---
title: "Comparison of the imperial and US customary measurement systems"
chunk: 2/2
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"
---

== Length ==
The international yard is defined as exactly 0.9144 metres. This definition was approved by the United States, Canada, the United Kingdom, South Africa, Australia and New Zealand through the international yard and pound agreement of 1959, and corresponds with the previous 1930s British and American definitions of 1 inch being 25.4 mm. In all systems, a yard is 36 inches.
The US survey foot and survey mile were maintained as separate units for surveying purposes to avoid the accumulation of error that would follow replacing them with the international versions, particularly with State Plane Coordinate Systems. The choice of unit for surveying purposes is based on the unit used when the overall framework or geodetic datum for the region was established; for example, much of the former British empire still uses the Clarke foot for surveying.
The US survey foot is defined so that 1 metre is exactly 39.37 inches, making the international foot of 0.3048 metres exactly two parts per million shorter. This is a difference of just over 3.2 mm, or a little more than one-eighth of an inch per mile. According to the National Institute of Standards and Technology, the survey foot is obsolete as of 1 January 2023, and its use discouraged.
The main units of length (inch, foot, yard and international mile) were the same in the US, though the US rarely uses some of the intermediate units today, such as the (surveyor's) chain (22 yards) and the furlong (220 yards).
At one time, the definition of the nautical mile was based on the surface area of the Clarke ellipsoid. While the US used the full value of 1853.256 metres, in the British Commonwealth, this was rounded to 6080 feet (1853.184 m). These have been replaced by the international version (which rounds the 60th part of the 45° to the nearest metre) of 1852 metres.

== Weight and mass ==

Traditionally, both Britain and the US used three different weight systems: troy weight for precious metals, apothecaries' weight for medicines and avoirdupois weight for almost all other purposes. However, apothecaries' weight has now been superseded by the metric system.
In all of these systems, the fundamental unit is the pound (lb), and all other units are defined as fractions or multiples of a pound. The tables of imperial troy mass and apothecaries' mass are the same as the corresponding United States tables, except for the British spelling "drachm" in the table of apothecaries' mass. The table of imperial avoirdupois mass is the same as the United States table up to one pound, but above that point, the tables differ.
One important difference is the widespread use in Britain of the stone of 14 pounds (6.35029318 kg) for body weight, whereas this unit is not used in the United States, although flour was sold by a barrel of 196 pounds (14 stone) until World War II.
Another difference arises in that Britain deleted the troy pound (373.2417216 g) and the pennyweight from the relevant statute on 1 January 1879, leaving only the troy ounce (31.1034768 g) and its decimal subdivisions, whereas the troy pound (of 12 troy ounces) and pennyweight are still legal in the United States, although they are no longer widely used.
The imperial system has a hundredweight defined as eight stone of 14 lb each, or 112 lb (50.80234544 kg), whereas a US hundredweight is 100 lb (45.359237 kg): in both systems, 20 hundredweights make a ton.
In the US, the terms long ton (2240 lb, 1016.0469088 kg) and short ton (2000 lb,  907.18474 kg) are used. The metric ton is the name used for the tonne (1000 kg, 2204.62262 lb), which is 1.58% less than the long ton and is 10.23% more than the short ton.
The US customary system also includes the kip, equivalent to 1,000 pounds of force, which is also occasionally used as a unit of weight of 1,000 pounds (usually in engineering contexts).

== See also ==
Conversion of units
History of measurement
Systems of measurement
Weights and measures

== Notes ==

== References ==