Both imperial units and US customary units derive from earlier English units. Imperial units were mostly used in the British Commonwealth and the former British Empire but in most Commonwealth 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. The effort is proceeding slowly due to the overwhelming financial cost of converting the existing infrastructure. US companies which trade internationally are more likely to use the metric system due to international standards and certifications such as ISO9000. The metric system is preferred in certain fields such as science, medicine, technology and the military. The building profession uses US customary units, though architects working internationally are increasingly adapting to the metric system.

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 being 100 lb. and 2000 lb. respectively.