The metric and imperial systems coexist across the world, and a handful of exact conversion factors cover most everyday needs. This reference lists those factors for length, mass, volume, and temperature, and provides a live two-way converter that applies each one — including the offset that makes temperature special.
How it works
Most unit conversions are a single multiplication because the two scales share a zero point: 0 cm is 0 inches. So inches equal centimetres divided by 2.54, and kilograms equal pounds times 0.45359237. Many of these factors are defined exactly by international agreement rather than measured.
Temperature is the exception. Celsius and Fahrenheit have different zero points, so the conversion needs both a slope and an offset:
F = C × 9/5 + 32
C = (F − 32) × 5/9Key conversion factors at a glance
| From | To | Exact factor |
|---|---|---|
| 1 inch | centimetres | 2.54 (exact) |
| 1 foot | metres | 0.3048 (exact) |
| 1 mile | kilometres | 1.609344 (exact) |
| 1 pound (lb) | kilograms | 0.45359237 (exact) |
| 1 stone | kilograms | 6.35029318 (exact) |
| 1 US fluid oz | millilitres | 29.5735296 |
| 1 US gallon | litres | 3.785411784 (exact) |
| 1 UK gallon | litres | 4.54609 (exact) |
The “exact” label means these are treaty-defined equalities, not measured approximations — so there is no rounding in the factor itself.
Worked examples
Length: 5 feet 11 inches in centimetres. First convert to inches: 5 × 12 + 11 = 71 inches. Multiply by 2.54 to get 180.34 cm.
Mass: A 175 lb person in kilograms: 175 × 0.45359237 = 79.38 kg.
Temperature: 37 °C (normal body temperature) to Fahrenheit: 37 × 9/5 + 32 = 66.6 + 32 = 98.6 °F.
Volume: A 2-litre bottle in US fluid ounces: 2,000 ml ÷ 29.5735 = approximately 67.6 fl oz.
Notes
Watch the gallon: a US liquid gallon (3.785 L) is about 20 percent smaller than a UK imperial gallon (4.546 L), which quietly distorts fuel-economy and recipe figures. Keep full precision through a calculation and round only at the final step.
The pound is also ambiguous — “pound” in everyday usage means the pound-mass (lb), while pound-force (lbf) is a separate unit used in engineering. For cooking and shipping the mass pound is almost always intended. Similarly, a fluid ounce is entirely different from an ounce of mass, so always check which is meant when converting recipe or bottle volumes.
The most common conversion mistakes
Miles per gallon vs litres per 100 km: These measure fuel economy in opposite directions — lower l/100 km is better, higher mpg is better. They cannot be converted by a simple ratio; the correct formula is l/100km = 282.48 / mpg_US. A US car rated 30 mpg uses approximately 7.84 l/100 km.
US oz vs UK oz: These are identical for mass (both 28.3495 g), but differ significantly for volume. A US fluid ounce is 29.574 ml, while a UK fluid ounce is 28.413 ml — a small but real difference that can matter in precise cooking or pharmaceutical contexts.
Celsius vs Kelvin: Kelvin is simply Celsius shifted by 273.15, with no scale change: K = °C + 273.15. For everyday temperature conversion Kelvin rarely appears, but it matters in scientific contexts where absolute temperature is required.
Stone vs pounds: The stone (14 lb) is commonly used for body weight in the UK and Ireland but almost never elsewhere. A person weighing 12 stone is 168 lb or 76.2 kg. When comparing health data or BMI values between UK and US sources, always check which unit the chart assumes.