Unit Converter

Seven families of units, converted exactly

This converter handles length, weight, volume, area, speed, data and temperature. Within each family every unit is defined by its size in terms of one base unit, so a conversion is a single multiplication and division and nothing accumulates rounding along the way. The definitions used are the exact international ones rather than the rounded values that circulate informally: an inch is exactly 2.54 cm, a mile exactly 1,609.344 m, a pound exactly 0.45359237 kg and a nautical mile exactly 1,852 m.

Temperature is the exception. It has offsets as well as scale, so it carries explicit conversion functions rather than a factor — you cannot convert Celsius to Fahrenheit by multiplying.

Worked conversions

A mile is 1.609344 km, so 100 km is 62.137119 miles. Six feet is 1.8288 m. Seventy kilograms is 154.3236 pounds, and a stone is 6.35029318 kg. One hundred kilometres per hour is 62.137119 mph, and a knot — one nautical mile per hour — is exactly 1.852 km/h.

Temperature: 100 °F is 37.7778 °C using (F − 32) × 5 ÷ 9. Zero Celsius is 32 °F. Minus forty is the one point where the two scales meet, which is why −40 °C and −40 °F are the same temperature. Absolute zero is 0 K, which is −273.15 °C, and 0 °R on the Rankine scale, which is 459.67 °R below the freezing point of water expressed in Fahrenheit.

Where the same name means two different things

Several unit names are ambiguous across countries or industries, and the converter lists both rather than picking one silently.

• A US gallon is 3.785411784 litres and an imperial gallon is 4.54609 litres — about 20% larger. A car rated at 40 mpg in the UK is roughly 33 mpg in US terms, on identical fuel consumption.
• US pints, quarts, cups, fluid ounces, tablespoons and teaspoons are all correspondingly smaller than their imperial counterparts. The volume list here is US measure unless it says otherwise.
• A gigabyte is 1,000 megabytes and a gibibyte is 1,024 mebibytes. A 1 TB drive holds 1,000,000,000,000 bytes, which an operating system reporting in binary units calls 931 GB. Nothing is missing; the two are counting in different bases.

Precision and rounding

Results are displayed to six significant figures, and a very small result falls back to significant digits rather than collapsing to zero — converting one milligram to tonnes shows 1e−9 rather than a flat 0. Internally the arithmetic is double-precision floating point, so a converted value may differ from the exact decimal in the fifteenth digit or beyond. That is invisible in any practical use and is a property of binary floating point rather than of the conversion factors, which are exact.

Where a conversion depends on something other than the unit, this tool will not guess. Volume to weight needs the density of the substance — that is what the cooking converter handles, ingredient by ingredient. Fuel economy in mpg and in litres per 100 km are reciprocals rather than a scale factor, which is why they live in the fuel cost calculator instead.