Unit Converter
Convert metric and imperial units instantly: length, weight, temperature, volume, area, speed, time and digital storage.
Runs 100% in your browser
Recipes from another country, a product listed in inches, a weather forecast in Fahrenheit or a phone that shows storage in gibibytes: everyday life is full of units. This free Unit Converter handles eight categories in one place, converts in both directions as you type and shows the value in every unit of the category at once. It uses the exact official conversion factors, so results match textbooks and engineering references.
How to use the unit converter
- Choose a category such as Length, Weight or Temperature.
- Pick the units in the From and To lists.
- Type a number in either box. The other box updates instantly, so you can convert in both directions.
- Check the table below for the same value in every other unit, or press the swap button to reverse the conversion.
The metric system and SI
Most of the world uses the International System of Units (SI), maintained by the International Bureau of Weights and Measures (BIPM) near Paris. Its base units include the metre, kilogram, second and kelvin, and since 2019 all of them are defined from fixed constants of nature, such as the speed of light and the Planck constant. Prefixes scale units by powers of ten: kilo means 1,000, centi one hundredth and milli one thousandth. That decimal structure is why metric conversions are just a shift of the decimal point.
Imperial and US customary units, such as the inch, foot, pound and gallon, are still used in the United States and in parts of daily life in the United Kingdom and elsewhere. Since the International Yard and Pound Agreement of 1959, these units are defined exactly in metric terms, which is what makes precise conversion possible.
Exact conversion factors used
| Unit | Exact value |
|---|---|
| 1 inch | 2.54 cm |
| 1 foot | 0.3048 m |
| 1 mile | 1.609344 km |
| 1 nautical mile | 1,852 m |
| 1 pound | 0.45359237 kg |
| 1 US gallon | 3.785411784 L |
| 1 imperial gallon | 4.54609 L |
| 1 acre | 4,046.8564224 m² |
| 1 hectare | 10,000 m² |
Some useful approximations for mental maths: 1 kg is about 2.2 lb, 1 km is about 0.62 miles, 1 litre is a little more than a US quart, and a hectare is about 2.47 acres.
Temperature is different
Temperature scales do not share a zero point, so you cannot convert them with a single multiplication. The tool uses the standard formulas: °F = °C × 9/5 + 32 and K = °C + 273.15. Water freezes at 0 °C (32 °F) and boils at about 100 °C (212 °F) at sea level, normal body temperature is around 37 °C (98.6 °F), and the two scales meet at −40. Kelvin, the SI unit, starts at absolute zero and is used in science.
Cooking and liquid volumes
Recipe units vary between countries. This converter uses US customary kitchen measures: a US cup is 236.6 ml, a tablespoon 14.8 ml and a teaspoon 4.9 ml. Many other countries use a metric cup of 250 ml and a 15 ml tablespoon (Australia uses a 20 ml tablespoon), so check where a recipe comes from. Note too that the US gallon and the imperial gallon differ by about 20 percent, which matters when comparing fuel economy figures.
Data: why your 1 TB drive shows 931 GB
Storage makers use decimal prefixes, so 1 GB is 1,000,000,000 bytes. Many operating systems count in binary multiples of 1,024, which the International Electrotechnical Commission names kibibytes (KiB), mebibytes (MiB) and gibibytes (GiB). A drive sold as 1 TB holds 1012 bytes, which is about 931 GiB, and that is the number Windows shows labelled as "GB". Nothing is missing; it is the same amount in a different unit. Internet speeds use bits, not bytes: 100 megabits per second is 12.5 megabytes per second.
Tips for quick checks
When a converted value looks wrong, check the direction first: converting a big unit into a small one should give a bigger number, such as 1 kilometre becoming 1,000 metres. Then check that you picked the right variant, for example US versus imperial gallons, or GB versus GiB.
Speed and time
Road signs use km/h in most countries and mph in a few, while aviation and shipping use knots (one nautical mile per hour, about 1.852 km/h). For time, a year here means a Julian year of 365.25 days, the average length used in astronomy, so "hours in a year" comes out at 8,766.
Accuracy and rounding
Results are calculated in double-precision floating point and shown with up to 10 significant digits, which removes tiny binary rounding errors (for example 0.1 + 0.2 shows as 0.3). Very large or very small values switch to scientific notation. For legal metrology, contracts or medical dosing, always follow the precision required by the relevant standard.
Frequently asked questions
How many centimetres are in an inch?
Exactly 2.54 cm. This has been the international definition since 1959.
How do I convert kg to pounds?
Multiply by about 2.20462. For example, 70 kg is about 154.3 lb. To go back, multiply pounds by 0.45359237.
What is the formula for Celsius to Fahrenheit?
Multiply by 9/5 and add 32. So 25 °C is 77 °F. From Fahrenheit, subtract 32 and multiply by 5/9.
Is a UK gallon the same as a US gallon?
No. An imperial gallon is 4.54609 litres and a US gallon is 3.785411784 litres, about 20 percent smaller.
What is the difference between GB and GiB?
A gigabyte (GB) is 1,000,000,000 bytes; a gibibyte (GiB) is 1,073,741,824 bytes, or 1,024 cubed.
Does the converter work offline?
Yes. All factors are built into the page, so once it has loaded you can convert without a connection.
Sources
- BIPM: The International System of Units (SI Brochure)
- NIST Special Publication 811: Guide for the Use of the SI
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement
- NIST: Prefixes for binary multiples
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By Abdullah Al-Janoubi · WorldScope Tools · Updated October 2026
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