Unit Converter

How the converter works

Each category has one base unit: the metre for length, the kilogram for mass, the cubic metre for volume, the square metre for area, metres per second for speed, the kilobyte for data size and the second for time. Every unit in that category is stored as a single number — how many base units one of it is worth — and conversions are done in two steps. The value you type is multiplied by the factor of the unit you selected as from, which expresses it in base units, and that result is divided by the factor of the unit you selected as to. Because both steps go through the same base, any pair of units in a category converts correctly without needing a separate combination for every possible pair, and the table below the inputs simply repeats the second step once for each unit in the list.

Temperature is handled by a different path. Celsius, Fahrenheit and Kelvin agree on the size of a degree in only some cases and disagree about where zero sits, so a single multiplier cannot describe them. The converter therefore sends temperatures through the standard formulas: °F = °C × 9/5 + 32, °C = (°F − 32) × 5/9, and K = °C + 273.15, with the Kelvin conversions chained through Celsius. Pressing Swap reverses the two units and recomputes immediately, which is the quickest way to answer the reverse of a question you have just asked.

Where the factors come from, and how much to trust the result

The factors used here are exact wherever an exact definition exists, and the tool does not round them. The international inch has been defined as exactly 25.4 millimetres since the 1959 agreement between the United States, the United Kingdom and the other Commonwealth countries, and this page uses that value unchanged. The international avoirdupois pound has been exactly 0.45359237 kilograms since the same agreement. US volumes follow from the exact inch: a US gallon is exactly 231 cubic inches, which is exactly 3.785411784 litres, and a fluid ounce is one 128th of that. Because those chains of exact definitions are preserved rather than pre-rounded, 1 mi = 1.609344 km exactly and 1 lb = 453.59237 g exactly.

Not every factor is defined. There is no exact length for a mean Earth year, no exact value for the mean solar day or the Julian year, and no exact megabyte, because a megabyte means 1 000 000 bytes in the SI decimal convention and 1 048 576 bytes in the binary convention. Where an exact definition does not exist the converter uses the standard average — 365.2425 days for the Gregorian year, 86 400 seconds for the day, and 1 048 576 bytes for the binary megabyte — and labels those rows so the assumption is visible. Treat results built on them as indicative rather than exact.

Display rounding is the other place where a figure can differ from your own arithmetic. Every number on this page is rounded to 12 significant digits and shown with thousands separators, which is deliberately a little less than the 15 to 17 decimal digits a JavaScript double can carry. That is what stops binary floating-point artefacts — the 0.30000000000000004 that 0.1 + 0.2 produces when printed raw — from appearing in an answer that should read 0.3. The rounding is for display only: each conversion in the table is computed from the original full-precision value, not from the rounded text of the row above it, so errors do not accumulate down the table.

How to use it

Start by choosing a Category: Length, Mass, Volume, Area, Speed, Temperature, Data size or Time. The from and to lists then fill with the units of that category and the conversion table underneath is rebuilt, so switching from Length to Data size never leaves a stale row behind. Type the number you want to convert into Value to convert; the result and the table update as you type, so there is nothing to submit. Decimal points, a leading minus sign and thousands separators are all accepted, and the value field is a text input with a numeric keyboard on phones so that a pasted figure such as 1,250.75 is not rejected.

Set From to the unit you have and To to the unit you want. If you need the opposite direction, either swap the two selects or press the ⇄ Swap button, which exchanges them and recalculates. The table is the fastest way to answer "how much is this in everything?" — it lists each unit of the current category with the same typed value expressed in it, as well as the factor relative to the base unit, so you can see at a glance whether a unit is larger or smaller than the one you started from. The hint line under the inputs always states the direction in force, in the form 5 metres = 16.4041994751 feet, so a mis-set select is easy to spot.

Everything here is keyboard reachable: Tab moves through the category, value and unit controls, the selects open with the usual arrow keys, and the swap button activates with Space or Enter. The updated result sits in a live region, so a screen reader announces each new value as it changes. The page is usable at 360 pixels wide, and because the whole tool is a single local script it keeps working with no network connection at all.