Two Systems, One World
Almost every country in the world uses the metric system (officially called the International System of Units, or SI) as its primary measurement system. The notable exceptions are the United States, which uses the US customary system, and Myanmar and Liberia, which have not fully adopted metric either. The United Kingdom uses a hybrid: metric for most official and scientific purposes, but imperial units persist in everyday conversation (people give their height in feet and inches, their weight in stones and pounds, and road distances in miles).
The result is a world where switching between measurement systems is a constant, practical necessity — in cooking, travel, medicine, engineering, and sport. Here are the conversions that come up most often.
Length and Distance
The most common confusion arises with miles versus kilometres for road distances and speeds.
- 1 mile = 1.609 kilometres — A quick approximation: multiply miles by 1.6 to get km. So 60 mph ≈ 96 km/h.
- 1 kilometre = 0.621 miles — Multiply km by 0.6 for a rough mile equivalent. A 5km run is about 3.1 miles.
- 1 inch = 2.54 centimetres — Exact by definition. Used constantly for clothing sizes, screen sizes, and engineering.
- 1 foot = 30.48 centimetres — Height in feet and inches is still the norm in the US and UK. 6 feet = 182.9 cm.
- 1 yard = 0.914 metres — Common in American football and some gardening contexts.
Practical tip for height: To convert feet/inches to centimetres, convert feet to inches first (multiply by 12), add the remaining inches, then multiply by 2.54. So 5'11" = (5 × 12) + 11 = 71 inches × 2.54 = 180.3 cm.
Weight and Mass
Three different systems for everyday weight — grams/kilograms, pounds/ounces, and stones/pounds — creates plenty of confusion.
- 1 kilogram = 2.205 pounds — A rough approximation: 1 kg ≈ 2.2 lbs. So 70 kg ≈ 154 lbs.
- 1 pound = 0.454 kilograms — Divide pounds by 2.2 for a quick kg estimate.
- 1 stone = 14 pounds = 6.35 kilograms — Used for body weight in the UK. 11 stone = 154 lbs = 70 kg.
- 1 ounce = 28.35 grams — Common in US recipes and for small weights.
Practical tip for cooking: US recipes often use cups, tablespoons, and ounces. 1 cup of water = 240 ml = 240 g (water only — density varies for other ingredients). 1 tablespoon = 15 ml. 1 teaspoon = 5 ml.
Temperature
Converting between Celsius and Fahrenheit is the conversion that trips people up most. The relationship is not a simple ratio — it requires both multiplication and addition.
- °C to °F: multiply by 9/5, then add 32. Or: multiply by 1.8, then add 32.
- °F to °C: subtract 32, then multiply by 5/9. Or: subtract 32, then divide by 1.8.
Useful reference points to memorise:
- 0°C = 32°F — Freezing point of water
- 20°C = 68°F — Comfortable room temperature
- 37°C = 98.6°F — Normal human body temperature
- 100°C = 212°F — Boiling point of water
Quick mental approximation for weather: Double the Celsius temperature and add 30 to get an approximate Fahrenheit value. 20°C → 40 + 30 = 70°F (actual: 68°F). Close enough for understanding a weather forecast.
Volume
Volume conversions are complicated by the fact that the US and UK define "pints" and "gallons" differently.
- 1 US gallon = 3.785 litres (1 UK gallon = 4.546 litres)
- 1 US pint = 473 ml (1 UK pint = 568 ml — significantly larger)
- 1 litre = 1.057 US quarts
- 1 US fluid ounce = 29.57 ml
Fuel efficiency: The US measures fuel economy in miles per gallon (mpg). Europe uses litres per 100 kilometres (L/100km). To convert: divide 235 by L/100km to get US mpg. A car that uses 7 L/100km gets about 33 mpg (US).
Area
- 1 acre = 0.405 hectares (1 hectare = 2.471 acres) — Common in land area descriptions
- 1 square mile = 2.59 square kilometres
- 1 square foot = 0.0929 square metres — US real estate lists areas in square feet; European listings use square metres
Quick Reference Table
- 1 km = 0.62 miles · 1 mile = 1.61 km
- 1 kg = 2.20 lbs · 1 lb = 0.45 kg
- 1 L = 0.26 US gal · 1 US gal = 3.79 L
- 1 m² = 10.76 ft² · 1 ft² = 0.093 m²
- 1 hectare = 2.47 acres · 1 acre = 0.40 ha
For any conversion not covered here, use our free Unit Converter — it handles hundreds of unit combinations across length, temperature, area, volume, weight, and time.