Common Unit Systems Converter

They share an ancestor in older English units, but diverged after American independence. Length and mass are now identical — the inch and the pound have the same definition in both. Volume is the real difference: Britain redefined its gallon in 1824 while the US kept the older wine gallon, leaving imperial liquid measures about 20% larger.

The United States uses US customary units for most everyday purposes. The United Kingdom is officially metric but retains imperial for road distances, draught beer and personal height and weight. Liberia and Myanmar are commonly cited as the only other non-metric countries, though both are moving toward SI.

SI stands for Système International d’Unités, adopted in 1960 as the modern form of the metric system. It defines seven base units — including the metre, kilogram and second — from which every other unit is derived. "Metric" is the older, looser term.

Yes, and have been since 1959. The international yard and pound agreement fixed the yard at exactly 0.9144 m and the pound at exactly 0.45359237 kg. Imperial units are therefore defined in terms of SI, which is why the conversions in this tool are exact rather than measured approximations.

The best-known case is the Mars Climate Orbiter, lost in 1999 when one team supplied thrust data in pound-force seconds while the receiving software expected newton seconds. The spacecraft approached the planet at the wrong altitude and was destroyed. It remains a standard example of why unit discipline matters in engineering.

Strictly, Fahrenheit is not an imperial unit — the imperial system never formally defined temperature. It is listed there because it is the scale still used alongside imperial measures in everyday British and American speech. The UK officially reports weather in Celsius.