Metric vs Imperial Units Explained
A plain-language overview of the two measurement systems.
Published by QuickUtilaUpdated 2 min read
There are two main families of measurement units in everyday use. The metric system (officially the International System of Units, or SI) is used by almost every country. The imperial system, and its close relative US customary units, are still common in the United States and — for some purposes — the United Kingdom.
The metric system: everything in tens
Metric units scale by powers of ten, and the same prefixes are used everywhere: kilo means 1,000, centi means 1/100 and milli means 1/1,000. So a kilometer is 1,000 meters, a centimeter is 1/100 of a meter and a milligram is 1/1,000 of a gram. Converting within the system is just a matter of moving the decimal point.
Imperial and US customary units
Imperial units grew out of traditional measures, so their relationships are less regular: 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile, 16 ounces in a pound and 14 pounds in a stone.
Since 1959 these units have been defined exactly in metric terms. An inch is exactly 2.54 cm and a pound is exactly 0.45359237 kg, which is why conversions between the systems can be precise.
Key conversion factors
| Imperial / US unit | Metric equivalent |
|---|---|
| 1 inch | 2.54 centimeters (exact) |
| 1 foot | 0.3048 meters (exact) |
| 1 mile | 1.609344 kilometers (exact) |
| 1 ounce | 28.349523125 grams (exact) |
| 1 pound | 0.45359237 kilograms (exact) |
| 1 US gallon | 3.785411784 liters (exact) |
| 1 imperial gallon | 4.54609 liters (exact) |
Watch out: US and imperial volumes differ
Lengths and weights are the same in the imperial and US systems, but volumes are not. A US gallon is about 3.785 liters, while an imperial (UK) gallon is about 4.546 liters — around 20% more. US pints and fluid ounces differ from imperial ones too. Always check which system a recipe or fuel figure uses.
QuickUtila’s Volume Converter uses US customary units and labels each one “US” to avoid this confusion.
Which system should you use?
- Science, medicine and most international trade use metric units.
- US recipes, road signs and product labels commonly use US customary units.
- In the UK, road distances are in miles and pints are used for milk and beer, while most other measures are metric.