There is a single technique that works for converting any unit into any other compatible unit. Once you understand it, you never have to memorise individual formulas again.
The core idea: multiply by 1
A conversion factor is a fraction equal to 1. Because 1 inch equals 2.54 centimetres, the fraction 2.54 cm / 1 in is equal to one. Multiplying by it doesn’t change the quantity — only the units it’s expressed in.
To convert 10 inches to centimetres:
10 in × (2.54 cm / 1 in) = 25.4 cmThe “in” units cancel, leaving centimetres. This is called dimensional analysis, and it is the method scientists and engineers use.
A step-by-step recipe
- Write down what you have, including its unit.
- Find the conversion factor between your unit and the target unit.
- Arrange the factor so the unit you want to cancel is on the bottom.
- Multiply and cancel the units.
- Round the result to a sensible precision.
Worked example: kilometres to miles
You know that 1 mile = 1.609344 km. To convert 5 km to miles, arrange the factor so kilometres cancel:
5 km × (1 mi / 1.609344 km) = 3.1069 miYou can check the answer instantly with the kilometres to miles converter.
Chaining conversions
If you don’t know a direct factor, go through an intermediate unit. To convert feet to centimetres, you might go feet → inches → centimetres:
2 ft × (12 in / 1 ft) × (2.54 cm / 1 in) = 60.96 cmThis is exactly how Units Verse works internally: every unit is defined against a single base unit, so any conversion is just “convert to the base, then convert to the target.” Read more in our conversion methodology.
Watch out for these traps
- Non-linear units. Temperature scales use an offset, not just a multiplier:
°F = °C × 1.8 + 32. See temperature scales explained. - US vs UK measures. A US gallon and an imperial gallon are different sizes — see US vs UK gallon.
- Rounding too early. Keep full precision until the final step to avoid compounding errors.