RunToolRunUnit Converter
Convert units easily.
How to Convert Units
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Length, weight, temperature, volume, area, speed and more.
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Type the number and choose source and target units.
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The conversion updates live as you type.
Exact conversions and approximate ones
Some conversions are exact by definition: an inch is precisely 25.4 millimetres, a pound is precisely 0.45359237 kilograms. These are definitions agreed by international standard, not measurements, so they carry no error at all.
Others are approximations that depend on conditions. Converting a volume of flour to a weight depends on how it is packed; converting fuel volume to mass depends on temperature. When a conversion involves a physical property rather than a defined ratio, the answer is an estimate.
Units with the same name and different values
A US gallon is 3.785 litres; an imperial gallon is 4.546 litres — a 20% difference under the same word. US and imperial pints and fluid ounces differ similarly. A US ton is 907 kg, a metric tonne is 1,000 kg, and a UK long ton is 1,016 kg.
These collisions are a genuine hazard in recipes, engineering specifications and shipping documents. When a figure crosses a border, the unit's origin matters as much as its name.
Temperature is not a simple ratio
Most conversions are multiplication. Temperature is not, because Celsius and Fahrenheit have different zero points as well as different scale sizes. Twenty degrees Celsius is not twice ten degrees Celsius in any physical sense, and doubling a Fahrenheit figure means something different again.
Kelvin is the exception that makes this concrete: it starts at absolute zero, so ratios in Kelvin are meaningful while ratios in Celsius are not. Temperature differences convert by ratio alone; temperature values need the offset.
Mass, weight and significant figures
Kilograms measure mass; pounds are commonly used for both mass and force. On Earth the distinction rarely matters, but in physics and engineering it does, and mixing the two is a classic source of error by a factor of roughly 9.8.
Precision deserves equal care. Converting 'about 5 miles' to 8.04672 kilometres implies a measurement accurate to the centimetre that nobody made. The converted figure should carry no more significant figures than the number you started with.
Why unit errors are expensive
Unit confusion has destroyed spacecraft, grounded aircraft with insufficient fuel and caused medication dosing errors. The failure mode is consistent: each party is internally correct and the mismatch sits at the boundary between them.
The defence is to write the unit next to every number, at every step, including intermediate results. Conversion runs in your browser and nothing is transmitted, but the arithmetic is the easy part — knowing which unit you were given is the hard one.
Converting in the right direction
A surprising share of conversion errors are simply inversions — multiplying where you should divide. The check that catches almost all of them is to ask whether the answer should be bigger or smaller before you look at it. Kilometres are shorter than miles, so a distance in kilometres must be the larger number.
This sounds trivial and it is exactly what people skip when working quickly. A metre is longer than a foot, a kilogram heavier than a pound, a litre smaller than a gallon: knowing which way each pair runs turns a wrong answer into something you notice immediately rather than something you pass on.