How to Convert Units Without Making Mistakes
Last updated 2026-10-20
The factor-label method
Write the quantity as a fraction and multiply by conversion factors written so the unwanted units cancel. To turn 5 miles into kilometres: 5 mi × (1.609344 km / 1 mi) = 8.05 km. The mile units cancel, leaving kilometres. Every converter on this site uses exact international definitions — 1 inch is exactly 2.54 cm, 1 pound is exactly 0.45359237 kg.
Temperature is different
Temperature scales have different zero points, so conversions are formulas, not factors: °F = °C × 9/5 + 32, and kelvin adds 273.15 to Celsius. Forgetting the +32 is the single most common conversion error — use the Temperature Converter for anything that matters.
Fuel economy flips the fraction
Miles per gallon measures distance per fuel; litres per 100 km measures fuel per distance — they are reciprocals, so a bigger mpg is better while a smaller L/100 km is better. The Fuel Consumption Converter handles the inversion and the US/imperial gallon difference (an imperial gallon is about 20% larger).
Sanity-check everything
Ask whether the answer should be bigger or smaller. A kilogram is heavier than a pound, so the number of pounds must be larger than the number of kilograms (×2.2). If your conversion goes the wrong way, you divided where you should have multiplied.