Metric vs Imperial: Your Complete Conversion Guide
You're reading a recipe that uses cups. Your scale shows kilograms. Your sat-nav speaks in miles, but road signs abroad use kilometres.
The world uses two main measurement systems — metric and imperial — and switching between them is something most people do more often than they realise. This guide explains both systems clearly, covers the most common conversions, and links you to free tools for each measurement type.
What Is the Metric System?
The metric system — officially the International System of Units (SI) — is the global standard. Almost every country uses it for science, medicine, and everyday life. Its main advantage: it is built on powers of 10. Moving between units means multiplying or dividing by 10, 100, or 1,000. No irregular conversion factors.
| Measurement | Common Metric Units | Key Relationship |
|---|---|---|
| Length | Millimetre (mm), Centimetre (cm), Metre (m), Kilometre (km) | 1 km = 1,000 m = 100,000 cm |
| Weight / Mass | Milligram (mg), Gram (g), Kilogram (kg), Tonne | 1 kg = 1,000 g |
| Volume | Millilitre (ml), Litre (L) | 1 L = 1,000 ml |
| Temperature | Celsius (°C), Kelvin (K) | 0°C = 273.15 K |
What Is the Imperial System?
The imperial system grew from historical British units and is still used daily in the United States, and partially in the UK. Imperial units have irregular conversion factors — 12 inches to a foot, 3 feet to a yard, 5,280 feet to a mile — which makes them harder to work with mathematically.
| Measurement | Common Imperial Units | Key Relationship |
|---|---|---|
| Length | Inch (in), Foot (ft), Yard (yd), Mile (mi) | 1 ft = 12 in | 1 mile = 5,280 ft |
| Weight | Ounce (oz), Pound (lb), Stone, Ton | 1 lb = 16 oz | 1 stone = 14 lb |
| Volume | Fluid ounce, Cup, Pint, Quart, Gallon | 1 US gallon = 4 quarts = 128 fl oz |
| Temperature | Fahrenheit (°F) | 32°F = 0°C | 212°F = 100°C |
The Most Common Conversions — With Quick Formulas
Length Conversions
Use the Length Converter for any distance or size conversion.
| 1 metre | = 3.281 feet | = 39.37 inches |
| 1 kilometre | = 0.621 miles |
| 1 mile | = 1.609 kilometres |
| 1 foot | = 0.305 metres | = 30.48 cm |
| 1 inch | = 2.54 centimetres |
| 1 centimetre | = 0.394 inches |
Weight & Mass Conversions
Use the Weight and Mass Converter for body weight, food, and shipping.
| 1 kilogram (kg) | = 2.205 pounds (lbs) |
| 1 pound (lb) | = 0.454 kg | = 16 ounces |
| 1 stone | = 6.35 kg | = 14 lbs |
| 1 ounce (oz) | = 28.35 grams |
| 1 gram (g) | = 0.035 ounces |
Temperature Conversions
Temperature is the trickiest conversion because it uses addition and subtraction, not just multiplication. Use the Temperature Converter for accurate results.
| Celsius → Fahrenheit | Multiply by 9/5, then add 32. Example: 25°C × 1.8 + 32 = 77°F |
| Fahrenheit → Celsius | Subtract 32, then multiply by 5/9. Example: (77°F − 32) × 0.556 = 25°C |
| Celsius → Kelvin | Add 273.15. Example: 100°C + 273.15 = 373.15 K |
| 0°C | = 32°F (water freezing point) |
| 100°C | = 212°F (water boiling point) |
| 37°C | = 98.6°F (normal human body temperature) |
Volume Conversions
Note: the US gallon and UK gallon are different sizes — a common source of confusion. Use the Volume Converter to avoid errors.
| 1 litre (L) | = 0.264 US gallons | = 0.220 UK gallons |
| 1 US gallon | = 3.785 litres |
| 1 UK gallon | = 4.546 litres |
| 1 cup (US) | = 236.6 ml |
| 1 fluid ounce (US) | = 29.57 ml |
Area Conversions
For property, flooring, and land measurements, use the Area Converter.
| 1 square metre (m²) | = 10.764 square feet |
| 1 square foot (ft²) | = 0.093 m² |
| 1 acre | = 4,047 m² | = 0.405 hectares |
| 1 hectare (ha) | = 2.471 acres | = 10,000 m² |
Speed Conversions
For travel, running, and engineering, use the Speed Converter.
| 1 km/h | = 0.621 mph |
| 1 mph | = 1.609 km/h |
| 1 m/s | = 3.6 km/h | = 2.237 mph |
| 1 knot | = 1.852 km/h | = 1.151 mph |
Which Countries Use Which System?
| Metric only | Most of Europe, Asia, Africa, South America, Australia |
| Primarily imperial (US customary) | United States |
| Mixed (metric official, imperial common) | United Kingdom, Canada |
| Science and medicine worldwide | Always metric (SI units) |
Quick Mental Maths Tricks
- °C to °F roughly: double the Celsius value, then add 30. (Close enough for everyday use.)
- km to miles roughly: multiply by 0.6, or divide by 1.6.
- kg to lbs quickly: multiply by 2.2.
- Litres to pints (UK): multiply by 1.76.
- For anything precise — shopping, cooking, science — use the free converters.
Common Mistakes When Converting Units
- Using US gallons when you need UK gallons — they differ by about 20%
- Forgetting that temperature uses addition and subtraction, not just multiplication
- Confusing fluid ounces (volume) with weight ounces — they are completely different units
- Mixing metric and imperial values in the same calculation
- Rounding too early and losing precision across multiple conversion steps
Frequently Asked Questions
Why does the US still use imperial units?
The US officially adopted the metric system in 1975 but never enforced it at a public level. Imperial units are embedded in everyday American life — roads, construction, body measurements, and retail all continue to use inches, miles, and pounds.
Is the UK metric or imperial?
Both. Road distances use miles. Body weight is often given in stone and pounds. But officially, the UK uses metric for trade, science, and most commercial labelling.