Metric vs imperial units: a practical guide

The metric system (SI) is based on powers of ten and is used by almost every country. The imperial and US customary systems are still used in daily life in the United States and, partly, in the United Kingdom.

The metric system is easier to scale — a kilometre is 1,000 metres — while imperial units, such as 12 inches to a foot and 5,280 feet to a mile, have to be memorised.

Length: 1 inch is 2.54 cm exactly, 1 foot is 0.3048 m and 1 mile is 1.609344 km. Weight: 1 pound is 0.45359237 kg and 1 ounce is about 28.35 g.

Volume: 1 US gallon is 3.785 litres, while 1 UK gallon is 4.546 litres, so always check which gallon is meant.

Temperature is the one conversion that is not a simple multiplication. Fahrenheit equals Celsius × 9/5 + 32, and Kelvin equals Celsius + 273.15.

So 100°C is 212°F and −40 is the point where the two scales meet. Nothing can be colder than absolute zero, −273.15°C.

Check the units on both sides before converting, and remember that fluid ounces measure volume while ounces measure weight. Data units have their own trap: a megabyte is 1,000,000 bytes but a mebibyte is 1,048,576.

For anything where precision matters, use a converter rather than rounded rules of thumb, and keep the full precision until the final result.