Converting units is memorising one equation and applying it two thousand ways. Most everyday conversions - grams to kilograms, inches to centimetres, litres to millilitres - are powers of ten or a single multiplier, and the handful of exceptions (temperature, knots, acres) follow one clear rule each.
The two conversion families
Decimal conversions are just place-shifting
Metric-to-metric conversion moves the decimal point: 1 kg = 1,000 g, 1 m = 100 cm, 1 L = 1,000 mL. No formula needed, just counting zeroes. This is why the metric system is the reference system nearly everywhere: the prefix (kilo-, milli-, centi-) tells you the factor.
Cross-system conversions use fixed multipliers
- Length: 1 inch = 2.54 cm, 1 foot = 30.48 cm, 1 mile = 1.609 km.
- Mass: 1 lb = 453.6 g, 1 kg = 2.2046 lb.
- Area: 1 acre = 4,046.86 m² (about 0.4 hectare).
- Volume: 1 imperial gallon = 4.546 L, 1 US gallon = 3.785 L - note the difference!
Units that are not simple multipliers
Temperature: C = (F - 32) / 1.8 and F = (C × 1.8) + 32 add a constant, so there is no one factor. Datasets on square units must multiply by the factor twice: 1 m² = 10,000 cm², because it counts a grid of 100 × 100. Squares and cubes are where most kitchen conversions go wrong.
Picking the right reference for context
Recipes, carpentry, fuel economy and shipping each carry their own conventions. Converting with the default factor is fine when the audience is local; when documents cross borders, state the unit explicitly - "six feet" and "6 ft" are unambiguous in a way that "6" is not.
Example walk-through
Convert 2.5 kg of flour to pounds: 2.5 × 2.2046 = 5.51 lb. A 10-foot shelf in centimetres: 10 × 30.48 = 304.8 cm. A 72°F summer day in Celsius: (72 - 32) / 1.8 = 22.2°C. The Unit Converter covers length, mass, temperature, area, volume and speed in one screen, and the Train Speed Checker handles km/h vs m/s for travel maths.