Exact factors, not rounded ones
Most conversion errors come from a factor someone half-remembered. These are defined values, not approximations:
- 1 inch = 25.4 mm exactly (so 1 ft = 0.3048 m, 1 mile = 1,609.344 m)
- 1 pound = 0.45359237 kg exactly (so 1 oz = 28.349523125 g)
- 1 US gallon = 3.785411784 litres exactly
- 1 acre = 4,046.8564224 m²; 1 hectare = 10,000 m² exactly
The inch and pound have been defined in metric terms since the international yard and pound agreement of 1959 — the metric value is the definition, not a conversion of it.
The gallon trap
A US gallon is 3.785 litres. An imperial gallon is 4.546 litres — about 20% larger. Both are listed separately here because recipes, fuel economy figures and container sizes routinely mix them without saying which is meant. US and imperial pints, quarts and fluid ounces differ too.
Temperature works differently
Length and weight conversions are a single multiplication. Temperature is not, because the scales have different zero points:
°F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9 · K = °C + 273.15
That offset is why you cannot convert a temperature difference the same way as a temperature. A rise of 10 °C is a rise of 18 °F, not 50 °F.
Quick reference
- 180 cm = 5 ft 10.9 in · 70 kg = 154.32 lb
- 25 °C = 77 °F · 98.6 °F = 37 °C
- 5 km = 3.107 miles · 1 litre = 0.264 US gallons
- 1 acre = 0.405 hectares · 1 m² = 10.764 ft²
If your conversion is really a percentage question rather than a unit one, the percentage calculator handles that side.