Metric and Imperial Conversions That Trip People Up

Imperial units have been defined in terms of metric ones since 1959, so almost every conversion factor is exact. The errors come from the handful of places where the names lie.

Almost every conversion is exact

People treat imperial-to-metric conversion as approximate. It is not. Since the 1959 international yard and pound agreement, the customary units have been defined in terms of metric ones, so the factors are exact by definition and any rounding is a choice you made, not a limitation of the units.

Everything else in the customary system is derived from those two definitions. A foot is 12 inches, so it is exactly 0.3048 m. A mile is 5,280 feet, so it is exactly 1.609344 km. No measurement was involved.

Length

The mental shortcut for kilometres to miles is to multiply by 0.6 and add a bit — 100 km is 62.1 miles. The length converter carries the full precision.

Weight and mass

A UK stone is 14 pounds. A US short ton is 2,000 lb, a UK long ton is 2,240 lb, and a metric tonne is 1,000 kg — three different things sharing one word, which is a recurring source of shipping errors.

Volume: the gallon problem

This is the conversion that catches people out, because the US and imperial systems use identical names for different quantities.

An imperial gallon is about 20% larger than a US gallon, which quietly ruins fuel-economy comparisons: 40 miles per imperial gallon is only about 33 miles per US gallon. Note also that the fluid ounce runs the other way — the imperial one is smaller. Use the volume converter and pick the system explicitly.

Temperature: the one with an offset

Every other conversion here is a single multiplication. Temperature is not, because the scales do not share a zero.

Useful anchors: −40 is the same number on both scales, 0°C is 32°F, 20°C is 68°F, body temperature 37°C is 98.6°F, and 100°C is 212°F. A temperature difference converts differently from a temperature reading — a rise of 10°C is a rise of 18°F, with no offset applied. The temperature converter handles readings.

Area and speed

An acre is 43,560 square feet, a number that only makes sense historically — it was a furlong by a chain. A hectare is a clean 10,000 square metres. For speed, 60 mph is 96.56 km/h and 100 km/h is 62.14 mph; see the area converter and the speed converter.

Four ways conversions go wrong

• Squaring the wrong factor. Area factors are the length factor squared and volume factors are it cubed. One foot is 0.3048 m, but one cubic foot is 0.3048³ = 0.0283 m³.
• Mixing US and imperial. Gallons, quarts, pints and fluid ounces all differ between the two. Recipes and fuel figures are the usual casualties.
• Rounding too early. Convert with full precision, round once at the end. Rounding 2.54 to 2.5 introduces a 1.6% error that then compounds.
• The US survey foot. Land surveys historically used a foot fractionally longer than the international one. It was formally retired at the end of 2022, but older survey data still carries it, and over long distances the difference is real.

Kitchen conversions are their own problem, because flour and sugar are sold by volume in one country and weight in another — that is covered separately in cups to grams for baking.