🔄 Universal Unit Conversion Calculator
Convert units of length, weight, and temperature instantly across metric and imperial systems.
Converting units, and the conversions worth memorising
Unit conversion is multiplication by a ratio equal to one. Since 1 inch is exactly 2.54 cm, the fraction 2.54 cm / 1 inch has a value of one, and multiplying by it changes the units without changing the quantity. Arranging the fraction so the unwanted unit cancels is the whole technique.
12 in × (2.54 cm / 1 in) = 30.48 cm
Length
| From | To | Multiply by |
|---|---|---|
| inches | centimetres | 2.54 (exact) |
| feet | metres | 0.3048 (exact) |
| yards | metres | 0.9144 (exact) |
| miles | kilometres | 1.609344 (exact) |
| nautical miles | kilometres | 1.852 (exact) |
Those are exact by definition, not measurement: since 1959 the international yard has been defined as exactly 0.9144 metres. A useful mental shortcut for miles to kilometres is to multiply by 1.6, or use consecutive Fibonacci numbers, since 5 miles is almost exactly 8 km and 8 miles is almost exactly 13 km.
Weight and mass
| From | To | Multiply by |
|---|---|---|
| pounds | kilograms | 0.45359237 (exact) |
| ounces | grams | 28.349523 |
| stone | kilograms | 6.35029 |
| US tons | tonnes | 0.907185 |
| troy ounces | grams | 31.1035 |
A troy ounce, used for precious metals, is about 10% heavier than the ordinary avoirdupois ounce. Gold quoted per ounce is always troy, which is why gold weights never quite reconcile with kitchen scales.
Volume: the US and imperial trap
A US gallon is 3.785 litres; an imperial gallon is 4.546 litres, about 20% larger. The same split runs through pints and fluid ounces, so a British pint is 568 ml while a US pint is 473 ml. Recipes are the most common casualty: a US cup is 237 ml, a metric cup is 250 ml, and an Australian tablespoon is 20 ml against 15 ml everywhere else.
| Unit | Millilitres |
|---|---|
| US fluid ounce | 29.57 |
| Imperial fluid ounce | 28.41 |
| US cup | 236.6 |
| Metric cup | 250 |
| US pint | 473.2 |
| Imperial pint | 568.3 |
| US gallon | 3,785 |
| Imperial gallon | 4,546 |
Temperature is different
Temperature scales have offset zero points, so conversion needs addition as well as multiplication. It is the one case where you cannot just multiply by a factor:
°C = (°F − 32) × 5/9 °F = °C × 9/5 + 32 K = °C + 273.15
The quick mental version for Fahrenheit to Celsius is to subtract 30 and halve, which gets within a couple of degrees across normal weather. The scales cross at −40, where both read the same, and body temperature is 37 °C or 98.6 °F.
Why unit errors are expensive
NASA lost the Mars Climate Orbiter in 1999 because one team worked in pound-force seconds and another in newton-seconds. In 1983 an Air Canada 767 ran out of fuel at altitude after a conversion error between pounds and kilograms during refuelling. The arithmetic is not hard; the discipline of writing units next to every number is what prevents the error.
A worked example
A recipe calls for 2 lb 6 oz of flour and 1.5 US cups of milk, and you have metric scales and a 180 °C oven. The flour: 2 lb is 907.2 g, 6 oz is 170.1 g, total 1,077 g. The milk: 1.5 × 236.6 = 355 ml. The recipe says 350 °F, which is (350 − 32) × 5/9 = 176.7 °C, so 175 or 180 on a domestic dial.
Frequently Asked Questions (FAQs)
How many millilitres are in a cup?
It depends which cup. A US cup is 236.6 ml, a metric cup is 250 ml and an old imperial cup was 284 ml. Recipes rarely say which they mean; if it is American, assume 237 ml, and if it is Australian or modern British, assume 250 ml.
What is the easiest way to convert Celsius to Fahrenheit in my head?
Double it and add 30. For 20 °C that gives 70 °F against the exact 68, which is close enough for weather. Reverse it by subtracting 30 and halving. The exact formulas use 9/5 and 5/9 rather than 2 and ½.
Why is a US gallon different from an imperial gallon?
They have separate histories. The US kept the older English wine gallon of 231 cubic inches, while Britain redefined its gallon in 1824 as the volume of ten pounds of water. The imperial gallon ended up about 20% larger, and the difference flows through pints and fluid ounces too.
Is the inch exactly 2.54 cm?
Yes, by international agreement since 1959. Before that, the US and British inches differed by a tiny amount. The US survey foot, used in some land measurements, was slightly different and was officially retired at the end of 2022.
How do I convert speed units?
Miles per hour to kilometres per hour, multiply by 1.609. Metres per second to kilometres per hour, multiply by 3.6. Knots to kilometres per hour, multiply by 1.852, since a knot is one nautical mile per hour.
What is the difference between mass and weight?
Mass is the amount of matter and does not change with location; weight is the force gravity exerts on it. A kilogram is a unit of mass and a newton is a unit of force. In everyday use the terms are interchangeable because gravity is effectively constant on Earth.