Unit Converter
| Unit | Value |
|---|
Checked against: 1 inch = 2.54 cm exactly; 1 lb = 0.45359237 kg exactly; 100 °F = 37.78 °C Last reviewed:
Sources and method
- Unit Conversion - National Institute of Standards and Technology (NIST)
- International yard and pound - Wikipedia
Calculations run in your browser. Nothing you enter is sent to a server or stored.
Pick a category, enter a value and choose the units to convert from and to. The converter covers length, weight, temperature, volume, area and speed, and lists the value in every unit of the category. Factors are the exact international definitions: 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, 1 US gallon = 3.785411784 L.
Most everyday conversions run between metric units and the US customary or British imperial ones, and that is where mistakes creep in: US and UK gallons differ by a fifth, an ounce of weight is not a fluid ounce, and temperatures need an offset as well as a multiplier. This converter uses the defined factors throughout, so results are as exact as the number of digits shown.
How conversion works
For length, weight, volume, area and speed, every unit has a fixed factor relative to one base unit (metres, kilograms, litres, square metres, metres per second). To convert, the value is multiplied by the factor of the unit you have and divided by the factor of the unit you want:
Result = Value × factor(from) ÷ factor(to)
So 10 km in miles is 10 × 1,000 ÷ 1,609.344 = 6.21371 miles.
Temperature is different, because the scales have different zero points as well as different step sizes. The converter goes through kelvin:
- °F = °C × 9/5 + 32
- °C = (°F − 32) × 5/9
- K = °C + 273.15
The conversions people look up most
| Unit | Equals | Status |
|---|---|---|
| 1 inch | 2.54 cm | Exact, by definition |
| 1 foot | 0.3048 m | Exact, by definition |
| 1 mile | 1.609344 km | Exact, by definition |
| 1 km | 0.621371 mi | Rounded to 6 significant figures |
| 1 pound | 0.45359237 kg | Exact, by definition |
| 1 kg | 2.20462 lb | Rounded to 6 significant figures |
| 1 ounce | 28.349523125 g | Exact, by definition |
| 1 stone | 14 lb = 6.35029318 kg | Exact, by definition |
| 1 US gallon | 3.785411784 L | Exact, by definition |
| 1 UK gallon | 4.54609 L | Exact, by definition |
| 1 US cup | 236.588 mL | Rounded to 6 significant figures |
| 1 acre | 4,046.86 m² | Rounded to 6 significant figures |
| 1 hectare | 10,000 m² = 2.47105 acres | Rounded to 6 significant figures |
| 1 knot | 1.852 km/h | Exact, by definition |
Temperature reference
| Celsius | Fahrenheit | Kelvin |
|---|---|---|
| -40 °C | -40 °F | 233.15 K |
| -18 °C | -0.4 °F | 255.15 K |
| 0 °C | 32 °F | 273.15 K |
| 10 °C | 50 °F | 283.15 K |
| 20 °C | 68 °F | 293.15 K |
| 25 °C | 77 °F | 298.15 K |
| 30 °C | 86 °F | 303.15 K |
| 37 °C | 98.6 °F | 310.15 K |
| 40 °C | 104 °F | 313.15 K |
| 100 °C | 212 °F | 373.15 K |
| 180 °C | 356 °F | 453.15 K |
| 200 °C | 392 °F | 473.15 K |
Traps to watch for
- US versus imperial volume. US gallons, quarts, pints and fluid ounces are all smaller than the UK versions, except the fluid ounce, which is slightly larger in the US (29.57 mL against 28.41 mL).
- Weight ounces versus fluid ounces. One measures mass, the other volume. They only match for water, and only approximately.
- Short tons, long tons and tonnes. A US (short) ton is 2,000 lb, a UK (long) ton 2,240 lb, and a metric tonne 1,000 kg (about 2,204.6 lb).
- Area and volume factors. Squaring or cubing a length factor is easy to forget: 1 m is 3.28 ft, but 1 m² is 10.76 ft².
- Temperature differences. A change of 10 °C is a change of 18 °F, with no +32, because the offset cancels out.
For body measurements, the BMI calculator and TDEE calculator accept metric or imperial units directly.
Frequently asked questions
How many centimetres are in an inch?
Exactly 2.54. Since the international yard and pound agreement of 1959, the inch has been defined as 25.4 mm, so 1 foot is exactly 30.48 cm and 1 mile exactly 1.609344 km.
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 (or 1.8) and add 32. 25 °C is 25 × 1.8 + 32 = 77 °F. To go the other way, subtract 32 and multiply by 5/9: 100 °F is 37.78 °C.
How many pounds are in a kilogram?
About 2.20462. The pound is defined as exactly 0.45359237 kg, so 1 kg = 1 ÷ 0.45359237 = 2.2046226 lb.
Is a US gallon the same as a UK gallon?
No. A US gallon is 3.785411784 litres; an imperial (UK) gallon is 4.54609 litres, about 20% more. US and UK pints and fluid ounces differ too, so recipes and fuel figures need the right one.
How do I convert km/h to mph?
Divide by 1.609344. 100 km/h is 62.14 mph. A quick estimate is to multiply by 0.62, or by 5/8.
How many square feet are in a square metre?
About 10.764. One square foot is exactly 0.09290304 m² (0.3048 m squared), so 1 m² = 10.7639 ft². An acre is 43,560 ft² or 4,046.86 m².