.280839895… ft, which never terminates and has to be cut off somewhere.
- Rounding to more digits than you measured is false precision. Converting a tape-measure reading of six feet into 1.8288 metres implies a tenth-of-a-millimetre measurement that never existed. Keep roughly the significant digits you started with.
Frequently asked questions
Are the conversion factors exact or rounded?
Exact where an exact definition exists — inch, pound, and the metric units are defined by international standard. Rounding happens only in the displayed result.
Why do US and UK gallons differ?
They are genuinely different units: a US gallon is 3.785 litres, an imperial gallon 4.546. Recipes and fuel figures crossing the Atlantic go wrong here more than anywhere else.
Which is right for cooking, weight or volume?
Weight, whenever the recipe offers it. A cup of flour can vary by 20% depending on how it was scooped; 120 grams is 120 grams.