How this calculator works
Length and mass units are the same in the imperial and US customary systems, and both rest on the 1959 agreement that made the international inch exactly 25.4 mm and the pound exactly 0.45359237 kg. Volume is the exception: the US gallon is 3.785411784 L while the imperial gallon is 4.54609 L, about 20 per cent larger, so this page offers both. Temperature needs an offset as well as a scale factor, because the Fahrenheit and Celsius scales place zero in different places. Choose a quantity and a direction, then pick the unit from the dropdown beside the number; the answer has its own unit selector, so a result can be read in millimetres, metres, inches or miles without re-entering anything.
Formula
1 in = 25.4 mm. 1 ft = 0.3048 m. 1 yd = 0.9144 m. 1 mi = 1,609.344 m. 1 oz = 28.349523125 g. 1 lb = 0.45359237 kg. 1 st = 6.35029318 kg. 1 imperial gallon = 4.54609 L. 1 US gallon = 3.785411784 L. degrees C = (degrees F - 32) x 5/9.
Worked example
6 ft is 1.8288 m. 180 lb is 81.6466 kg. 1 imperial gallon is 4.54609 L, while 1 US gallon is only 3.78541 L. 68 degrees F is 20 degrees C. Going the other way, 1.8 m is 5.90551 ft and 80 kg is 176.370 lb.
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Frequently asked questions
Is an imperial gallon the same as a US gallon?
No. The imperial gallon is 4.54609 L and the US gallon 3.785411784 L, so the imperial gallon is about 20 per cent larger. Both are offered on the volume inputs and outputs, so check which one a recipe or fuel figure means.
Are US and imperial inches and pounds the same?
Yes, since the 1959 international agreement. The inch is exactly 25.4 mm and the pound exactly 0.45359237 kg in both systems, so only volume and some older liquid names differ.
Why does temperature need a different formula?
The scales have different zero points. Water freezes at 0 degrees Celsius but 32 degrees Fahrenheit, and 100 Celsius degrees span the same range as 180 Fahrenheit degrees. The conversion therefore needs both a factor of 5/9 and the 32-degree offset.
What is a stone?
A stone is exactly 14 pounds, or 6.35029318 kg, and it is still used for body weight in the United Kingdom and Ireland. It is offered on the imperial mass inputs and outputs.