Metric vs imperial units
Which definitions are exact, where US and imperial units quietly disagree, and the conversions that are worth knowing by heart.
Most unit conversions are not approximations. Since the 1959 international yard and pound agreement, the imperial and US customary units have been defined in terms of the metre and the kilogram, which means every length and mass conversion between the two systems is exact by definition. Where conversions go wrong, it is almost never the arithmetic: it is that two units share a name, or that a difference has been treated like an absolute value.
Exact definitions
| Unit | Equals | Note |
|---|---|---|
| 1 inch | 25.4 mm | Exact, by the 1959 international yard and pound agreement |
| 1 yard | 0.9144 m | Exact |
| 1 mile | 1609.344 m | Exact: 1760 yards |
| 1 pound | 0.453 592 37 kg | Exact |
| 1 US gallon | 3.785 411 784 L | Exact: defined as 231 cubic inches |
| 1 imperial gallon | 4.546 09 L | Exact: about 20% larger than the US gallon |
Where the systems diverge
US and imperial units share names but not sizes
This is the single biggest trap in "imperial" conversion, because the two systems use identical words. A US gallon is 3.785 L; an imperial gallon is 4.546 L, 20% larger. A US pint is 16 US fluid ounces (473 mL); an imperial pint is 20 imperial fluid ounces (568 mL). Confusingly, the imperial fluid ounce is the smaller of the two (28.41 mL against 29.57 mL), so you cannot even reason about which pint is bigger from the ounce count alone. If a recipe, fuel figure or tank capacity does not say which gallon it means, the number is ambiguous by a fifth.
There are three different tons
A short ton is 2000 pounds (907.185 kg) and is what "ton" means in the United States. A long ton is 2240 pounds (1016.047 kg) and is the traditional British ton, still used for shipping displacement. A tonne, or metric ton, is exactly 1000 kg. The short and long ton differ by 12%, and both are commonly written "ton" without qualification. Freight, emissions and commodity figures are the usual places this bites.
The US had two different feet until 2022
The international foot (0.3048 m exactly) and the US survey foot (1200/3937 m) differ by about 2 parts per million. That is invisible over a room and about 3 mm over a kilometre, but land surveys and state plane coordinate systems are measured over tens of kilometres, where it becomes metres of real disagreement. NIST retired the US survey foot at the end of 2022; older survey data still carries it.
Fahrenheit and Celsius need two different formulas
To convert a temperature, °F = °C × 9/5 + 32. To convert a temperature *difference*, the offset does not apply: a change of 1 °C is a change of 1.8 °F, not 33.8 °F. Absolute values and deltas are different quantities that happen to share a unit symbol, and mixing them is the most common temperature bug there is. Kelvin sidesteps the problem entirely, which is why scientific work uses it: 0 K is absolute zero, −273.15 °C, −459.67 °F.
Who uses what
| Region | System | In practice |
|---|---|---|
| United States | US customary | Metric is legal and standard in science, medicine and the military; everyday commerce is customary. |
| United Kingdom | Mixed | Officially metric, but road signs use miles, draught beer is sold in imperial pints, and body weight is often in stones. |
| Canada | Mixed | Metric since the 1970s, but construction lumber, ovens and body measurements often stay imperial. |
| Most of the world | Metric (SI) | Adopted as the sole official system, with aviation as a global exception: altitude is in feet almost everywhere. |
Convert between them
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Related: precision and rounding explains why two correct converters can disagree in the fourth decimal place, and conversion mistakes covers the failures these ambiguities have actually caused.