How to use the unit converter
- Choose a category: Length, Weight, Temperature, Volume, Speed, Area, Time or Data.
- Type a number in Value. Commas (1,000) and scientific notation (1.5e6) work. Use a period for decimals.
- Pick the unit you have in From and the unit you want in To. The arrows button swaps the two.
- Read the result. The line under it shows the exact ratio, and the Quick reference table lists common amounts for the same pair.
How the conversions work
Every length, weight, volume, speed, area, time and data unit is stored as a fixed multiple of one base unit: meter, kilogram, liter, meter per second, square meter, second or byte. The tool multiplies your value by the From factor and divides by the To factor. The inch (exactly 0.0254 m), the pound (exactly 0.45359237 kg) and the US gallon (exactly 3.785411784 L) are the internationally agreed definitions listed in NIST Special Publication 811, Appendix B.8. Results show up to 12 significant digits.
Temperature is different because the scales start at different zero points. The tool converts through Celsius: °F = °C × 9/5 + 32 and K = °C + 273.15. For data, KB, MB, GB, TB and PB are powers of 1,000, while KiB, MiB, GiB, TiB and PiB are powers of 1,024, as defined in the NIST binary prefix reference.
Worked example
The page opens on Length with Value 1, From Miles (mi) and To Kilometers (km). It shows 1.609344 Kilometers (km) and the formula line "1 mile = 1.609344 kilometers". To convert any other distance, multiply the miles by 1.609344. Type 5 and the result is 8.04672.
Limits and edge cases
- Months and years are averages (30.436875 days and 365.2425 days). For exact calendar spans between two dates, use the date difference calculator.
- A volume cannot become a weight without a density. For cups to grams, use the cooking measurement converter.
- Fluid ounce, cup, pint, quart and gallon are US customary units. The UK pint and UK gallon are listed separately. Ton (US) is the 2,000 lb short ton, Ton (UK) is the 2,240 lb long ton, and the metric ton is 1,000 kg.
- Temperatures below absolute zero (0 K, -273.15 °C, -459.67 °F) are converted but flagged. For temperatures only, try the temperature converter.
- Values above 1e21 or below 1e-6 appear in scientific notation. Results that overflow are reported as out of range instead of showing a wrong number.
- For drive and file sizes in more detail, see the data storage converter.
Frequently asked questions
How many kilometers are in a mile?
One mile is exactly 1.609344 kilometers. So 5 miles is 8.04672 km, and a 26.2 mile marathon is about 42.165 km.
What is the formula for Celsius to Fahrenheit?
Multiply the Celsius temperature by 9/5 (1.8), then add 32. For example, 100 °C × 1.8 + 32 = 212 °F. To go the other way, subtract 32 and multiply by 5/9.
How many ounces are in a pound?
A pound has exactly 16 avoirdupois ounces, so one ounce is 28.349523125 grams. That is a weight ounce. A fluid ounce measures volume, and 16 US fluid ounces make one US pint.
Why is 1 TB not 1,024 GB?
In SI units, 1 TB is 1,000 GB because kilo, mega, giga and tera mean powers of 1,000. The 1,024 steps belong to the binary units, so 1 TiB is 1,024 GiB. Operating systems that label a binary count as GB are really showing GiB, which is why a 1 TB drive appears to hold about 931 GB.
What is the difference between a US gallon and a UK gallon?
A US gallon is 3.785411784 liters and an imperial (UK) gallon is 4.54609 liters, about 20 percent larger. Choose UK gallon in the Volume category when you need the imperial measure.
Sources: NIST SP 811, Appendix B.8 conversion factors; NIST, prefixes for binary multiples.
Related reading: Unit Conversion Cheat Sheet: The Most Common Conversions