Common unit systems

The International System of Units is the modern metric system and the global standard for science and trade. It is built on base units such as the meter, kilogram, second and kelvin, scaled by powers of ten with prefixes like kilo-, centi- and milli-.

The British Imperial system defines units such as the mile, yard, foot, pound and imperial gallon. It is still used in the United Kingdom for some everyday measures.

Used in the United States, US customary units share many names with Imperial units but differ in some volumes - a US gallon is smaller than an imperial gallon.

Many fields keep their own units - nautical miles and knots at sea, carats for gemstones, BTUs for heating, electronvolts in physics.