Two Systems, One World
The metric system uses powers of ten and a small set of base units. The imperial system — and the closely related US customary system — grew from older English units with less consistent relationships: 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile. Most of the world uses metric for everything; the United States uses US customary units for everyday life and metric for science and medicine; the UK uses a mix.
Key Conversion Factors
Because the two systems were defined independently, converting between them always uses a fixed, memorizable factor. These factors are exact by definition — for example, one inch is defined as exactly 2.54 centimeters, and one pound as exactly 0.45359237 kilograms.
| Quantity | Imperial | Metric | Factor |
|---|---|---|---|
| Length | 1 inch | = 2.54 cm | ×2.54 |
| Length | 1 foot | = 0.3048 m | ×0.3048 |
| Length | 1 mile | ≈ 1.609 km | ×1.60934 |
| Weight | 1 pound | ≈ 0.4536 kg | ×0.453592 |
| Volume | 1 gallon (US) | ≈ 3.785 L | ×3.78541 |
Watch for 'Same Name, Different Size'
Some units share a name but differ between systems. A US gallon (3.785 L) is smaller than an imperial gallon (4.546 L). A US fluid ounce (29.57 mL) differs from an imperial fluid ounce (28.41 mL). A US (short) ton is 2,000 lb while a UK (long) ton is 2,240 lb. Always confirm which version a source means before converting.