Unit Converter
Convert length, weight, temperature, volume, area, speed, data and more (as you type).
1 cm = 0.3937008 in
What are you converting?
Pick a category, then type a number. Everything converts as you type. There is no button to press.
This converts between metric and imperial units across length, weight, volume, area, temperature and speed.
Most metric-imperial conversions are exact by definition, so the results are precise rather than approximate.
How unit systems relate
Nearly every unit conversion is one multiplication. What makes them look arbitrary is that the factors are exact definitions, not measurements: an inch is defined as exactly 2.54 cm, a pound as exactly 0.45359237 kg, and a US gallon as exactly 3.785411784 litres.
Those definitions come from international agreement, mostly the 1959 international yard and pound agreement. US customary units are legally defined in terms of metric units, which means the US is metric underneath its everyday measurements.
The one that behaves differently is temperature, because the scales have different zero points as well as different step sizes. Every other conversion is a scale factor; temperature needs an offset too.
None of these are rounded. They are the definitions from which every other imperial unit is derived.
What to enter
- Category
- Length, weight, volume, area, speed or temperature. Each has its own set of units.
- Value and source unit
- What you are converting from.
- Target unit
- What you want it in.
The seven SI base units
- metre (m)
- Length. Defined by the speed of light.
- kilogram (kg)
- Mass. Redefined in 2019 from the Planck constant, retiring the physical prototype.
- second (s)
- Time. Defined by a caesium-133 transition frequency.
- ampere, kelvin, mole, candela
- Electric current, temperature, amount of substance and luminous intensity.
- Everything else
- Derived from these seven. A newton is kg·m/s²; a joule is a newton-metre.
What this assumes
US customary and imperial units are not identical. A US gallon is 3.785 L and an imperial gallon is 4.546 L, which is a 20% difference.
Conversions here are for measurement units. Currency, which changes constantly, is a different kind of problem entirely.
How to calculate unit conversion
Multiply by the factor, divide to reverse, and check the direction of your answer.
- reverse
- Divide by the same factor, or multiply by its reciprocal
- temperature
- The exception: needs an offset as well as a factor
Identify both units precisely. US gallon or imperial gallon? Short ton or metric tonne? Ounce or troy ounce? These pairs differ substantially.
Multiply by the factor. 10 inches × 2.54 = 25.4 cm.
Sanity-check the direction. A centimetre is smaller than an inch, so the centimetre figure must be larger. Getting the direction backwards is the most common error.
Keep enough precision. Round at the end, not in the middle. Rounding 2.54 to 2.5 introduces a 1.6% error before you have even started.
See a worked example: the same volume in two gallons
- Volume
- 10 gallons
US gallons: 10 × 3.785411784 = 37.85 litres.
Imperial gallons: 10 × 4.54609 = 45.46 litres.
The same word, a 20% difference. This is why US and UK fuel economy figures are not comparable without converting.
A US car doing 30 mpg is doing about 36 mpg in imperial gallons, purely from the unit.
37.85 L or 45.46 L, depending on the gallon
Frequently asked questions
For the main ones, yes. An inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg, both by international agreement since 1959.
They are definitions, not measurements. That is also why the numbers look so oddly precise: they were chosen to reconcile the two systems, not derived from anything natural.
Mostly volume. A US gallon is 3.785 L and an imperial gallon is 4.546 L, about 20% larger. US and imperial pints, quarts and fluid ounces differ correspondingly.
Length and weight are the same in both since 1959. It is volume where the two systems genuinely diverge, and where recipes and fuel figures go wrong.
Because the scales have different zero points as well as different step sizes. Every other conversion is a single multiplication.
Celsius to Fahrenheit needs × 9/5 and + 32. For temperature *differences*, only the 9/5 applies, since a difference has no zero point to reconcile.
Extensively, and more than most people realise. Science, medicine, the military, and much of manufacturing all use metric routinely.
US customary units are also legally *defined* in metric terms, so the metric system underpins everything. The 1975 Metric Conversion Act made the switch voluntary, and everyday use never followed.
Yes, famously. The Mars Climate Orbiter was lost in 1999 because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds.
The Gimli Glider is the other well-known case: a 1983 Air Canada 767 ran out of fuel mid-flight after a pounds-versus-kilograms error during refuelling. It landed safely on a former airbase.
Round at the end, not during the calculation. Rounding 2.54 to 2.5 introduces a 1.6% error before you begin.
Match the precision of your input. A measurement of "about 6 feet" does not become more accurate by being reported as 182.88 cm.
Problems people actually run into
Converting in the wrong direction
Dividing when you should multiply gives an answer wrong by the square of the factor, and it can look plausible enough to accept.
Sanity-check the size: a centimetre is smaller than an inch, so a length in centimetres must be a bigger number. If it is not, the direction is wrong.
Assuming a gallon is a gallon
A US gallon is 3.785 L and an imperial gallon is 4.546 L, a 20% difference under the same name. The same applies to pints, quarts and fluid ounces.
Check which system a figure uses before converting, particularly with recipes, fuel economy and anything published outside the US.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026