Unit Conversion Basics
Unit conversion trips up students, travellers, and engineers alike. The good news: every conversion is just multiplication or a short formula. Here are the essentials worth memorising.
Last updated July 23, 20265 min read
Length
- 1 inch = 2.54 cm exactly
- 1 foot = 0.3048 m
- 1 mile = 1.60934 km
- 1 metre ≈ 3.281 feet
Weight and mass
- 1 pound = 0.45359 kg
- 1 kilogram ≈ 2.2046 lb
- 1 ounce = 28.3495 g
- 1 stone = 6.35029 kg
Temperature
Temperature is the odd one — it uses an offset, not just a ratio. That is why you cannot simply multiply.
Example: °F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9 · K = °C + 273.15
Volume
- 1 US gallon = 3.78541 L
- 1 UK gallon = 4.54609 L
- 1 US cup = 236.588 mL
- 1 fluid ounce (US) = 29.5735 mL
Tip: US and UK gallons are different — always check which system a recipe or spec sheet uses.
Precision and rounding
Conversion factors above are precise enough for daily life. For engineering, hold at least six significant figures and round only at the final step.