Quick answer: Unit conversion multiplies by a fixed factor, and many of the factors are exact by definition: 1 inch is 2.54 cm, 1 pound is 0.45359237 kg and 1 mile is 1.609344 km. Temperature is the exception because it also shifts the zero point: °C = (°F − 32) × 5 ⁄ 9.
Recipes use cups, road signs use kilometers or miles, weather forecasts use Celsius or Fahrenheit, and storage is measured in gigabytes or gibibytes. A good converter saves you from doing the maths in your head, or getting it wrong.
Choose a type of measurement, type a value, and the result appears straight away. The table underneath shows the same value in every other unit of that type, which is handy when you are not sure which unit you need.
How the conversions work
Each unit is defined by an exact factor relative to a base unit for its type, and the tool multiplies by the ratio between the two. Several everyday factors are exact by international definition:
1 mile = 1,609.344 m (exactly)
1 pound = 0.45359237 kg (exactly)
1 US gallon = 3.785411784 liters (exactly)
Temperature is not a simple multiplication, so it uses formulas: °F = °C × 9/5 + 32 and K = °C + 273.15. Results are shown to up to ten significant digits.
Things that cause confusion
- US and imperial gallons differ: a US gallon is about 3.785 liters and an imperial gallon about 4.546 liters. This tool shows both.
- A kilobyte (KB) is 1,000 bytes, while a kibibyte (KiB) is 1,024 bytes. The same applies to MB and MiB, and GB and GiB.
- Ounces can measure weight or fluid volume. Pick the type first, then the unit.
A worked example
Example. 1 mile is 1.609344 kilometers. A speed of 100 km/h is about 62.14 mph. A room temperature of 72 °F is 22.22 °C. A 256 GB phone holds 256,000 MB using decimal units, or about 238.4 GiB when measured in binary units.
Exact conversion factors
| Unit | Exact value |
|---|---|
| 1 inch | 2.54 centimeters |
| 1 foot | 0.3048 meters |
| 1 yard | 0.9144 meters |
| 1 mile | 1.609344 kilometers |
| 1 pound | 0.45359237 kilograms |
| 1 ounce (weight) | 28.349523125 grams |
| 1 US gallon | 3.785411784 liters |
| 1 UK (imperial) gallon | 4.54609 liters |
| 1 US fluid ounce | 29.5735295625 milliliters |
| 1 acre | 4,046.8564224 square meters |
These conversions are exact by international agreement, not rounded.
Common mistakes to avoid
- Confusing US and UK units. A US gallon is 3.785 liters and a UK gallon is 4.546 liters, and the two “pints” also differ.
- Rounding early. Keep several digits during a calculation and round only the final answer.
- Mixing mass and weight, or volume and weight. Grams and cups need the density of the ingredient, which is why the cooking converter asks what you are measuring.
Go deeper: read our guide Cups to Grams: A Baking Conversion Guide.
Frequently asked questions
How do I convert kilometers to miles?
Divide kilometers by 1.609344, or multiply by about 0.6214. Pick Length, choose kilometers as the From unit and miles as the To unit, and the result appears instantly.
How accurate are the conversions?
They use exact defined factors, and the display rounds to ten significant digits. That is far more precision than any everyday use requires.
Why does my 256 GB drive show less space on my computer?
Manufacturers use decimal gigabytes (1 GB = 1,000,000,000 bytes), while some operating systems report in binary units and label them GB. 256 GB is about 238 GiB.
Can I convert between metric and imperial cooking measures?
Yes. Use the Volume type for cups, tablespoons, teaspoons and milliliters, and Weight for grams and ounces. Note that cup sizes vary between countries; this tool uses the US cup (about 237 ml).
People also search for this as: unit conversion calculator, metric to imperial converter, length converter, weight converter, temperature converter Celsius Fahrenheit, convert miles to km.
Sources and further reading
- International yard and pound (Wikipedia)
- NIST Guide for the Use of the International System of Units (SI)
Formulas on this page are checked by automated tests against independent references. See how we test our tools.