Science
Unit Converter
Six categories with exact conversion factors — including the awkward ones, like 1 US gallon being 3.785411784 litres rather than a round 3.8.
How to use it
- Pick a category — length, weight, temperature, area, volume or speed.
- Choose the unit you have and the unit you want.
- Type the value; the conversion updates as you type.
Some conversion factors are definitions, not measurements
The inch has been exactly 25.4 millimetres since the 1959 international yard and pound agreement. A pound is exactly 0.45359237 kilograms. These are not approximations refined by experiment — they are legal definitions, so converting 12 inches to millimetres has an exact answer rather than a rounded one. Older factors still circulating in textbooks, such as 25.4001, predate that agreement.
Why two systems persist
The metric system was designed to be internally consistent: every unit is a power of ten from a base, and mass, length and volume are linked by water at its densest. Imperial units grew out of trade and body measurements, which is why there are 12 inches to a foot and 16 ounces to a pound. The United States adopted metric units as legal definitions in 1866 but never mandated their use in daily life, so both systems run in parallel — and conversion errors remain a real source of engineering failures.
Where conversions actually go wrong in a kitchen
Volume-to-weight is not a fixed conversion. A US cup of flour is about 120 grams; a cup of sugar is about 200; a cup of honey is 340. Recipes that give cups assume a specific ingredient, which is why professional recipes weigh instead. If a conversion result looks wrong by 40%, the ingredient density is almost always the reason.
The other silent error is the gallon. A US gallon is 3.785 litres, an imperial gallon is 4.546 — a 20% difference that turns a fuel-economy comparison into nonsense. Oven temperatures have the same problem at the margins: 180 °C is 356 °F, and fan ovens run about 20 °C hotter than the dial says.
Frequently asked questions
Are the factors exact?
The imperial ones are: an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg by international agreement. Results are rounded to eight decimals.
Why is temperature separate?
Temperature scales have offsets, not just scale factors, so they cannot use the same multiply-and-divide rule.
Which gallon is used?
The US gallon (3.785411784 L). The imperial gallon is 4.54609 L — about 20% larger, the classic source of confusion.
Is a US fluid ounce the same as an imperial one?
No. US 29.57 ml, imperial 28.41 ml — a 4% difference that matters in a 500 ml recipe.
Why is a nautical mile not a normal mile?
It is defined as one minute of latitude, which is why it divides cleanly into degrees on a chart. That is also why aviation and shipping still use it.