Unit converter

Pick a category, type a number, and every unit in that category is converted at once. That is deliberately more than you asked for — the second question is usually another unit in the same family, and the arithmetic is already done.

The factors here are the exact legal definitions rather than rounded approximations, which matters more than it sounds for anything that compounds. The inch has been exactly 25.4 millimetres since the international agreement of 1959, and the pound exactly 0.45359237 kilograms — so a conversion chain through several units does not accumulate error. Temperature is handled separately from everything else because it is the only category where the scales do not share a zero point: converting Celsius to Fahrenheit needs an offset as well as a factor, and a converter built only on factors gets it silently and badly wrong.