<P> The SI includes two classes of units which are defined and agreed internationally . The first of these classes includes the seven SI base units for length, mass, time, temperature, electric current, luminous intensity and amount of substance . The second class consists of the SI derived units . These derived units are defined in terms of the seven base units . All other quantities (e.g. work, force, power) are expressed in terms of SI derived units . </P> <P> Both imperial units and US customary units derive from earlier English units . Imperial units were mostly used in the former British Empire and the British Commonwealth, but in all these countries they have been largely supplanted by the metric system . They are still used for some applications in the United Kingdom but have been mostly replaced by the metric system in commercial, scientific, and industrial applications . US customary units, however, are still the main system of measurement in the United States . While some steps towards metrication have been made (mainly in the late 1960s and early 1970s), the customary units have a strong hold due to the vast industrial infrastructure and commercial development . </P> <P> While imperial and US customary systems are closely related, there are a number of differences between them . Units of length and area (the inch, foot, yard, mile etc .) are identical except for surveying purposes . The Avoirdupois units of mass and weight differ for units larger than a pound (lb). The imperial system uses a stone of 14 lb, a long hundredweight of 112 lb and a long ton of 2240 lb . The stone is not used in the US and the hundredweights and tons are short: 100 lb and 2000 lb respectively . </P> <P> Where these systems most notably differ is in their units of volume . A US fluid ounce (fl oz), about 29.6 millilitres (ml), is slightly larger than the imperial fluid ounce (about 28.4 ml). However, as there are 16 US fl oz to a US pint and 20 imp fl oz per imperial pint, the imperial pint is about 20% larger . The same is true of quarts, gallons, etc . Six US gallons are a little less than five imperial gallons . </P>

Si units are part of a system of measurement used only in north america