Two unit mix-ups have become engineering folklore. In 1999 NASA’s Mars Climate Orbiter was lost when thruster data was produced in pound-force seconds by one team while another team’s software expected newton-seconds. In 1983 an Air Canada Boeing 767 ran out of fuel in flight because the fuel load had been calculated in pounds instead of kilograms. The crew glided it to a safe landing at Gimli, Manitoba. Both failures came from a number that travelled without its unit.
The exact definitions
Imperial and US customary units are defined from metric ones. These four are exact, not rounded:
- 1 inch = 25.4 millimetres
- 1 yard = 0.9144 metres
- 1 mile = 1,609.344 metres
- 1 pound = 0.45359237 kilograms
Every other conversion in the Toolhouse unit converter is built from these, which is why the results can be trusted to the digits shown.
Mental shortcuts
- Miles to kilometres: divide by 5, then multiply by 8. A marathon of 26.2 miles gives 41.9 km. The exact figure is 42.165 km, and the official marathon distance is 42.195 km.
- Kilometres to miles: multiply by 0.62.
- Kilograms to pounds: multiply by 2.2.
- Litres to US gallons: divide by 3.8.
- Celsius to Fahrenheit: double it and add 30. For 20°C that gives 70°F, close to the exact 68°F. The exact rule is multiply by 1.8 and add 32.
- Fahrenheit to Celsius: subtract 32, then divide by 1.8.
Same name, different size
| Unit | US | UK or metric |
|---|---|---|
| Gallon | 3.785 litres | 4.546 litres |
| Pint | 473 ml | 568 ml |
| Fluid ounce | 29.57 ml | 28.41 ml |
| Cup | 236.6 ml | 250 ml (metric cup) |
| Ton | 907 kg (short ton) | 1,016 kg (long ton), 1,000 kg (tonne) |
A UK gallon is about 20% larger than a US gallon, so a fuel economy figure in “miles per gallon” means different things on each side of the Atlantic. A recipe that says “1 pint” can be off by a fifth if it comes from the other country.
Habits that prevent mistakes
- Write the unit next to every number, in notes, spreadsheets and messages.
- Convert once, at the start or the end, not repeatedly in the middle.
- Round only the final answer. Rounding at each step compounds the error.
- Sanity-check the size of the result. A bag of flour is not 45 kilograms.
Try it yourself: Open the unit converter