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How to use the unit converter
There are two ways in, and both give the same answers.
- Pick a quantity (pressure, torque, energy and so on), type a value, then choose the From and To units. The result, the conversion factor and the same value in every other unit of that quantity appear at once.
- Or skip the menus and type the whole thing in the free-form box at the bottom, then press Convert or Enter. This is the route for compound units and for sums of mixed units.
The value box also accepts expressions such as 3/4, 1e6 or 2^10. The converter covers 19 quantities: length, area, volume, mass, time, speed, acceleration, force, pressure, energy, power, torque, temperature, angle, frequency, data, density, flow and electric charge.
| Type this in the free-form box | You get |
|---|---|
60 mi/h to m/s | 26.8224 m/s |
9.81 m/s^2 to ft/s^2 | 32.18503937 ft/s² |
5 ft + 3 in to cm | 160.02 cm |
2 kWh to MJ | 7.2 MJ |
100 degF to degC | 37.77777778 °C |
Write powers with ^ and a division with /. Temperatures are written degC, degF, K and degR. Results show 10 significant digits.
How a conversion factor works
A conversion factor is a fixed ratio that equals 1 once you include the units. Since 1 in is exactly 25.4 mm, the fraction 25.4 mm / 1 in is just a way of writing 1. Multiply by it and the value is unchanged, only its unit label moves: 3 in × 25.4 mm/in = 76.2 mm. The "in" cancels. If your units do not cancel, you used the factor upside down.
Compound units chain the same trick. 60 mi/h to m/s needs two factors, 1609.344 m per mile and 3600 s per hour: 60 × 1609.344 / 3600 = 26.8224 m/s. The free-form box does that chain for you, which is why it can take ft/s^2 or lbf ft without a special menu entry.
Why temperature is different
Most units share a zero: zero metres is zero feet. Celsius, Fahrenheit and kelvin do not. Their zeros sit at different physical temperatures, so converting needs a scale and an offset: °F = °C × 1.8 + 32. That is why the converter shows a note instead of a single factor for temperature. Two checks: 0 °C is 32 °F, and 100 °C is 212 °F. The scales also agree at exactly -40.
The offset matters only for a reading. 10 °C on a thermometer is 50 °F. But a temperature rise of 10 °C is a rise of 10 × 1.8 = 18 °F, with no 32 added. Kelvin is the odd one out in a useful way: 0 °C = 273.15 K, and a rise of 1 K equals a rise of 1 °C.
Look-alike units that are not the same
- Gallons. The US gallon is about 3.785 L. The imperial (UK) gallon is 4.54609 L, which is 1.200949926 US gallons. A "10 gallon" tank is 10 US gal or 12.00949926 US gal depending on which country wrote the spec. In the converter,
galis US andimpgalis imperial. - Tons. The US short ton is 2000 lb = 907.18474 kg. The UK long ton is 2240 lb = 1016.046909 kg. The metric tonne is 1000 kg. The three differ by up to 12%.
- Bytes. 1 kB is 1000 B, but 1 KiB is 1024 B. At gigabyte scale the gap grows: 1 GiB = 1,073,741,824 B, so a 500 GB drive holds 465.6612873 GiB, which is what many operating systems display.
- Gauge and absolute pressure. A tyre gauge reads pressure above the atmosphere. Absolute pressure adds the atmosphere back. This converter changes the number from one unit to another and nothing more, so it will not add atmospheric pressure for you. 32 psi gauge is about 32 + 14.69594878 = 46.69594878 psi absolute at standard atmosphere. Add it yourself first, ideally with local pressure.
Common engineering conversion factors
Every value below was produced by this converter, so they match what you get above. Each row reads "1 of the first unit equals this much of the second".
| Quantity | 1 of this | equals |
|---|---|---|
| Length | 1 in | 25.4 mm |
| Length | 1 ft | 0.3048 m |
| Length | 1 mi | 1.609344 km |
| Mass | 1 lb | 0.45359237 kg |
| Volume | 1 US gal | 3.785411784 L |
| Acceleration | 1 g₀ | 9.80665 m/s² |
| Force | 1 lbf | 4.448221615 N |
| Pressure | 1 psi | 6.894757293 kPa |
| Pressure | 1 bar | 14.50377377 psi |
| Pressure | 1 atm | 101.325 kPa |
| Torque | 1 lbf·ft | 1.355817948 N·m |
| Energy | 1 kWh | 3.6 MJ |
| Energy | 1 BTU | 1.055055853 kJ |
| Power | 1 hp (mechanical) | 0.7456998716 kW |
Horsepower comes in more than one kind. hp here is mechanical (745.7 W). PS is metric horsepower, so 1 PS = 0.73549875 kW. A European motor rated in "hp" may mean PS.
Worked examples
1. Tyre pressure: 32 psi in kPa and bar
One psi is 6.894757293 kPa, so 32 × 6.894757293 = 220.6322334 kPa. A bar is 100 kPa, so the same pressure is 2.206322334 bar. European placards often print bar, phone apps print kPa, and a US door sticker prints psi. All three describe the same air, and the table under the result lists every other pressure unit too.
2. Torque spec: 250 N·m in lbf·ft
A workshop manual gives 250 N·m for a wheel hub nut, and your torque wrench reads in foot-pounds. One newton metre is 0.7375621493 lbf·ft, so 250 × 0.7375621493 = 184.3905373 lbf·ft. If the wrench reads in inch-pounds, multiply by 12 for 2212.686448 lbf·in. In the unit boxes torque units are written N m, lbf ft and lbf in, with a space and no dot.
3. Battery energy: 5000 mAh at 3.7 V in Wh
A milliamp-hour is a unit of charge, not energy. To get energy you need the voltage. Step one: 5000 mAh is 5 Ah, or 18,000 C (coulombs). Step two: energy = charge × voltage, so 5 Ah × 3.7 V = 18.5 Wh. Check in joules: 18,000 C × 3.7 V = 66,600 J, and 66,600 J ÷ 3600 = 18.5 Wh. The free-form box does it in one line, because it multiplies the quantities and keeps the units.
Reading the result and common mistakes
- Converting a rise as if it were a reading. Use the offset formula for a thermometer value, and multiply by 1.8 alone for a difference between two temperatures.
- Mixing gauge and absolute pressure. Gas laws and vapour tables need absolute pressure. Add atmosphere first, then convert.
- Mass versus force. A kilogram-force (
kgf) is the weight of 1 kg under standard gravity: 9.80665 N. A pound-force is 4.448221615 N. Do not read lb as lbf in a torque or stress calculation. - Decimal versus binary prefixes. kB, MB and GB are powers of 1000. KiB, MiB and GiB are powers of 1024. Disk makers use the first, many operating systems the second.
- Keeping every digit. The display shows 10 significant digits. Round to the precision of your input: 220.6322334 kPa from a gauge good to 1 psi is "about 221 kPa".
For a conversion buried in longer maths, convert the pieces here, then carry on in the scientific calculator, which has its own Units tab.
Frequently asked questions
How do I convert psi to kPa?
Multiply by 6.894757293. So 30 psi is about 206.8 kPa, and 32 psi is 220.6322334 kPa. To go the other way, divide by the same number.
Why does temperature conversion not use a single factor?
Celsius, Fahrenheit and kelvin start at different zero points, so you need an offset as well as a scale: °F = °C × 1.8 + 32. A reading and a difference convert differently, because the offset cancels in a difference.
Is a US gallon the same as an imperial gallon?
No. A US gallon is about 3.785 L and an imperial gallon is 4.54609 L, so 10 imperial gallons is 12.00949926 US gallons. Use gal for the US unit and impgal for the imperial one.
What is the difference between a short ton, a long ton and a tonne?
A short ton is 2000 lb (907.18474 kg), a long ton is 2240 lb (1016.046909 kg) and a tonne is 1000 kg. In this converter they are ton, longton and tonne.
Can I convert compound units like m/s² to ft/s²?
Yes. Type it in the free-form box, for example 9.81 m/s^2 to ft/s^2, which gives 32.18503937 ft/s². You can also mix units in a sum, such as 5 ft + 3 in to cm, which gives 160.02 cm.
Does it convert gauge pressure to absolute pressure?
No. It only changes the unit of the number you give it. To get absolute pressure, add the atmospheric pressure to the gauge reading yourself (about 14.69594878 psi or 101.325 kPa at standard conditions), then convert.