Unit conversion
Unit converter
Pick a category, pick two units, get the number. Every other unit in the category is listed underneath, so you can see the whole picture rather than one isolated figure.
Every unit in this category
Why conversions still catch people out
Most conversions are a single multiplication, which makes them feel too simple to check. That confidence is the problem. In 1999 NASA lost the Mars Climate Orbiter because one team supplied thruster impulse in pound-force seconds while the receiving software expected newton-seconds; the spacecraft entered the atmosphere at the wrong altitude and broke up. A factor of 4.45 destroyed a 125 million dollar mission.
Smaller versions happen constantly. Gallons are different sizes in the US and the UK. A ton comes in three varieties. Ounces measure both weight and volume, and they are not related.
The units that share a name but not a size
| Name | US | Imperial / metric | Difference |
|---|---|---|---|
| Gallon | 3.785 l | 4.546 l | UK is 20% bigger |
| Pint | 473 ml | 568 ml | UK is 20% bigger |
| Fluid ounce | 29.57 ml | 28.41 ml | US is 4% bigger |
| Ton | 907 kg (short) | 1016 kg (long) / 1000 kg (tonne) | Up to 12% |
Note the reversal in the third row: the UK gallon is larger, but the UK fluid ounce is smaller, because the two systems divide their gallons differently — 128 US fluid ounces to the US gallon, 160 imperial to the imperial gallon.
Kilobyte or kibibyte
Storage has two competing definitions. Drive manufacturers use decimal units, where a gigabyte is 1,000,000,000 bytes. Operating systems have traditionally used binary units of 1,073,741,824 bytes but labelled them “GB” anyway. That is why a 1 TB drive shows up as roughly 931 GB in Windows — nothing is missing, the two figures just count differently.
The IEC introduced the binary prefixes kibi, mebi, gibi and tebi in 1998 to settle the ambiguity. macOS and Linux distributions have largely moved to decimal reporting; Windows still labels binary values with decimal names. Both are offered here so you can convert either way.
Temperature is the odd one out
Every other conversion on this page is a single multiplication, because the scales share a zero. Temperature scales do not. Celsius sets zero at the freezing point of water, Fahrenheit at a brine mixture, and Kelvin at absolute zero, so converting needs both a scale factor and an offset.
This is also why a temperature difference converts differently from a temperature. A rise of 10 °C is a rise of 18 °F, not 50 °F. The offset applies to points on the scale, not to intervals.
Where the numbers come from
The imperial-to-metric factors here are exact by definition, not approximations. Since the 1959 international yard and pound agreement, an inch is exactly 25.4 mm and a pound is exactly 0.45359237 kg. Similarly the calorie is defined as exactly 4.184 joules and the nautical mile as exactly 1852 metres. Only a handful of values, such as the average month and the speed of sound, are genuinely approximate.
Common questions
How many cm are in an inch?
Exactly 2.54, by international agreement since 1959. That makes a foot exactly 30.48 cm and a mile exactly 1609.344 m. Because the definition is exact rather than measured, conversions between the systems carry no rounding error at the definition level.
Why does my 1 TB hard drive show less than 1 TB?
The manufacturer counts a terabyte as 10^12 bytes; Windows displays the same capacity divided by 2^40 but still labels it TB. The drive holds what it says — the two systems are counting in different bases. A 1 TB drive is about 931 of the binary units Windows shows.
Is a fluid ounce the same as an ounce?
No. A fluid ounce measures volume and an ounce measures weight, and they only coincide for a liquid with the density of water. Eight fluid ounces of honey weighs about 12 ounces; eight fluid ounces of flour weighs about 4.5. This is the single most common source of error in recipes written in US customary units.
What temperature is the same in Celsius and Fahrenheit?
Minus 40. Setting C equal to F in the conversion formula gives C = C × 9/5 + 32, which solves to −40. It is the only point where the two scales agree, which makes it a handy sanity check on any conversion routine.