---
title: "Comparison of the imperial and US customary measurement systems"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Comparison_of_the_imperial_and_US_customary_measurement_systems"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:36:09.542342+00:00"
instance: "kb-cron"
---

Both the British imperial measurement system and United States customary systems of measurement derive from earlier English unit systems used prior to 1824 that were the result of a combination of the local Anglo-Saxon units inherited from Germanic tribes and Roman units.
Having this shared heritage, the two systems are quite similar, but there are differences. The US customary system is based on English systems of the 18th century, while the imperial system was defined in 1824, almost a half-century after American independence.

== Volume ==
Volume may be measured either in terms of units of cubic length or with specific volume units. The units of cubic length (the cubic inch, cubic foot, cubic mile, etc.) are the same in the imperial and US customary systems, but they differ in their specific units of volume (the bushel, gallon, fluid ounce, etc.). 
The US customary system has a set of units for fluids, it has a different measure for dry goods for the pint, quart and barrel, and a peck and bushel for dry goods only. The imperial system has only one set of measurements, defined independently of the US customary system.
By the end of the 18th century, various systems of volume measurement were in use throughout the British Empire. Wine was measured with units based on the wine gallon of 231 cubic inches (3.785 L), while beer was measured with units based on an ale gallon of 282 cubic inches (4.621 L) and grain was measured with the Winchester measure, with a gallon of approximately 268.8 cubic inches (one eighth of a Winchester bushel or 4.405 L). In 1824, these units were replaced with a single system based on the imperial gallon. Originally defined as the volume of 10 pounds (4.54 kg) of distilled water (under certain conditions), then redefined by the Weights and Measures Act 1985 to be exactly 4.54609 L (≈277.42 cu in), the imperial gallon is 1.62% smaller than the pre-1824 ale gallon.
The Winchester measure was made obsolete in the British Empire, but remained in use in the US until the 1990s. In the British Empire, the Winchester bushel was replaced with an imperial bushel of eight imperial gallons, with the subdivisions of the bushel being maintained. 
As with US dry measures, the imperial system divides the bushel into 4 pecks, 32 quarts or 64 pints: the imperial quart and imperial pint are 3.21% larger than their US dry counterparts, whereas the imperial peck and imperial bushel were deleted from the relevant UK statute in 1968.
Fluid measure is not as straightforward. The American colonists adopted a system based on the 231-cubic-inch wine gallon for all fluid purposes: this became the US gallon, being legally adopted in 1836. Both the imperial and US fluid gallon are divided into 4 quarts, 8 pints or 32 gills. However, whereas the US gill is divided into four US fluid ounces, the imperial gill is divided into five imperial fluid ounces. Thus, while the imperial fluid ounce is 3.924% smaller than the US fluid ounce, the imperial gallon, quart, pint and gill are all 20.095% larger than their US counterparts. 
One avoirdupois ounce of water has an approximate volume of one imperial fluid ounce at 62 °F (16.7 °C): this convenient fluid ounce to avoirdupois ounce relation does not exist in the US system, as one US fluid ounce is 4.318% larger than the avoirdupois ounce.
One noticeable comparison between the imperial system and the US system is between some Canadian and American beer bottles: many Canadian brewers package their beer in 12 imperial fl oz bottles, which are 341 mL each, while American brewers package their beer in 12 US fl oz bottles, which are 355 mL each. Consequently, Canadian bottles are labelled as 11.5 fl oz in US units when imported into the United States.
Because the standard size of Canadian beer bottles predates the adoption of the metric system in Canada, the bottles are still sold and labelled in Canada as 341 mL. Canned beer in Canada is sold and labelled in 355 mL cans (12.5 imperial fl oz), and when exported to the United States, they are labelled as 12 fl oz.