Unit Converters

Unit conversion looks trivial until an ambiguous unit costs real money. Most of the factors here are exact by international definition rather than approximate, and the errors that matter come from picking the wrong unit rather than from arithmetic.

Exact definitions worth knowing

An inch is exactly 25.4 millimetres and a pound is exactly 0.45359237 kilograms — both fixed by the 1959 international agreement, not measured. The metre itself is defined from the speed of light, which is exactly 299,792,458 metres per second.

The units that are genuinely ambiguous

Tons. A US short ton is 2,000 lb, a UK long ton is 2,240 lb and a metric tonne is 1,000 kg. They differ by up to 12 percent, which is a costly ambiguity in shipping and commodities.

Ounces. A troy ounce, used for precious metals, is about 31.1 g; the ordinary avoirdupois ounce is about 28.35 g. Gold quoted per ounce always means troy.

Miles. A nautical mile is 1,852 m, not 1,609.344 m, and it underlies the knot used in aviation and at sea.

Temperature is the one with an offset

Length, weight and speed conversions are simple ratios, so they work identically on a measurement and on a difference. Temperature is not: °F = °C × 9/5 + 32. That offset applies to a reading but not to a difference. A rise of 10 °C is a rise of 18 °F, not 50 °F. Applying the full formula to a temperature difference is a standard engineering error.

Finally, do not let conversion invent precision. Reporting "about 5 feet" as 1.524 metres implies millimetre accuracy you never had.