How to use the unit conversion calculator
- Pick a category, such as length, temperature or digital storage.
- Enter the value you want to convert.
- Choose the unit to convert from and the unit to convert to.
- Read the result, the conversion factor, and a table with the same value in every unit of that category.
How unit conversion works
Most conversions are a single multiplication. Every unit has a fixed size in a base unit, such as meters for length or kilograms for mass, and you scale by the ratio of the two sizes:
Temperature is the exception because the scales start at different zero points, so you have to add or subtract an offset as well as scale. The calculator converts through kelvin, which starts at absolute zero. Fuel economy needs care too: miles per gallon measures distance per fuel, while liters per 100 km measures fuel per distance, so one is the reciprocal of the other.
Worked example
The calculator starts by converting 10 kilometers to miles:
- 1 kilometer is 1,000 meters, so 10 km is 10,000 m.
- 1 mile is exactly 1,609.344 m.
- 10,000 ÷ 1,609.344 = 6.213712 miles, shown to seven significant figures.
For temperature, 98.6 °F becomes (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37 °C. For fuel economy, 30 mpg (US) becomes 235.215 ÷ 30 = 7.84 L/100 km. For storage, a 1 TB drive holds 1,000,000,000,000 bytes, and 1,000,000,000,000 ÷ 1,073,741,824 = 931.3 GiB, which is the size Windows reports.
Exact definitions behind the factors
| Unit | Exact value |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot | 0.3048 m |
| 1 mile | 1,609.344 m |
| 1 pound | 0.45359237 kg |
| 1 US gallon | 231 cubic inches = 3.785411784 L |
| 1 imperial gallon | 4.54609 L |
| 1 acre | 4,046.8564224 m² |
| 1 kilocalorie | 4,184 J |
| 1 British thermal unit (IT) | 1,055.05585262 J |
| 1 standard atmosphere | 101,325 Pa |
The inch, foot, yard and pound were fixed in metric terms by the 1959 international agreement, so these conversions are exact, not rounded. Other factors, such as pounds per square inch (6,894.757 Pa) and mechanical horsepower (745.7 W), follow from them. The full list is in NIST's Guide to the SI, Appendix B.
Units that are easy to mix up
- US and imperial gallons. An imperial gallon is about 20% larger than a US gallon, so UK mpg figures look higher for the same car.
- Decimal and binary storage. Drive makers use GB (1,000³ bytes); Windows uses GiB (1,024³ bytes) but labels them GB.
- Fluid ounces and ounces. A fluid ounce measures volume and an ounce measures weight. A US fluid ounce of water weighs about 1.04 oz.
- Calories. Food labels use kilocalories, written Calories with a capital C. One food Calorie is 1,000 small calories.
- Short and long tons. A US short ton is 2,000 lb, a UK long ton is 2,240 lb and a metric tonne is 1,000 kg (2,204.6 lb).
Frequently asked questions
How do I convert Fahrenheit to Celsius in my head?
Subtract 30 and halve it for a quick estimate: 80 °F gives about 25 °C (the exact answer is 26.7 °C). For the exact value, subtract 32 and multiply by 5/9. Going the other way, double the Celsius figure and add 30, or multiply by 9/5 and add 32 for the exact answer.
Why does my 1 TB drive show as 931 GB?
The drive maker counts 1 TB as 1,000,000,000,000 bytes, but Windows divides by 1,024 three times and calls the result GB. 1,000,000,000,000 ÷ 1,073,741,824 is 931.3, so no space is missing. Choose digital storage in the calculator to convert between the two.
How accurate are the conversions?
The factors are exact where the units are defined exactly, which covers length, mass, area, volume and most others. Results are rounded to seven significant figures for display. Months and years use the average Gregorian calendar, 30.436875 and 365.2425 days, so a specific month may be a day longer or shorter.
How do I convert mpg to L/100 km?
Divide 235.215 by the miles per US gallon. A car rated at 30 mpg uses 235.215 ÷ 30 = 7.84 liters per 100 km. For imperial (UK) mpg, divide 282.481 by the figure instead.
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