How to use the unit converter
- Choose a category from the buttons: length, mass, temperature, volume, pressure, data size and so on.
- Type a value. Decimals, negatives and scientific notation (
3e8) all work, and commas are ignored. - Pick the From and To units. The result updates as you type; the ⇆ button swaps the two units.
- Read the formula under the result. For simple units it shows the multiplication factor both ways; temperatures show the full formula with offset; fuel economy shows the reciprocal.
- Scan the table of every unit in the category, and click any row to make it the target unit. Set precision to fixed decimals or significant digits for reports and homework.
Quick reference by category
Each category name opens that converter directly. Values are rounded to six significant digits unless marked exact.
| Category | Units | SI or standard unit | Common conversions |
|---|---|---|---|
| Length | 21 | metre (m) | 1 in = 2.54 cm (exact) 1 ft = 0.3048 m 1 mi = 1.60934 km 1 km = 0.621371 mi |
| Mass / weight | 18 | kilogram (kg) | 1 lb = 0.45359237 kg (exact) 1 kg = 2.20462 lb 1 oz = 28.3495 g 1 st = 6.35029 kg |
| Temperature | 5 | kelvin (K) | 0 °C = 32 °F 100 °C = 212 °F 98.6 °F = 37 °C 0 °C = 273.15 K |
| Area | 12 | square metre (m²) | 1 ac = 4046.86 m² 1 ha = 2.47105 ac 1 ft² = 0.092903 m² 1 mi² = 2.58999 km² |
| Volume | 24 | cubic metre (m³); litre in daily use | 1 gal (US) = 3.785411784 L (exact) 1 gal (UK) = 4.54609 L 1 cup (US) = 236.588 mL 1 fl oz (US) = 29.5735 mL |
| Speed | 8 | metre per second (m/s) | 1 mph = 1.60934 km/h 1 km/h = 0.621371 mph 1 kn = 1.852 km/h 1 m/s = 3.6 km/h |
| Time | 14 | second (s) | 1 d = 24 h 1 wk = 168 h 1 yr = 365.2425 d 1 h = 3600 s |
| Pressure | 15 | pascal (Pa) | 1 atm = 14.6959 psi 1 bar = 14.5038 psi 1 psi = 6.89476 kPa 1 atm = 101.325 kPa |
| Energy | 13 | joule (J) | 1 kcal = 4.184 kJ 1 kWh = 3.6 MJ 1 BTU = 1.05506 kJ 1 cal = 4.184 J |
| Power | 12 | watt (W) | 1 hp = 745.7 W 1 kW = 1.34102 hp 1 PS = 735.499 W 1 TR = 3.51685 kW |
| Force | 11 | newton (N) | 1 lbf = 4.44822 N 1 kgf = 9.80665 N 1 kN = 224.809 lbf 1 N = 100,000 dyn |
| Frequency | 10 | hertz (Hz) | 1 rpm = 0.0166667 Hz 1 kHz = 1000 Hz 1 GHz = 1000 MHz 1 rad/s = 0.159155 Hz |
| Angle | 9 | radian (rad) | 1 rad = 57.2958° 1° = 0.0174533 rad 1 turn = 360° 1 grad = 0.9° |
| Density | 12 | kilogram per cubic metre (kg/m³) | 1 g/cm³ = 1000 kg/m³ 1 lb/ft³ = 16.0185 kg/m³ 1 kg/m³ = 0.062428 lb/ft³ 1 lb/gal (US) = 119.826 kg/m³ |
| Fuel economy | 6 | none (L/100 km or km/L in metric countries) | 30 mpg (US) = 7.84049 L/100 km 8 L/100 km = 29.4018 mpg (US) 1 mpg (UK) = 0.832674 mpg (US) 1 mpg (US) = 0.425144 km/L |
| Data size | 19 | bit / byte (IEC prefixes for powers of 2) | 1 KiB = 1024 B 1 GB = 0.931323 GiB 1 TB = 0.909495 TiB 1 B = 8 bit |
| Data transfer rate | 11 | bit per second (bit/s) | 100 Mbit/s = 12.5 MB/s 1 Gbit/s = 125 MB/s 1 MB/s = 8 Mbit/s 1 kbit/s = 1000 bit/s |
| Electric current | 8 | ampere (A) | 1 A = 1000 mA 1 mA = 1000 µA 1 kA = 1000 A 1 abA = 10 A |
| Astronomical distance | 15 | metre; au, light-year and parsec in astronomy | 1 au = 149,597,870.7 km (exact) 1 ly = 9.4607 × 10¹² km 1 pc = 3.26156 ly 1 ly = 63241.1 au |
Metric, US customary and imperial: why three systems?
The metric system (SI) is used by almost every country and by science worldwide. Units scale by powers of ten with prefixes such as kilo (103), mega (106), milli (10−3) and micro (10−6), so converting within the system is just moving the decimal point. Since 2019 all seven SI base units are defined from fixed constants of nature: the metre from the speed of light, the kilogram from the Planck constant, the second from caesium atoms.
US customary units (inch, foot, mile, pound, US gallon, Fahrenheit) grew out of English units used before 1824. Since the 1959 international yard and pound agreement they are defined in metric terms: an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg. The United States still uses them for everyday life, construction and many industries.
The British imperial system was standardised in 1824 and shares lengths and weights with US customary units, but volumes differ. An imperial gallon is 4.54609 litres, an imperial pint is 20 imperial fluid ounces (568 mL) against the US pint of 16 US fluid ounces (473 mL), and the imperial fluid ounce is about 4% smaller than the US one. The UK now uses metric officially but keeps miles, pints of beer and milk, and stones for body weight.
| Quantity | Metric | US customary | Imperial (UK) |
|---|---|---|---|
| Gallon | – | 3.785 L (128 US fl oz) | 4.546 L (160 imp fl oz) |
| Pint | – | 473 mL | 568 mL |
| Fluid ounce | – | 29.57 mL | 28.41 mL |
| Cup | 250 mL (metric cup) | 236.6 mL | 284 mL (rarely used) |
| Ton | 1,000 kg (tonne) | 2,000 lb = 907 kg (short ton) | 2,240 lb = 1,016 kg (long ton) |
| Fuel economy | L/100 km | miles per US gallon | miles per imperial gallon |
| Body weight | kilograms | pounds | stones and pounds |
Conversions people get wrong
- Temperature is not a multiplication. Celsius and Fahrenheit have different zero points, so °F = °C × 1.8 + 32. Doubling 10 °C does not double the heat. See the temperature converter for charts and oven temperatures.
- Square and cubic units scale by the square and cube. 1 m = 3.28 ft, but 1 m² = 10.76 ft² and 1 m³ = 35.31 ft³.
- Fuel economy is reciprocal. Improving from 10 to 20 mpg saves more fuel than going from 30 to 60 mpg over the same distance; L/100 km shows that directly.
- Bits vs bytes. Internet speeds are sold in megabits per second; downloads show megabytes per second. Divide by 8: a 100 Mbit/s connection tops out at 12.5 MB/s.
- kB vs KiB. SI prefixes mean powers of 1,000 and IEC prefixes (KiB, MiB, GiB) mean powers of 1,024. The gap grows with size: 2.4% at kilo, 7.4% at giga, 10% at tera.
- Calories vs kilocalories. A food “Calorie” is a kilocalorie, 4.184 kJ. Nutrition labels outside the US often list both kcal and kJ.
- Horsepower has several definitions. Mechanical hp is 745.7 W, metric PS 735.5 W and electrical hp exactly 746 W. Car specs in Europe often use PS; in the US, hp.
- Torr and mmHg differ slightly. 1 torr is exactly 1/760 atm; 1 mmHg is 133.322387415 Pa by convention, about 0.000014% more. For blood pressure and weather they are interchangeable.
Exact definitions used
These factors are exact by definition, so conversions built on them carry no rounding error beyond floating-point arithmetic: inch 0.0254 m, foot 0.3048 m, mile 1,609.344 m, nautical mile 1,852 m, pound 0.45359237 kg, US gallon 3.785411784 L (231 cubic inches), imperial gallon 4.54609 L, standard gravity 9.80665 m/s² (so 1 kgf = 9.80665 N and 1 lbf = 4.4482216152605 N), standard atmosphere 101,325 Pa, thermochemical calorie 4.184 J, electronvolt 1.602176634 × 10−19 J, speed of light 299,792,458 m/s, astronomical unit 149,597,870,700 m, and the Julian light-year 9,460,730,472,580,800 m. Derived units follow from these: psi is 6,894.757293168 Pa, mechanical horsepower is 550 ft·lbf/s = 745.69987158 W, and a parsec is 648,000/π au, about 3.0857 × 1016 m. The Mach unit uses the speed of sound in the International Standard Atmosphere at sea level (340.294 m/s); real Mach numbers depend on air temperature.
Common everyday conversions
| Conversion | Rule of thumb | Exact or precise value |
|---|---|---|
| Kilometres to miles | × 0.6 (or × 5 ÷ 8) | × 0.621371 |
| Kilograms to pounds | × 2.2 | × 2.20462 |
| Centimetres to inches | ÷ 2.5 | ÷ 2.54 |
| Litres to US gallons | ÷ 4 (a little more) | ÷ 3.78541 |
| °C to °F | × 2 + 30 | × 1.8 + 32 |
| Square metres to square feet | × 10.8 | × 10.7639 |
| Hectares to acres | × 2.5 | × 2.47105 |
| Bar to psi | × 14.5 | × 14.5038 |
| kW to hp | × 1.34 | × 1.34102 |
| Knots to km/h | × 1.85 | × 1.852 (exact) |
Looking for something else? Try the percentage calculator, number base converter, date difference or time zone converter. For a longer walkthrough read How to Use Unit Converter: Practical Guide and Best Practices.
Frequently asked questions
How many units does this converter support?
243 units in 19 categories: length, mass and weight, temperature, area, volume, speed, time, pressure, energy, power, force, frequency, angle, density, fuel economy, data size, data transfer rate, electric current and astronomical distance.
How accurate are the conversions?
Every factor uses the exact legal definition where one exists, such as 1 inch = 0.0254 m, 1 pound = 0.45359237 kg and 1 US gallon = 3.785411784 L. Calculations use double-precision numbers and show up to 14 significant digits, which is far beyond what everyday or engineering work needs.
What is the difference between a US gallon and an imperial gallon?
A US gallon is 3.785411784 litres and an imperial (UK) gallon is 4.54609 litres, about 20% larger. US and imperial pints, quarts and fluid ounces differ too, so recipes, fuel economy figures and drink sizes from the UK and US are not interchangeable.
Why does my 1 TB drive show as 931 GB?
Drive makers use SI prefixes, where 1 TB is 10^12 bytes. Windows reports sizes in binary units (powers of 1,024) but labels them GB, so 10^12 bytes appears as 931 GB, which is really 931 GiB. Choose Data size to convert between GB and GiB.
Is weight the same as mass?
Strictly, no. Mass is the amount of matter, measured in kilograms or pounds; weight is the force of gravity on it, measured in newtons or pound-force. In everyday use weight means mass, so the Mass / weight category converts kg, lb, oz and stone. Use Force for newtons and pound-force.
How do I convert mpg to L/100 km?
Divide 235.215 by the US mpg figure, or 282.481 by the UK mpg figure. 30 mpg (US) is 7.84 L/100 km. Because the scales are reciprocal, the Fuel economy category handles this automatically, and 0 converts to infinity.
Can I link to a specific conversion?
Yes. The address bar updates as you convert, for example /unit-converter?mode=temp&from=F&to=C&v=350. Copy link shares the exact category, units and value. Older links such as ?mode=weight or ?mode=astronomy still open the right category.
Does the converter work offline?
Yes. All unit definitions and calculations are in the page itself, so once it has loaded it keeps working without a connection, and nothing you type is sent anywhere.