Overview

The Imperial units, often called the Imperial system, are a set of standardized measures for length, area, volume and mass that were formalized in Great Britain in the early 19th century. They were created to rationalize and replace a variety of older English units and were adopted across the British Empire. Although many countries later converted to the metric system, Imperial units remain familiar in everyday language and in a number of practical contexts.

Units and structure

The system groups units into families for different kinds of measurement. Common length units include the inch, foot, yard and mile. Areas are frequently expressed in square yards or acres. Volume is handled with distinct liquid measures such as the gallon, pint and fluid ounce, while mass and weight use the ounce, pound and ton, alongside the traditional stone and hundredweight in certain regions.

  • Length: inch, foot, yard, mile
  • Area: square yard, acre
  • Volume: gallon, quart, pint, fluid ounce
  • Mass/weight: ounce, pound, stone, hundredweight, ton

History and development

The modern Imperial system traces back to an Act of Parliament passed in 1824 that sought to standardize the many local and historic English measures into a single legal framework. Over the following decades the system was used throughout the British Empire and became the de facto set of measures in many Commonwealth countries. During the 20th century most of those countries moved to the metric system, a process known as metrication, though the pace and extent of change varied by country.

Modern use and notable distinctions

Today Imperial units persist in a number of specific settings. The United Kingdom uses miles for road distances and retains colloquial uses such as describing a person's height in feet and inches and body weight in stones and pounds. Pubs and breweries commonly use pints. Aviation typically measures altitude in feet. Nevertheless, scientific, industrial and international trade measurements overwhelmingly use the metric system.

Important differences exist between Imperial units and the related U.S. customary units: for example the Imperial gallon is larger than the U.S. gallon, and the sizes of the pint and fluid ounce differ between the two systems. Some units, such as the yard and the foot, are equivalent in both systems. Because two systems with similar unit names coexist, conversions are a frequent source of confusion when comparing historical records or products from different countries.

Further information

For introductions and legal histories, consult general references on measurements and the 19th-century reforms: measurement overview, the 1824 Act. For information about Commonwealth adoption and later metrication see Commonwealth usage and metrication background. For a comparison with U.S. customary units and specific conversion notes, see differences from U.S. units.