Most countries use the metric system. The United States, along with a handful of others, still uses imperial units for everyday life. If you travel between metric and imperial countries, follow US recipes in a metric kitchen, or work with international colleagues, knowing the key conversions saves constant searching.
The metric system is based on powers of 10 — prefixes like kilo (×1000), centi (÷100), and milli (÷1000) apply consistently across all units. Imperial units have no such logic: 12 inches in a foot, 3 feet in a yard, 1760 yards in a mile.
Length
| Metric | Imperial | Approx. conversion |
|---|---|---|
| 1 cm | 0.39 inches | 1 inch ≈ 2.54 cm |
| 1 metre | 3.28 feet | 1 foot ≈ 30.5 cm |
| 1 metre | 1.09 yards | 1 yard ≈ 91.4 cm |
| 1 km | 0.62 miles | 1 mile ≈ 1.61 km |
Quick rule: To convert km to miles, multiply by 0.6 (or divide by 1.6). A 10 km run is about 6.2 miles.
Weight and mass
| Metric | Imperial | Approx. conversion |
|---|---|---|
| 1 gram | 0.035 oz | 1 oz ≈ 28.35 g |
| 100 g | 3.5 oz | — |
| 1 kg | 2.2 lb | 1 lb ≈ 454 g |
| 1 kg | 35.3 oz | — |
Quick rule: To convert kg to pounds, multiply by 2.2. A 70 kg person weighs about 154 lb.
Temperature
Temperature is where metric and imperial diverge most dramatically for everyday use:
- Celsius to Fahrenheit: multiply by 9/5 then add 32. Or: multiply by 2 and add 30 (approximate)
- Fahrenheit to Celsius: subtract 32 then multiply by 5/9
| Celsius (°C) | Fahrenheit (°F) | Context |
|---|---|---|
| 0°C | 32°F | Water freezes |
| 20°C | 68°F | Room temperature |
| 37°C | 98.6°F | Body temperature |
| 100°C | 212°F | Water boils |
| 180°C | 356°F | Common baking temp |
Volume
| Metric | Imperial (US) | Approx. |
|---|---|---|
| 1 ml | 0.034 fl oz | — |
| 250 ml | 1 cup (US) | — |
| 1 litre | 4.23 cups | 1 US pint ≈ 473 ml |
| 1 litre | 2.11 US pints | 1 UK pint ≈ 568 ml |
| 1 litre | 0.26 US gallons | 1 US gallon ≈ 3.79 L |
US vs UK volumes: A US cup is 237 ml; a UK metric cup is 250 ml. A US pint is 473 ml; a UK pint is 568 ml. Recipes from different countries use different cup and pint sizes — always check the origin.
Speed
| km/h | mph | Context |
|---|---|---|
| 50 km/h | 31 mph | Typical urban speed limit |
| 100 km/h | 62 mph | Common motorway/highway limit |
| 120 km/h | 75 mph | High motorway speed |
Quick rule: To convert km/h to mph, multiply by 0.62 (or divide by 1.6). The same factor as km to miles.