Unit Conversion Calculator — Length, Weight, Temperature

Select a category, enter your value, and convert any unit instantly.

Almost Every Conversion Is One Multiplication

Length, mass, volume, area, speed and energy all work the same way. Each unit has a fixed ratio to every other unit measuring the same quantity, so converting means multiplying by that ratio once.

Value in new unit = Value in old unit × Conversion factor

The only decision is which way round to apply it. Going to a smaller unit produces a bigger number, and going to a larger unit produces a smaller one. If a metre becomes 0.001 kilometres, the answer must be smaller; if it becomes 100 centimetres, it must be bigger. Checking the direction before reading the digits catches most errors immediately.

These factors are exact by definition rather than measured approximations — the inch was defined as precisely 2.54 centimetres by international agreement, so the conversion carries no error at all. Any imprecision in your answer comes from your input or your rounding, not from the factor.

Temperature Is the Exception

Every other conversion is a pure scaling, because zero means the same thing in both units — zero kilograms is zero pounds. Temperature scales do not share a zero point, so they need an offset as well as a scale factor.

°F = (°C × 9 ÷ 5) + 32
°C = (°F − 32) × 5 ÷ 9
K = °C + 273.15

The offset is why you cannot convert a temperature by multiplying alone, and why the shortcut that works everywhere else fails here. It also produces a genuinely useful consequence: a difference in temperature converts without the offset. A rise of 10 °C is a rise of 18 °F, not 50 °F, because the 32 cancels when you subtract two converted values.

Mixing those two up is the classic temperature error. Converting a thermostat setting uses the full formula; converting how much the temperature changed uses only the ratio.

Kelvin behaves differently again, having a true zero, so it scales proportionally with Celsius and needs only the offset of 273.15.

Do Not Report More Precision Than You Had

A conversion cannot add accuracy that the original measurement did not contain, and calculators encourage exactly that by returning many decimal places.

Measuring a room as 12 feet — to the nearest foot — and converting gives 3.6576 metres. Reporting that figure claims precision to a tenth of a millimetre from a measurement good to about 15 centimetres. The honest answer is 3.7 metres, or 3.66 at most.

The working rule is to keep roughly as many significant figures as your input had, and to round only at the very end. Rounding at each intermediate step compounds the error, particularly in a chain of two or three conversions.

Where Unit Errors Do Real Damage

• Two different gallons. A US gallon is about 3.785 litres and an imperial gallon about 4.546 — a difference of roughly 20%. Fuel economy figures quoted in miles per gallon are not comparable between the two systems without saying which is meant.
• Weight and mass in recipes. Fluid ounces measure volume and ounces measure weight, and they are not interchangeable. A cup of flour and a cup of water weigh very different amounts.
• Ambiguous tons. Metric tonnes, short tons and long tons all exist and differ by up to about 10%.
• Converting twice. Chaining conversions through an intermediate unit accumulates rounding at each step. Convert directly where a factor exists.
• Assuming a system from a country. Several countries use metric officially and imperial conversationally, so a figure given without units cannot be inferred from where it came from.

For fuel economy specifically, where the two gallons and the reversed L/100 km scale all collide, our fuel cost calculator handles the conversions directly. For currency, which is a rate rather than a fixed factor, use the currency converter.

Unit Conversion Questions