Unit converters: metric, US and imperial

Metric, US customary and imperial: which is which

Three families of units turn up in everyday life. The metric system, formally the International System of Units (SI), builds everything from base units such as the meter, the kilogram and the second, and scales them by powers of ten with prefixes like kilo- and milli-. It is the official system in almost every country and the language of science everywhere.

US customary units (inches, feet, miles, pounds, US gallons, Fahrenheit) are the everyday system of the United States. Imperial units grew out of the same English measures but were standardized separately in the United Kingdom in 1824, which is why some of them share names with US units while differing in size.

Why the conversions are exact

Until the 1950s the inch and the pound differed very slightly between the United States, the United Kingdom, Canada and other countries. In 1959 the national standards laboratories of six English-speaking nations agreed on an international yard of exactly 0.9144 m and an international pound of exactly 0.45359237 kg. From those two numbers come the inch (2.54 cm), the foot (0.3048 m), the mile (1,609.344 m) and the ounce. The US gallon is defined as 231 cubic inches and the imperial gallon as 4.54609 liters, so both are tied to the meter as well.

The result is that nearly every conversion on these pages is a definition, not a measurement. Where the converters show long decimals, the digits come from dividing by an exact factor, and the working line under each result states the factor used. Temperature and fuel economy are the two exceptions to plain multiplication: temperature scales have different zero points, and liters per 100 km is the reciprocal of distance per unit of fuel.

Who uses what

Most of the world uses metric units for nearly everything. The United States uses customary units for daily life while science, medicine and much of industry work in metric. The United Kingdom is officially metric but still uses miles and miles per hour on road signs, pints for draught beer, and stone for body weight. Canada and Australia began converting to metric in the 1970s, though feet and pounds still appear in conversation, and cooks everywhere meet cups and spoons whose size depends on the country a recipe came from.