Conversions that catch people out
Most unit mistakes come from a handful of traps. The converter handles them, but it helps to know they're there:
- UK and US gallons are different. An imperial gallon is 4.546 L; a US gallon is 3.785 L. Fuel economy in "miles per gallon" from a US source will look about 17% worse than the UK figure for the same car.
- Gauge vs absolute pressure. A tyre gauge reading 2.2 bar means 2.2 bar above atmospheric, so about 3.2 bar absolute. Converters (including this one) convert the number you give; they can't tell which kind it is.
- Temperature differences aren't temperatures. A rise of 10 °C is a rise of 18 °F, not 50 °F. Convert a change by multiplying by 9/5 only; the +32 applies to actual temperatures.
- Calories on food labels are kilocalories. A "200 Calorie" snack is 200 kcal, or about 837 kJ.
- kW vs kWh. Kilowatts are a rate (power); kilowatt-hours are an amount (energy). A 2 kW heater running for 3 hours uses 6 kWh.
There's a longer list with worked examples in unit conversion mistakes engineers still make.
Where the conversion factors come from
Every unit is converted through its SI base unit (metre, kilogram, pascal, joule, watt and so on), so converting psi to mmHg goes psi → Pa → mmHg. Many factors are exact by definition:
| Unit | Exact value | Defined by |
|---|---|---|
| 1 inch | 0.0254 m | International yard and pound agreement, 1959 |
| 1 pound | 0.45359237 kg | Same agreement |
| 1 UK gallon | 4.54609 L | Weights and Measures Act 1985 |
| 1 US gallon | 231 in³ = 3.785411784 L | US customary definition |
| 1 bar | 100,000 Pa | Definition |
| 1 standard atmosphere | 101,325 Pa | Definition |
| 1 thermochemical calorie | 4.184 J | Definition |
| 1 electronvolt | 1.602176634 × 10⁻¹⁹ J | 2019 SI redefinition (fixes the elementary charge) |
Others, like psi (6,894.757… Pa) and mechanical horsepower (745.6999… W), follow from these exact definitions and are calculated to full double precision. Results are shown to 7 significant figures, which is more than any practical measurement justifies. See significant figures for how many to keep.
The 2019 SI redefinition, briefly
Since May 2019 all seven SI base units are defined by fixing the numerical values of physical constants instead of by physical objects. The kilogram used to be the mass of a platinum-iridium cylinder kept near Paris; it's now defined through the Planck constant h = 6.62607015 × 10⁻³⁴ J·s exactly. In practice nothing changed for everyday conversions, but it means the definitions can no longer drift.