Most of the world uses the metric system; the United States still relies mainly on US customary units, and the UK mixes both. If you cook from foreign recipes, shop online, travel, or read a medical chart, you will convert units sooner or later. This guide covers the factors worth knowing and the mistakes to avoid.
The metric system, built on powers of ten, was designed in France in the 1790s and is now the standard for science and for nearly every country. The imperial and US customary systems developed from older traditional measures. The two overlap confusingly: a US gallon is not an imperial gallon, and a "ton" can mean three different things.
The inch is defined as exactly 2.54 centimetres, so every imperial length is tied to the metric system. A quick mental estimate: multiply miles by 1.6 to get kilometres, or divide kilometres by 1.6 to get miles.
Height is usually given in feet and inches, such as 5 ft 7 in. Convert in two steps: first change everything to inches (5 × 12 + 7 = 67), then multiply by 2.54 to get centimetres (67 × 2.54 = 170.18 cm). Going the other way, divide centimetres by 2.54 to get total inches, then split into feet and inches. 180 cm ÷ 2.54 = 70.87 inches, which is 5 ft 10.87 in, or about 5 ft 11 in.
Pay attention to the ton. A metric tonne is 1,000 kg, a US short ton is 2,000 pounds (about 907 kg), and a UK long ton is 2,240 pounds (about 1,016 kg). Shipping, steel and cargo quotes can differ by several percent depending on which one is meant.
Length and weight convert by multiplying. Temperature does not, because the scales have different zero points. Use these formulas:
Remember that normal body temperature is about 37 °C (98.6 °F), water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level. A handy rough check: double the Celsius value and add 30 for an approximate Fahrenheit figure. It is within a few degrees in everyday ranges.
Recipes are where conversion mistakes cost the most. Cups measure volume, grams measure mass, and the same cup holds different weights of different ingredients. A cup of flour weighs roughly 120 to 125 g, a cup of sugar about 200 g, and a cup of water about 237 g. So there is no single "cups to grams" factor; you need the ingredient. For liquids, 1 US cup is about 237 ml, 1 tablespoon about 15 ml and 1 teaspoon about 5 ml. An imperial pint (568 ml) is larger than a US pint (473 ml), another trap when using British recipes.
Property listings and land sales often use hectares, acres or square feet. Square units scale with the square of the length factor: since 1 m ≈ 3.281 ft, 1 m² ≈ 3.281² ≈ 10.76 ft², not 3.28.
When precision matters, such as medication, engineering or legal measurements, use the exact defined factors and have a second source confirm the result.
Try it yourself: the Unit Converter for length, weight, temperature, volume, area and height →