About Conversion

Every page holds one family of units. Type into any field and the rest fill in as you go — no page reloads, no button hunting. On most pages the fields are added together first, which is what makes "5 ft 9 in" or "2 days 6 hours" a single calculation instead of two.

Two families work differently because the mathematics differs. Temperature scales each have their own zero, so adding two of them is meaningless — that page takes one value and rewrites it on every scale. Fuel economy is worse still: litres per 100 km and miles per gallon are reciprocals, so the page inverts rather than scales.

Conversion factors follow the international definitions rather than rounded textbook values: the inch is exactly 25.4 mm, the pound exactly 0.45359237 kg, the nautical mile exactly 1852 m, standard atmosphere exactly 101 325 Pa. Where a unit is inherently approximate — an averaged calendar month, Mach at sea level, metric versus mechanical horsepower — the page says so on the spot instead of hiding it.

Results are shown to a precision you choose, from 4 to 12 significant figures. Clicking a result copies the plain number with its symbol, ready to paste into a spreadsheet. Your precision choice, language and theme are remembered in your own browser and nowhere else.

Found a factor you think is wrong, or a unit that should be here? That kind of correction is welcome — the unit tables are kept in one file precisely so they can be checked and fixed quickly.