Convert length, weight, temperature, area, volume, speed and data. Every factor is an exact definition, the working is shown beside the result, and nothing you type leaves your browser.
The same amount in each unit of the category, at the precision chosen above.
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Each unit is defined once, from the agreement or standard that fixes it. None of the factors is rounded, so the only rounding is the one you choose for display.
1 inch is 25.4 mm exactly, fixed by the international yard and pound agreement of 1959. The foot (12 in), yard (3 ft) and mile (1,760 yd) follow from it: this is the international foot, not the US survey foot of 1200/3937 m, which the US retired at the end of 2022. The nautical mile has been 1,852 m exactly since 1929.
1 pound is 0.45359237 kg exactly, from the same 1959 agreement. An ounce is a sixteenth of a pound, a stone is 14 pounds, and a tonne is 1,000 kg. Strictly these are units of mass; weight is the everyday word for them.
Kelvin is the base. Celsius adds 273.15 to reach it; Fahrenheit adds 459.67 and multiplies by 5/9. The offset goes on before the factor, so temperatures take the same route through the base unit as everything else, and the familiar °F = °C × 9/5 + 32 falls out of it.
Each unit is the square of its length unit: a square inch is 25.4² = 645.16 mm². An acre is 4,840 square yards, which makes it 4,046.8564224 m² exactly, and a hectare is 10,000 m².
1 US gallon is 231 cubic inches, which is 3.785411784 L exactly. The quart, pint, cup, fluid ounce, tablespoon and teaspoon are 1/4, 1/8, 1/16, 1/128, 1/256 and 1/768 of it. The UK gallon is 4.54609 L exactly, set by the Weights and Measures Act 1985.
Length over time. A mile per hour is 1,609.344 m in 3,600 s, which is 0.44704 m/s exactly. A knot is one nautical mile per hour. 1 km/h is 5/18 m/s, a fraction with no finite decimal form, so it is kept as a fraction rather than cut short.
A byte is 8 bits. KB, MB, GB and TB go up by 1,000 at each step, as the SI prefixes say and as drives are labelled. KiB, MiB, GiB and TiB go up by 1,024, the binary prefixes of IEC 80000-13, which is how Windows and many Linux tools report sizes; macOS, iOS and Android count in thousands.
Results are shown to the number of significant figures you pick, from 3 to 12, counted from the first non-zero digit, with trailing zeros dropped. The maths always runs at full precision, so the setting changes how much of the answer you see, never the answer. A figure that would need more than 20 decimal places is shown as a power of ten.
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Every unit is stored once as its size in the category's base unit, taken from its exact definition: 1 inch is 0.0254 m, 1 pound is 0.45359237 kg, 1 US gallon is 3.785411784 L. A value is multiplied by the size of the unit it is in and divided by the size of the unit you want. 5 feet is 5 × 0.3048 = 1.524 m, and 1.524 ÷ 0.01 = 152.4 cm. The route is shown beside every result, so you can check it.
Through kelvin, with an offset added before the scale factor. Celsius adds 273.15. Fahrenheit adds 459.67 and multiplies by 5/9. So 68 °F is (68 + 459.67) × 5/9 = 293.15 K, and 293.15 − 273.15 = 20 °C. Body temperature of 98.6 °F works out to exactly 37 °C, and minus 40 is the same temperature on both scales.
It sets how many digits are shown, counted from the first non-zero one, so at 6 figures 152.4 stays 152.4, 0.0328084 is shown in full and 1,609.344 becomes 1,609.34. Pick 8 or more to see an exact definition such as a mile in metres without rounding. It changes only the display: the maths always uses the full factors, and the value you typed is never rounded.
KB, MB, GB and TB use powers of 1,000, as the SI prefixes define them and as drives are labelled. KiB, MiB, GiB and TiB use powers of 1,024, the binary units of IEC 80000-13, which is how Windows and many Linux tools report sizes; macOS, iOS and Android count in thousands. A 1 TB drive holds 1,000,000,000,000 bytes, which is 931.323 GiB: the gap you see when a new drive looks smaller than the box said.
The US customary ones, all defined as fractions of the US gallon of 3.785411784 L: a cup is 236.588 mL, a fluid ounce 29.5735 mL, a tablespoon 14.7868 mL and a teaspoon 4.92892 mL. The UK gallon is a separate unit of 4.54609 L, about 20% larger than the US gallon. Metric recipes use a 5 mL teaspoon, a 15 mL tablespoon and a 250 mL cup, close enough for cooking but not the same.
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