Converting units is just multiplying by the right factor — except temperature, which is the odd one out. Knowing a handful of factors covers almost every everyday conversion; the rest is arithmetic. Here are the ones worth memorising, with worked examples, plus a free unit converter for the rest.
How unit conversion works
To convert, multiply by a factor that turns the old unit into the new one. If 1 inch = 2.54 cm, then to go from inches to cm you multiply by 2.54, and to go back you divide by 2.54. The only trick is knowing which way to multiply — set it up so the units you don't want cancel out.
Length
- 1 inch = 2.54 cm — so 10 in × 2.54 = 25.4 cm.
- 1 foot = 30.48 cm (12 inches).
- 1 mile = 1.609 km — a 5 km run is about 3.1 miles.
- 1 metre = 3.281 feet.
Weight
- 1 kg = 2.205 lb — so 70 kg × 2.205 ≈ 154 lb.
- 1 pound = 0.454 kg.
- 1 stone = 6.35 kg (14 lb), common for body weight in the UK.
Temperature (the odd one out)
Temperature scales don't share a zero, so you can't just multiply — you scale and shift:
- °C → °F: (°C × 9/5) + 32. So 20°C = (36) + 32 = 68°F.
- °F → °C: (°F − 32) × 5/9. So 98.6°F = (66.6) × 5/9 = 37°C.
Handy anchors: 0°C = 32°F (freezing), 37°C ≈ 98.6°F (body), 100°C = 212°F (boiling).
FAQ
How many cm in an inch? Exactly 2.54 cm.
How do I convert kg to pounds? Multiply kilograms by 2.205.
Why is temperature different? The scales have different zero points, so you scale and then add or subtract an offset, not just multiply.
Is my data private? Yes — the converter runs in your browser and stores nothing.