Metric vs Imperial vs US Customary: How the Systems Differ

• Metric / SI — a decimal system: units of the same quantity differ by powers of ten, marked with prefixes (kilo-, centi-, milli-). Its base and derived units are set out in the SI units guide.
• Imperial — the system defined in British law; its units are now defined in terms of metric ones (UK Weights and Measures Act 1985, Schedule 1).
• US customary — related historically to the British units but separately defined (NIST Handbook 44, Appendix C).

The US does not use the Imperial system. For length and mass the two systems agree; for volume they do not.

Since the international yard and pound definitions adopted in 1959, both systems define:

• 1 inch = 25.4 mm exactly, so foot, yard and mile are the same in both.
• 1 pound = 0.45359237 kg exactly, so the ounce (28.349523125 g) is the same in both.

The US "survey foot", a slightly different foot once used for land surveying, was retired by NIST at the end of 2022, leaving one foot in US use.

Volume is the main difference, and tons and stones add two more:

• Length — inch, foot, yard, mile vs millimetre, metre, kilometre. Same in Imperial and US. Use inches to centimetres or miles to kilometres.
• Mass — ounce, pound (and in Imperial, stone) vs gram and kilogram. Use pounds to kilograms or stones to kilograms.
• Volume — differs between Imperial and US; see the table above and gallons to litres.
• Temperature — Fahrenheit is used alongside these systems and Celsius alongside metric; the SI unit is the kelvin. Fahrenheit and Celsius have different zero points, so converting needs an offset as well as a factor: Fahrenheit to Celsius.
• Road speed — mph vs km/h; 1 mph = 1.609344 km/h exactly. mph to km/h.
• Fuel economy — miles per gallon (US or Imperial gallon) vs litres per 100 km, which are reciprocal; see the MPG vs L/100 km guide.

• Treating "Imperial" and "US" as the same for gallons, pints or fluid ounces.
• Mixing up fluid ounces (volume) and ounces (mass).
• Converting a temperature *difference* with the reading formula: a change of 10 °C is a change of 18 °F, not 50 °F.
• Assuming "ton" means tonne.
• Comparing MPG figures from US and UK sources without converting the gallon.

Neither system is more accurate: precision comes from the measurement, not the unit. The metric system's practical advantage is that converting within it only moves the decimal point. A step-by-step method for working conversions by hand is in the unit conversion guide for students.