Metric / imperial conversions: pitfalls to know
The coexistence of metric and imperial systems is an inexhaustible source of errors — some anecdotal, others costly. In 1999, NASA's Mars Climate Orbiter probe crashed on Mars because one software had calculated thrust in pound-force where another expected newtons. Cost: 327 million dollars. On a smaller scale, failed recipes, misguided orienteering races, or mis-converted pharmaceutical prescriptions occur daily. This guide lists common conversions and pitfalls to know.
Lengths: inch, foot, yard, mile
The basics: 1 inch = 2.54 cm exactly (modern definition), 1 foot = 12 inches = 30.48 cm, 1 yard = 3 feet = 91.44 cm, 1 mile = 1,609.344 m. The mile is often approximated to 1.6 km, introducing a 0.6% error — acceptable for conversation, not for a flight plan.
Frequent pitfall: the nautical mile (used in aviation and marine) equals 1,852 m, about 15% more than the statute mile. Confusing the two on a crossing can lead far from the intended destination. Similarly, "km/h" and "knot" (one nautical mile per hour) are not interchangeable.
Masses: ounce, pound, stone
1 pound (lb) = 453.592 g. 1 ounce (oz) = 28.3495 g. 1 stone = 14 pounds = 6.35 kg, primarily used in the UK for body weight. An English person saying they weigh 11 stone weighs about 70 kg.
Major pitfall: the "troy" ounce used for precious metals equals 31.1035 g, about 10% more than the ordinary avoirdupois ounce. A conversion error on a gold purchase can cost hundreds of euros.
Volumes: the great pitfall
This is where confusions are most costly. The US gallon and the imperial gallon (British, traditional English-speaking Canadian) don't equal: 1 US gallon = 3.785 litres, 1 imperial gallon = 4.546 litres. The difference is 20%.
Concrete consequence: a vehicle showing 30 mpg (miles per gallon) in the US consumes 7.84 L/100 km, but the same value in "imperial mpg" corresponds to 9.42 L/100 km. Comparing fuel economy across markets requires knowing which gallon is used.
In cooking, the American "cup" equals 236.6 ml, the Canadian "cup" 227.3 ml, the British "cup" 284.1 ml, and the metric cup 250 ml. For baking where proportions matter, these 5-20% gaps change the result.
Temperatures: the tricky conversion
The formula is well known: °C = (°F − 32) × 5/9 and °F = °C × 9/5 + 32. But many forget that scales do not share the same zero, which makes temperature "differences" specific. A 10 °C rise corresponds to an 18 °F rise, not a 10 °F rise.
For Kelvin, the rule is simple: K = °C + 273.15. A difference in K equals a difference in °C. Kelvin has no "degree": say "300 kelvins", not "300 degrees Kelvin".
Areas: acres, hectares, square yards
1 hectare = 10,000 m² = 2.471 acres. 1 acre = 4,046.86 m². 1 square yard = 0.836 m². In Belgium and France, the hectare is the agricultural reference; in Quebec, notarial deeds still frequently use measurements in square feet for buildings and arpents for land inherited from the French regime.
A Quebec arpent (the Paris arpent, historical) equals 0.342 hectare, or 3,418.89 m². Not to be confused with the common French arpent (0.512 ha) used elsewhere.
Best practices
- Always note the unit with the number, never the number alone. "100" means nothing if we don't know whether it's km or miles.
- In programs, work in internal SI units (meters, kilograms, seconds, kelvins) and convert only at display time.
- For any important recipe or prescription, use a verified conversion tool rather than mental approximation.
- Beware of rounded approximations: 1 mile ≈ 1.6 km is accurate to 0.6%; on 1000 km that's a 6 km gap.
To avoid errors, use our dedicated converters: length, weight, temperature and area.