Direct comparison
Metric vs imperial
The metric system is a decimal system of measurement built on the SI units; the imperial system is an older set of British units such as feet, pounds and pints with irregular conversion factors.
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Side-by-side comparison
| Dimension | Metric (SI) | Imperial |
|---|---|---|
| What it is | A decimal system of measurement based on the International System of Units (SI). | A traditional system of British weights and measures standardised in 1824. |
| Conversion basis | Powers of ten — units scale by 10, 100, 1,000 and so on. | Irregular factors — 12, 3, 14, 16, 8 depending on the unit. |
| Length | Millimetre, centimetre, metre, kilometre. | Inch, foot, yard, mile. |
| Mass or weight | Gram, kilogram, tonne. | Ounce, pound, stone, ton. |
| Volume | Millilitre, litre. | Fluid ounce, pint, quart, gallon. |
| Temperature | Degrees Celsius (kelvin in science). | Degrees Fahrenheit. |
| Where used | The official system in almost every country and in all science. | Mainly the United States; partial everyday use in the UK. |
| Ease of calculation | Simple — shift the decimal point to convert between units. | Harder — each conversion uses a different, memorised factor. |
| Status in science | The required system for all scientific and technical work. | Not used in science; converted to metric for analysis. |
Why science uses metric everywhere
Science adopted the metric system, and now the SI, because base-ten scaling makes calculation and communication reliable across borders. Converting kilometres to metres means moving a decimal point, not recalling that a mile is 5,280 feet. The SI also defines every unit against a fixed, universal standard rather than a physical object, so a kilogram measured in one laboratory means exactly the same as a kilogram measured in another. The famous loss of NASA’s Mars Climate Orbiter in 1999 — caused by one team using imperial units and another metric — is the textbook reminder of why a single, coherent system matters.
Common questions
FAQ
Is the metric system the same as SI?+
They are closely related but not identical. The metric system is the broad family of decimal measurement systems that began in France in the 1790s. The International System of Units (SI), agreed in 1960 and redefined in 2019, is the modern, formally standardised version of it, with seven base units. In everyday speech "metric" and "SI" are often used interchangeably.
Why does the United States still use imperial?+
For historical and practical reasons. The US inherited British units and never fully switched, despite the metric system being legal there since 1866. Industry, infrastructure and everyday habits are built around imperial units, so the cost and disruption of changing have repeatedly outweighed the push to convert. US science, medicine and the military nonetheless use metric.
Does the UK use metric or imperial?+
The UK uses both. It is officially metric for trade, science and most regulation, but imperial units survive in daily life: road signs in miles, beer and milk in pints, and body weight in stones and pounds. This mixed usage is a legacy of a gradual, incomplete switch from imperial to metric over the past century.
Going deeper
