Imperial and US customary measurement systems

Lore & Background

The imperial and US customary measurement systems both evolved from English units, which were themselves a combination of Roman, Carolingian, and Saxon measures. The US Customary system was developed after the American Revolution, using a subset of English units from the Thirteen Colonies. The imperial system was developed and used in the United Kingdom and its empire beginning in 1824. Most units have been adapted since the Norman Conquest. The foot used by craftsmen supplanted a longer agricultural foot, which was reduced to 10/11 of its former size, causing the rod to become 16.5 rather than 15 agricultural feet. The furlong and acre remained relatively unchanged once the acre became a measure of land size rather than value. Three principal pounds were used in England: the troy pound (5760 grains) for precious metals, the apothecaries' pound (also 5760 grains) for pharmacists, and the avoirdupois pound (7000 grains) for general purposes. The troy and apothecaries' pounds are divided into 12 ounces of 480 grains, while the avoirdupois pound has 16 ounces of 437.5 grains. The gallon differs between the two systems: the US gallon is 83.26742% of the imperial gallon, based on the pre-1826 English wine gallon. A US dry gallon, 96.8939% of an imperial gallon, is no longer used. In 1960, the two countries agreed on common definitions of the yard and pound based on the metre and kilogram, creating a slight difference between the international system and the US surveyors system, the latter deprecated in 2023.

Reader's Guide

The imperial and US customary systems both descend from an earlier English system of measurement, which itself drew from Ancient Roman, Carolingian, and Saxon units. Governments were among the earliest adopters of standardized weights and measures, often for tax collection; in the United States, the Treasury Department, rather than Congress, took the lead in establishing a standard system. The US customary system developed after the American Revolution from a subset of English units used in the Thirteen Colonies. It remains the predominant system in the United States and its territories, except in Puerto Rico and Guam, where Spanish rule introduced the metric system. The imperial system was developed in the United Kingdom and its empire beginning in 1824, though the metric system has since replaced it to varying degrees in former imperial territories.

Most units have been adapted since the Norman Conquest. Linear measures changed the least: the yard replaced the ell, and the chain was derived in England. The craftsmen’s foot supplanted a longer agricultural foot, which was reduced in size, causing the rod, pole, or perch to become based on the shorter foot. The furlong and acre remained relatively stable once the acre became a measure of land size rather than value. Three principal pounds were used in England: the troy pound (5760 grains) for precious metals, the apothecaries’ pound (also 5760 grains) for pharmacists, and the avoirdupois pound (7000 grains) for general use. The troy and apothecaries’ pounds are divided into 12 ounces of 480 grains, while the avoirdupois pound has 16 ounces of 437.5 grains.

The gallon differs between the two systems: the US gallon, based on the pre-1826 English wine gallon, is about 83.27% of the imperial gallon. A US dry gallon once existed but is no longer used or listed in statute. After US independence, customary units evolved separately, while the UK overhauled its system in 1826, creating different gallons. Later efforts to align the pound and yard definitions between the two countries used copies of British standards from 1855, but these were of poor quality compared to metric standards. In 1960, the US and UK agreed on common definitions of the yard and pound based on the metre and kilogram, which created a slight divergence in US linear measure—the international system and the surveyors system—until the latter was

Did You Know?

• In 1960, the US and UK agreed to common definitions of the yard and pound based on the metre and kilogram.
• The US Customary system is predominant in the United States and its territories, except Puerto Rico and Guam.