Reference · 2026-07-15
Despite the metric system being the international standard, millions of people encounter imperial measurements daily. The United Kingdom itself uses a curious mix — distances in miles, fuel in litres, body weight in stones, cooking in both grams and ounces. Americans measure almost everything in imperial units, while most of the rest of the world uses metric exclusively.
If you cook with American recipes, follow fitness advice from US-based creators, or travel between countries, unit conversion is a practical life skill. Understanding the logic behind both systems makes conversion faster and less error-prone.
The most frequent conversions people search for involve length. Here are the key relationships:
- 1 inch = 2.54 centimetres (exact)
- 1 foot = 30.48 centimetres
- 1 mile = 1.609 kilometres
- 1 metre = 3.281 feet
A useful shortcut: to convert miles to kilometres, multiply by 1.6. For a rough estimate, add 60% to the number. So 100 miles is roughly 160 km.
Weight conversions come up constantly in cooking, fitness and shipping:
- 1 pound (lb) = 453.6 grams
- 1 stone = 6.35 kilograms (14 pounds)
- 1 ounce = 28.35 grams
- 1 kilogram = 2.205 pounds
For cooking, remembering that 1 ounce is roughly 28 grams covers most recipe conversions. For body weight, divide kilograms by 6.35 to get stones.
The Fahrenheit to Celsius conversion trips people up because it involves both multiplication and subtraction:
- °C = (°F − 32) × 5/9
- °F = (°C × 9/5) + 32
Key reference points: 0°C = 32°F (freezing), 100°C = 212°F (boiling), 37°C = 98.6°F (body temperature), 180°C = 356°F (common baking temperature).
Cooking measurements cause the most confusion because the US cup system differs from metric measuring:
- 1 US cup = 236.6 ml
- 1 UK tablespoon = 15 ml
- 1 US tablespoon = 14.8 ml
- 1 US fluid ounce = 29.57 ml
- 1 litre = 4.227 US cups
You do not need to memorise exact conversion factors for everyday use. These approximations work well enough:
1. Kilometres to miles: divide by 1.6, or multiply by 0.6. So 80 km/h is roughly 50 mph.
2. Kilograms to pounds: double it and add 10%. So 70 kg ≈ 140 + 14 = 154 lbs.
3. Celsius to Fahrenheit: double the Celsius and add 30. So 20°C ≈ 40 + 30 = 70°F (actual: 68°F).
4. Litres to gallons: divide by roughly 4.5 for UK gallons or 3.8 for US gallons.
Britain officially adopted metric decades ago, but the switch was never fully completed, which is why the UK is one of the few places in the world where you will hear both systems used in the same sentence. Road distances and speed limits stayed in miles and mph largely because replacing every road sign in the country was judged too expensive and disruptive. Draught beer and cider are sold in pints by law — a rare case where an imperial unit is actually the legally mandated one. Body weight is commonly given in stones, a unit that barely exists outside the UK and Ireland. Meanwhile, packaged food, fuel and most scientific and medical contexts use metric exclusively. The result is a genuinely bilingual measurement culture, which is exactly why conversion literacy matters more for UK residents than for people in fully metric or fully imperial countries.
For scientific work, engineering, medical dosages or any safety-critical application, always use exact conversion factors and a reliable calculator. The approximations above are fine for cooking and everyday life, but not for anything where precision affects outcomes.
Our free Unit Converter handles all of these conversions instantly — length, weight, temperature, volume, speed, data storage and more. For recipe-specific conversions between cups, grams and ounces, the dedicated Cooking Converter accounts for ingredient density rather than using a single flat conversion factor, and the Speed Converter covers mph, km/h, knots and more for anything vehicle or travel related. No formulas to remember, no rounding errors.