Metric vs Imperial: A Practical Guide to Converting Units in Your Head
Easy mental shortcuts for kilometers, pounds, liters and temperatures, plus why the US and UK still use different gallons and pints.
Almost every country uses the metric system officially, but the United States still uses US customary units day to day, and the UK mixes both: miles on road signs, pints in pubs, stones for body weight, but liters at the petrol pump. If you travel, cook from foreign recipes or shop online internationally, a few mental shortcuts save a lot of time.
Length
- Inches to centimeters: multiply by 2.5 (exact: 2.54). A 12-inch pizza is about 30 cm.
- Feet to meters: divide by 3, then subtract a little. 6 ft ≈ 1.8 m.
- Miles to kilometers: multiply by 1.6. 50 mph ≈ 80 km/h.
- Kilometers to miles: multiply by 0.6. A 5 km run is about 3.1 miles.
A neat trick: consecutive Fibonacci numbers (3, 5, 8, 13, 21, 34) approximate the miles-to-kilometers ratio. 5 miles ≈ 8 km, 8 miles ≈ 13 km, 21 miles ≈ 34 km.
Weight
- Kilograms to pounds: double it and add 10%. 70 kg → 140 + 14 = 154 lb (exact: 154.3).
- Pounds to kilograms: halve it and subtract 10%. 180 lb → 90 − 9 = 81 kg (exact: 81.6).
- Ounces to grams: multiply by 28. A 16 oz steak is about 450 g.
- Stone to kilograms: multiply by 6.35. 11 stone ≈ 70 kg.
Volume
- Liters to US gallons: divide by 4 (exact: 3.785). 40 liters ≈ 10.5 gallons.
- A US cup is about 240 ml; a metric cup is 250 ml.
- A US pint (473 ml) is smaller than a UK pint (568 ml). Ordering a pint in London gets you about 20% more beer than in New York.
- A US gallon (3.785 L) is smaller than a UK gallon (4.546 L), which is why UK mpg figures look better for the same car.
Temperature
- Celsius to Fahrenheit: double and add 30. 25 °C → 80 °F (exact: 77 °F).
- Fahrenheit to Celsius: subtract 30 and halve. 70 °F → 20 °C (exact: 21.1 °C).
- Remember a few anchors: 0 °C = 32 °F (freezing), 10 °C = 50 °F (cool), 20 °C = 68 °F (room), 30 °C = 86 °F (hot), 37 °C = 98.6 °F (body).
Area
- Square meters to square feet: multiply by 10 and add 8%. 50 m² ≈ 538 ft².
- Hectares to acres: multiply by 2.5 (exact: 2.471).
Why the differences exist
The metric system was created in France in the 1790s to replace hundreds of inconsistent local units with a decimal system based on nature. Britain developed the imperial system in 1824, standardizing its own traditional units. The US had already become independent and kept the older English units, which is why US gallons and pints differ from British ones. Today all of these units are legally defined in terms of metric ones; for instance, the inch has been exactly 2.54 cm since 1959.
When precision matters, such as in medicine, engineering or baking, use exact conversions. The converters on this site give exact values to ten significant figures and show every unit at once.