The metric system is a decimal system: units scale by powers of ten, so 1 kilometer is 1,000 meters. Imperial and US customary units scale by irregular factors, such as 12 inches per foot, 16 ounces per pound and 5,280 feet per mile. Both measure the same things; metric is simpler to calculate with, and the two are linked by exact conversion factors.
What is the difference between metric and imperial?
| Metric (SI) | Imperial / US customary | |
|---|---|---|
| Structure | Decimal; prefixes (milli-, centi-, kilo-) scale by 10, 100, 1,000 | Mixed ratios: 12, 3, 5,280, 16, 2, 4 |
| Length | millimeter, centimeter, meter, kilometer | inch, foot, yard, mile |
| Mass | gram, kilogram | ounce, pound |
| Volume | milliliter, liter | fluid ounce, cup, pint, quart, gallon |
| Temperature | degree Celsius, kelvin | degree Fahrenheit |
| Governing body | International: the 1875 Treaty of the Meter | National definitions |
NIST describes the SI as the international standard for measurement, with seven base units: meter, kilogram, second, ampere, kelvin, mole and candela. Everything else in the system is derived from them. The International System of Units entry has the short definition.
How do the units line up?
| Quantity | Imperial / US | Metric | Exact? |
|---|---|---|---|
| Length | 1 inch | 25.4 mm (2.54 cm) | Exact |
| Length | 1 foot | 0.3048 m | Exact |
| Length | 1 mile | 1.609344 km | Exact |
| Mass | 1 pound | about 0.4536 kg | Rounded |
| Mass | 1 ounce | about 28.35 g | Rounded |
| Volume | 1 US gallon | about 3.785 L | Rounded |
| Volume | 1 US fluid ounce | about 29.57 mL | Rounded |
NIST marks the length factors as exact: the inch, foot, yard and mile are defined through the meter. The mass and volume factors in its table are given to six or seven digits. In practice that means US customary units are not independent of metric: a US inch is 2.54 cm by definition. See also the US customary units entry.
Converters for each pair are available at mm to inches, feet to meters, pounds to grams and gallons to liters; the full list is under unit converters.
Worked examples
Example 1: length
A 5 mile run: 5 × 1.609344 = 8.05 km. A 10 km race: 10 ÷ 1.609344 = 6.21 miles.
Example 2: mass
A 70 kg person: 70 ÷ 0.4535924 = 154.3 lb. A 5 lb bag: 5 × 0.4535924 = 2.27 kg. The kg to lbs tool handles other values.
Example 3: temperature
Temperature is the one conversion that is not a pure factor, because the scales start at different points: °F = °C × 1.8 + 32. So 20 °C is 20 × 1.8 + 32 = 68 °F. The full method is in the Celsius to Fahrenheit guide.
Example 4: moving within one system
In metric, 2.5 km = 2,500 m = 250,000 cm: only the decimal point moves. In customary units, 2.5 miles = 2.5 × 5,280 = 13,200 feet = 158,400 inches (× 12), which takes two different multipliers.
Why do the systems cause mistakes?
Two units can share a name and still differ. The US gallon (about 3.785 L) is not the British imperial gallon, which UK legislation fixes at exactly 4.54609 L, about 20% larger. An imperial fluid ounce is 1/160 of that gallon (28.41 mL), while a US fluid ounce is 1/128 of a US gallon (29.57 mL), so a recipe or label from another country can be off by several percent. Weight ounces and fluid ounces are also different quantities that share a word; see how many ounces in a cup.
Mixing systems inside one calculation is the larger risk. NASA reports that Mars Climate Orbiter was lost on 23 September 1999 because of a navigation error caused by a failure to translate English units to metric: ground software used English units while onboard software used metric. The numbers were each correct in their own unit; the mistake was the missing conversion.
When is each system used?
Metric is the everyday system in most of the world and the standard in science. The United States, one of the original signatories of the 1875 Treaty of the Meter, continues to use US customary units for everyday measures such as road distance, cooking and body height, while using SI in science and many industries. For work that crosses the boundary, convert once, at the edge of the calculation, and keep the units written next to every number.
Quick tips
- Memorize the exact length factors (25.4 mm per inch, 1.609344 km per mile); they never need rounding.
- Treat temperature separately from other conversions.
- Check which gallon or ounce a source means before converting volumes.
- Write units with every number in calculations; unit mismatches show up when they do not cancel.