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Unit Hub Online is a free collection of browser-based converters for everyday tasks, study and engineering reference. All calculations stay on your device.
Definitions and sources
Our static factors follow international unit definitions and the NIST Guide to the SI conversion tables. Traditional UK Imperial definitions also follow Schedule 1 of the Weights and Measures Act 1985. CSS typography relationships follow W3C CSS Values and Units. Binary data prefixes follow powers of 1,024; decimal prefixes follow powers of 1,000.
Everyday definitions
US liquid gallon = 3.785411784 L; Imperial gallon = 4.54609 L. Each liquid pint is one eighth of its gallon and each quart one quarter. US customary tablespoon = 1/256 US gallon; teaspoon = 1/768 US gallon. Metric spoons here mean exactly 5 mL and 15 mL; other regional spoon sizes are not implied. A cubic yard is exactly 27 cubic feet.
Conventional millimetre of mercury uses mercury density 13,595.1 kg/m³ and standard gravity 9.80665 m/s²: 1 mmHg = 133.322387415 Pa. Torr is separately defined as 1/760 of 101,325 Pa. Data rates use eight bits per byte, with decimal powers of 1,000 and binary powers of 1,024.
Fuel conversions derive from the exact international mile (1.609344 km) and the named gallon: L/100 km = 100 × litres per gallon ÷ kilometres per mile ÷ mpg. For km/L, L/100 km = 100 ÷ km/L. Fuel values must be positive. Mixed measurements use 12 inches per foot or 14 pounds per stone; negative mixed values are not supported.
UK, US and metric units
Imperial units are labelled UK in the selectors. UK and US gallons, pints, quarts, fluid ounces, hundredweights and miles per gallon have separate definitions. International inches, feet, yards, miles and avoirdupois pounds are shared between the UK and US. Traditional units include chains, furlongs, roods, gills, pecks and bushels. Plain ounces are avoirdupois; select Troy ounce for precious-metal mass.
Cooking measures use explicitly named definitions: metric teaspoons are 5 mL and metric tablespoons are 15 mL; US customary spoons differ. Regional and historical cups and spoons can vary, so match the definition in your recipe. This site covers a curated set of units across 18 categories; it does not cover every historical, regional or specialist unit.
What accuracy means here
Fixed definitions such as 1 inch = 0.0254 metre and 1 pound = 0.45359237 kilogram are exact definitions. Computer calculations use IEEE 754 double-precision arithmetic, which can introduce small rounding errors. Displays use your chosen number of significant figures; choosing more digits does not make an uncertain measurement more accurate. Extremely large or small results can exceed the supported numerical range and return an error.
Clear boundaries
Temperature conversions apply to absolute temperatures. Values below absolute zero are rejected. Pressure values retain their original gauge or absolute reference. Cups and gallons are labelled by system. Typography em and rem require font-size context. Time is a fixed duration, not a calendar calculation.
No live exchange rates, density guesses, variable regional land measures or context-free Mach conversions are included. For safety-critical work, independently verify the definitions and calculation.
Your saved conversions
Favourites are stored in your browser’s local storage. They do not sync between devices. Conversion still works if local storage is unavailable.
Using the tool
Edit either value to convert in that direction. Change a unit to recalculate from the last edited value. Swap reverses the selected units while retaining the displayed source quantity. Copy result copies the result shown, including its unit. Copy link includes your measurement and settings in a shareable URL; anyone with that link can read them. Search accepts categories, unit names and simple pairs such as “kg to lb”.