Metric vs Imperial Units
Most of the world measures in metric; the US mainly uses US customary units, and the UK uses a mix. Understanding how each system is built makes switching between them much easier.
How the metric system is built
The metric system (SI) has one base unit per quantity — meter, kilogram, second, liter for volume in everyday use — and scales it by powers of ten with prefixes: milli (1/1,000), centi (1/100), kilo (1,000). Converting within the system is just moving the decimal point: 1,250 m = 1.25 km. Units also link up: 1 liter of water has a mass of about 1 kg and occupies 1,000 cm³.
How imperial and US units are built
Imperial and US customary units grew from traditional measures, so the ratios vary: 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile; 16 ounces in a pound, 14 pounds in a stone. Since 1959 they have been officially defined in metric terms — the inch is exactly 2.54 cm — so conversions between the systems are exact.
Who uses what
- Almost all countries use metric officially.
- The United States uses US customary units for most everyday purposes, but metric in science, medicine and much of industry.
- The UK uses metric for most trade, but miles and mph on roads, pints for beer and milk, and stones for body weight.
- Canada is officially metric, but feet, inches and pounds are common informally.
Key equivalents
| Imperial / US | Metric | Quick estimate |
|---|---|---|
| 1 inch | 2.54 cm | 2.5 cm |
| 1 foot | 30.48 cm | 30 cm |
| 1 yard | 0.9144 m | a little under 1 m |
| 1 mile | 1.609 km | 1.6 km |
| 1 pound | 453.6 g | a bit under ½ kg |
| 1 ounce | 28.35 g | 30 g |
| 1 US gallon | 3.785 L | 4 L |
| 1 US cup | 236.6 mL | ¼ L |
| 32 °F | 0 °C | freezing point |
Mental conversion tricks
- km → miles: multiply by 6 and divide by 10 (100 km ≈ 60 mi).
- kg → lb: double and add 10% (50 kg ≈ 110 lb).
- °C → °F: double and add 30 (20 °C ≈ 70 °F; exact 68 °F).
- meters → feet: multiply by 3 and add 10% (10 m ≈ 33 ft).