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Metric and Imperial Conversions That People Get Wrong
Exact conversion factors for length, mass, area and volume, why area and volume surprise people, and the difference between US and Imperial measures.
Updated: 2026-10-19 · 7 min read
Most unit conversions are a single multiplication. The errors come from somewhere else: using an approximate factor where an exact one exists, converting area as if it were length, and assuming that a gallon is a gallon.
Two systems, one exact definition
Since 1959 the international yard and pound have been defined in terms of the metre and the kilogram, which means the imperial units are not independent — they are exact multiples. An inch is exactly 25.4 millimetres, a pound is exactly 0.45359237 kilograms.
Because the definitions are exact, a conversion between any two supported units can be done without accumulating error, as long as the factors are applied in full and rounding happens only at the end.
The factors worth memorising
A small set of numbers covers almost every everyday conversion. Keeping them exact avoids the slow drift that comes from chaining rounded approximations.
- Length: 1 in = 25.4 mm · 1 ft = 0.3048 m · 1 yd = 0.9144 m · 1 mi = 1609.344 m
- Mass: 1 lb = 0.45359237 kg · 1 oz = 28.349523125 g · 1 st = 14 lb
- Area: 1 acre = 4046.8564224 m² · 1 ha = 10 000 m² · 1 ft² = 0.09290304 m²
- Volume: 1 US gal = 3.785411784 L · 1 Imp gal = 4.54609 L · 1 US cup = 236.5882365 mL
- Speed: 1 m/s = 3.6 km/h · 1 mph = 1.609344 km/h · 1 knot = 1.852 km/h
Why area and volume surprise people
A length conversion uses the factor once. An area conversion uses it twice, and a volume conversion three times. That is easy to state and easy to forget in practice.
One metre is 3.28084 feet, so one square metre is 10.7639 square feet, not 3.28. One cubic metre is 35.3147 cubic feet. A room measured 5% short in both directions is about 10% short in area, which is why flooring and paint quantities are so often wrong.
The same logic applies to metric prefixes: a square kilometre is a million square metres, not a thousand. Kilo means a thousand in one dimension and a million in two.
US and Imperial are not the same
The United States kept the pre-1824 British measures for volume; the United Kingdom adopted the Imperial system. The names stayed the same and the quantities did not.
A US gallon is 3.785 litres; an Imperial gallon is 4.546 litres, about 20% larger. A US cup is 236.6 mL; an Imperial cup is 284.1 mL. A US fluid ounce is 29.57 mL; an Imperial fluid ounce is 28.41 mL — the only case where the US unit is larger.
This is why a British recipe converted with American cups comes out dry, and why fuel economy figures cannot be compared across the two systems without converting the gallon first.
Rounding at the right moment
Round once, at the end. Converting feet to metres, rounding to two decimals, then converting to centimetres and rounding again can shift a result by a centimetre or more on a long measurement.
For construction, work in millimetres or in the smallest unit involved, and only present a rounded figure at the end. For cooking, the opposite applies: precision beyond a gram is meaningless, and rounding early keeps the numbers usable.