How the converter works
Every unit is stored as its exact size in a metric base unit: meters for length, square meters for area, cubic meters for volume, kilograms for weight and meters per second for speed. The number you type is multiplied by the size of the first unit and divided by the size of the second. Nothing is rounded until the answer is shown, to 12 significant digits, so you see 3.048 rather than 3.0480000000000005.
Result = value × (size of the from-unit ÷ size of the to-unit)
Temperature works differently, because the scales start at different zeros. The converter goes through Celsius using the formulas in NIST Special Publication 811, Appendix B.8:
°C = (°F − 32) ÷ 1.8 · °F = °C × 1.8 + 32 · K = °C + 273.15
A temperature below absolute zero (0 K, −273.15 °C or −459.67 °F) does not exist, so the converter says so instead of giving a number.
Where the numbers come from
The definitions come from NIST Handbook 44 (2026), Appendix C, the general tables of units used for US weights and measures, and from NIST SP 811. All of these are exact:
| Unit | Definition |
|---|---|
| Inch | 2.54 centimeters |
| Foot | 12 inches = 0.3048 meter |
| Mile | 5,280 feet = 1,609.344 meters |
| Nautical mile | 1,852 meters |
| Acre | 43,560 square feet = 4,046.8564224 square meters |
| US gallon | 231 cubic inches = 128 US fluid ounces = 3.785411784 liters |
| US cup | 8 US fluid ounces, half a US pint |
| US tablespoon / teaspoon | ½ US fluid ounce / ⅓ tablespoon |
| Pound (avoirdupois) | 7,000 grains = 0.45359237 kilogram |
| Ounce | 1/16 pound = 28.349523125 grams |
| Troy ounce | 480 grains, with 1 grain = 64.79891 milligrams |
| Short ton / long ton | 2,000 pounds / 2,240 pounds |
| Mile per hour | 0.44704 meter per second |
The British units come from Schedule 1 of the UK Weights and Measures Act 1985: the imperial gallon is 4.54609 liters, the imperial pint 0.56826125 liter, the imperial fluid ounce 1/20 of a pint, and a stone 14 pounds.
The survey foot is gone
Until 2022, US land surveys used the survey foot, 1200/3937 meter, which is about 2 parts per million longer than the international foot. NIST retired it: December 31, 2022 was the last day it should be used. This converter uses the international foot everywhere, including for acres and square miles, so an acre is 4,046.8564224 square meters rather than the survey value of about 4,046.8726. The gap is about 162 square centimeters (25 square inches) per acre, which only matters for old deeds and survey records.
Worked example: a recipe from the UK
A British recipe asks for 1 imperial pint of milk and an oven at 180 °C. One imperial pint is 568.26125 mL, or about 2.4 US cups (568.26125 ÷ 236.5882365 = 2.40190). The oven: 180 × 1.8 + 32 = 356 °F, so set it to 350 °F or 360 °F.
Questions people ask
How many centimeters are in an inch?
Exactly 2.54. NIST defines the inch as 2.54 centimeters and the foot as 12 inches, or exactly 0.3048 meters, so 10 feet is exactly 3.048 meters and 12 inches is exactly 30.48 centimeters.
How do I convert pounds to kilograms?
Multiply pounds by 0.45359237, which is the exact definition of the avoirdupois pound. 150 lb × 0.45359237 = 68.0388555 kg. To go back, divide kilograms by 0.45359237, or multiply by about 2.20462.
How do I convert Fahrenheit to Celsius?
Subtract 32, then divide by 1.8: (°F − 32) ÷ 1.8. So 72 °F is 40 ÷ 1.8 = 22.2 °C. From Celsius to Fahrenheit, multiply by 1.8 and add 32. Temperature is not a simple ratio: 0 °C is 32 °F, not 0 °F, so a single factor never works.
Is a US gallon the same as a UK gallon?
No. A US gallon is 231 cubic inches, or 3.785411784 liters. The imperial (UK) gallon is 4.54609 liters, about 20% larger. The fluid ounces differ too: a US gallon holds 128 US fluid ounces, an imperial gallon 160 imperial fluid ounces, and the imperial fluid ounce is about 4% smaller than the US one.
Why does a nutrition label say a cup is 240 mL?
For nutrition labels the FDA rounds household measures: a cup means 240 mL, a tablespoon 15 mL, a teaspoon 5 mL and a fluid ounce 30 mL (21 CFR 101.9(b)(5)(viii)). The customary US cup used here is half a US pint, 236.5882365 mL. The difference is under 2%.
Which foot does this use, the international foot or the survey foot?
The international foot, exactly 0.3048 meters. The U.S. survey foot (1200/3937 meters, about 2 parts per million longer) was used for land surveying until NIST declared it obsolete on January 1, 2023. Acres and square miles here are based on the international foot too, as in NIST Handbook 44.
Can I link to a conversion?
Yes. The address bar updates as you type, for example /home/measuring/unit-converter/?from=ft&to=m&v=10. Copy that link and the page opens with the same units and number.