Two Systems, One World
The metric system (SI – Système International) is used by every country in the world for science and by almost every country for daily life. The imperial system – miles, pounds, Fahrenheit, gallons – persists primarily in the United States, with partial use in the UK and a few other countries.
If you travel, work internationally, or consume media from both sides of the Atlantic, you will constantly need to convert between them. Here are the key conversions worth memorizing – and the formulas for the ones worth calculating.
Length
| Metric | Imperial | Conversion |
|---|---|---|
| 1 kilometre (km) | 0.621 miles | ÷ 1.609 |
| 1 metre (m) | 3.281 feet | × 3.281 |
| 1 centimetre (cm) | 0.394 inches | × 0.394 |
| 1 millimetre (mm) | 0.039 inches | × 0.039 |
- 1 km ≈ 5/8 of a mile
- 1 inch = exactly 2.54 cm
- 6 feet ≈ 1.83 metres (useful for heights)
Weight / Mass
| Metric | Imperial | Conversion |
|---|---|---|
| 1 kilogram (kg) | 2.205 pounds (lb) | × 2.205 |
| 1 gram (g) | 0.035 ounces (oz) | × 0.035 |
| 1 metric tonne | 2,205 pounds | – |
- 1 kg ≈ 2.2 lb
- 100g ≈ 3.5 oz
- A pound ≈ 454g
Temperature
Temperature is the trickiest conversion because it is not just multiplication – there is also an offset.
Celsius to Fahrenheit: F = (C × 9/5) + 32 Fahrenheit to Celsius: C = (F − 32) × 5/9
Useful reference points:
| °C | °F | Context |
|---|---|---|
| −40 | −40 | Same in both scales |
| 0 | 32 | Freezing point of water |
| 20 | 68 | Comfortable room temperature |
| 37 | 98.6 | Normal human body temperature |
| 100 | 212 | Boiling point of water |
Volume
Volume is where the two systems diverge most confusingly, because the US gallon and the UK (imperial) gallon are different sizes.
| Metric | US customary | Imperial (UK) |
|---|---|---|
| 1 litre (L) | 2.113 US pints | 1.760 UK pints |
| 1 litre | 0.264 US gallons | 0.220 UK gallons |
| 1 millilitre (mL) | 0.034 fl oz (US) | 0.035 fl oz (UK) |
- 1 litre ≈ 1 US quart (actually 1.057 qts)
- A US gallon ≈ 3.785 litres; a UK gallon ≈ 4.546 litres
- A standard 500mL water bottle ≈ 1 US pint
Area
| Metric | Imperial |
|---|---|
| 1 m² | 10.764 sq ft |
| 1 hectare (ha) | 2.471 acres |
| 1 km² | 0.386 sq miles |
Speed
- 1 km/h = 0.621 mph
- 1 mph = 1.609 km/h
- 100 km/h ≈ 62 mph (motorway comparison)
- 60 mph ≈ 97 km/h
Converting Without Memorizing
For precise conversions across all unit types – length, weight, temperature, volume, area, speed, pressure, energy, and more – use the Unit Converter on this site. All calculations are done in your browser instantly.
The One You Must Know by Heart
If you only memorize one: 1 inch = 2.54 cm (exact, defined by international treaty). Everything else follows from the right formula.