Metric vs. Imperial: How the Two Measurement Systems Compare
Where each system is used, the conversion factors worth knowing, how US and British units differ, and the pitfalls that catch people out.
Last reviewed on October 2, 2026
Metric (SI) units scale by powers of ten (1 km = 1,000 m); imperial and US customary units use irregular ratios (1 mile = 1,760 yards = 5,280 feet).
Key exact factors: 1 inch = 2.54 cm, 1 foot = 0.3048 m, 1 mile = 1.609344 km, 1 pound = 0.45359237 kg. US and UK gallons, pints and tons are different sizes.
Most of the world measures distance in kilometres, weight in kilograms and volume in litres. A handful of places — most prominently the United States — keep the imperial-derived “US customary” units. Anyone working across borders, recipes or technical specifications eventually has to switch between the two, and the place it most often goes wrong is not the arithmetic but the assumptions.
What “Metric” Actually Means
The modern metric system is the International System of Units (SI). It is built on seven base units — metre, kilogram, second, ampere, kelvin, mole and candela — and a fixed set of decimal prefixes. Every other metric unit is a base unit, a combination of base units, or one of those scaled by a power of ten. That is why going from millimetres to metres to kilometres only means moving the decimal point. See the SI base units and prefixes table for the full list.
The SI is maintained by the International Bureau of Weights and Measures (BIPM). Scientific publishing, engineering tolerances and trade documentation are almost all in SI, even in countries that use other units day to day.
What “Imperial” Actually Means
Strictly, “imperial” refers to the British Imperial system set out in the Weights and Measures Act of 1824. The system used in the United States — feet, pounds, US gallons, US cups — descends from the same English roots but diverged before 1824, so the two are similar but not identical. Since 1959, both have been defined in terms of metric units: the inch is exactly 2.54 cm and the pound exactly 0.45359237 kg.
You will also meet older units: the chain and furlong in horse racing, the bushel in US agriculture, and the troy ounce (31.1035 g) for precious metals.
US Customary vs British Imperial
Length and weight units match between the two systems. Volume units and the ton do not:
| Unit | United States | United Kingdom (imperial) | Difference |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | UK is 20% larger |
| Pint | 473 mL (16 US fl oz) | 568 mL (20 imp fl oz) | UK is 20% larger |
| Fluid ounce | 29.57 mL | 28.41 mL | US is 4% larger |
| Ton | Short ton: 2,000 lb (907 kg) | Long ton: 2,240 lb (1,016 kg) | Metric tonne is 1,000 kg |
| Hundredweight | 100 lb | 112 lb | — |
| Body weight | Pounds | Stones and pounds | 1 st = 14 lb |
Where Each System Is Used Today
| Where | System | Notes |
|---|---|---|
| Most of the world | Metric (SI) | Europe, Latin America, Africa, Asia, Australia, New Zealand use metric for daily life. |
| United States | US customary | Roads, cooking, body weight, construction. Science, medicine and the military use SI. |
| United Kingdom | Mixed | Miles and pints on roads and in pubs; metric in shops, schools and engineering; body weight in stones. |
| Canada | Mostly metric | Imperial lingers for height, body weight, cooking and building materials. |
| Aviation | Mixed by convention | Altitude in feet, distance in nautical miles, speed in knots almost everywhere. |
| Shipping | Nautical units | Nautical miles and knots; cargo in metric tonnes. |
Conversion Factors Worth Knowing
Most everyday conversions need only a small set of factors. Many are exact by international agreement, which is why a careful converter keeps full precision internally and rounds only for display.
| Imperial / US unit | Metric equivalent | Exact? | Converter |
|---|---|---|---|
| 1 inch (in) | 2.54 cm | Exact | in → cm |
| 1 foot (ft) | 0.3048 m | Exact | ft → m |
| 1 yard (yd) | 0.9144 m | Exact | — |
| 1 mile (mi) | 1.609344 km | Exact | mi → km |
| 1 nautical mile | 1.852 km | Exact | — |
| 1 ounce (oz) | 28.349523125 g | Exact | oz → g |
| 1 pound (lb) | 0.45359237 kg | Exact | lb → kg |
| 1 stone (st) | 6.35029318 kg | Exact | st → kg |
| 1 US fluid ounce | 29.5735 mL | ≈ | mL → fl oz |
| 1 US cup | 236.588 mL | ≈ | cups → mL |
| 1 US gallon | 3.785411784 L | Exact | gal → L |
| 1 imperial gallon | 4.54609 L | Exact | — |
| 1 acre | 4,046.856 m² (0.4047 ha) | ≈ | — |
| 1 mph | 1.609344 km/h | Exact | mph → km/h |
| 1 psi | 6.894757 kPa (0.0689 bar) | ≈ | psi → bar |
| 1 horsepower (mechanical) | 745.7 W | ≈ | hp → kW |
The Mistakes People Actually Make
- US vs UK gallon. If a British recipe calls for a gallon of stock, do not multiply by 3.785. The two gallons differ by almost a litre.
- Short ton vs long ton vs metric tonne. Each is a different mass. In freight or commodity work, “ton” is ambiguous unless qualified.
- Fluid ounces are not ounces. A fluid ounce is a volume; an avoirdupois ounce is a weight. They share a name and little else.
- Square and cubic units do not scale linearly. A square mile is not 1.609 km² but about 2.59 km². A cubic foot is about 0.0283 m³. For area, square the linear factor; for volume, cube it.
- Temperature has an offset. Fahrenheit is not a multiple of Celsius — use the formula. See the temperature scales guide.
- Fractions vs decimals. US construction uses fractional inches (3⁄16, 7⁄8). Mixing those with decimal millimetres in one drawing is a classic source of error.
A Worked Example
A US recipe calls for 2½ pounds of chuck roast at 350 °F for 3 hours. Your kitchen has a metric scale and a Celsius oven.
- 2.5 lb × 0.45359 kg/lb ≈ 1.134 kg. Round to 1.1 kg.
- 350 °F = (350 − 32) × 5⁄9 ≈ 176.7 °C. Set 175–180 °C, or about 160 °C with the fan on.
- 3 hours stays 3 hours — time is the same in both systems.
Each step uses a different rule: a factor for mass, a formula with an offset for temperature, and no change for time. Treating “convert” as one operation is what causes mix-ups.
A Decision Checklist
- Is the source US or UK? Same word, possibly different unit.
- Is the unit linear, square or cubic? Square or cube the factor accordingly.
- Does the unit have an offset (temperature)? Use the formula.
- Does the answer need to be exact (legal, customs, engineering) or approximate (cooking, travel)?
- What precision does the job actually need? Building plans need millimetres; trip planning rarely needs better than the nearest kilometre.
Popular Metric ↔ Imperial Converters
- km to miles1 km = 0.621371 mi
- kg to lbs1 kg = 2.20462 lb
- cm to inches1 in = 2.54 cm exactly
- metres to feet1 m = 3.28084 ft
- litres to gallons1 US gal = 3.785 L
- km/h to mph100 km/h ≈ 62 mph
Frequently Asked Questions
Is imperial the standard for international aviation?
Not officially, but in practice largely yes. ICAO rules are based on SI yet allow non-SI units for flying: almost every country uses feet for altitude, knots for airspeed and nautical miles for distance. A few countries, notably China, use metric flight levels. Fuel and cargo are increasingly measured in kilograms and litres.
What imperial units are still commonly used in metric countries?
Inches for screen sizes, wheel rims and pipe threads; feet for aircraft altitude; knots and nautical miles at sea and in the air; troy ounces for gold; and, in the UK, miles on road signs and pints of beer and milk.
Is US customary the same as imperial?
No. Length and weight units are identical (both defined from the 1959 international inch and pound), but volumes differ: a US gallon is 3.785 L and an imperial gallon is 4.546 L, and a US pint is 473 mL against 568 mL in the UK. The US ton is 2,000 lb; the UK long ton is 2,240 lb.
Which countries do not use the metric system?
The United States is the main country that uses non-metric units in daily life. Liberia and Myanmar are often listed alongside it, though both use metric units in many contexts. The UK and Canada are officially metric but keep some imperial units by custom.
What is the difference between metric and imperial weight?
Metric weight uses the gram and kilogram (1 kg = 1,000 g). Imperial and US customary weight uses the ounce, pound and stone (16 oz = 1 lb, 14 lb = 1 st). 1 kg ≈ 2.2046 lb and 1 lb = 0.45359237 kg exactly. A metric tonne (1,000 kg) is close to, but not the same as, a UK long ton or a US short ton.
How can I remember which is metric and which is imperial?
Metric units step by powers of ten and share prefixes — milli, centi, kilo — so a kilometre, kilogram and kilolitre are all 1,000 of something. Imperial units have their own names and irregular ratios: 12 inches in a foot, 3 feet in a yard, 16 ounces in a pound.
Does imperial make more sense than metric?
Imperial units are often sized to the human body and divide neatly by 2, 3 and 4, which suits carpentry and cooking. Metric wins for calculation: every conversion is a shift of the decimal point, and units for length, volume and mass link together (1 litre of water weighs 1 kilogram). That is why science and international trade standardised on metric.