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
[
edit
]
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 one set of units for fluids and another set for dry goods. The imperial system has only one set defined independently of, and subdivided differently from, its US counterparts.
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), 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.
[a]
Originally defined as the volume of 10
pounds
(4.54 kg) of distilled water (under certain conditions),
[b]
then redefined by the
Weights and Measures Act 1985
to be exactly
4.546
09
L
(277.4 cu in), the imperial gallon is close in size to the old ale gallon.
The Winchester measure was made obsolete in the British Empire but remained in use in the US.
[c]
The Winchester bushel was replaced with an imperial bushel of eight imperial gallons. The subdivisions of the bushel were maintained. As with US dry measures, the imperial system divides the bushel into 4 pecks, 8 gallons, 32 quarts or 64 pints. Thus, all of these imperial measures are about 3% larger than are their US dry-measure counterparts.
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 fluid gallon. Both the imperial and US fluid gallon are divided into 4 quarts, 8 pints or 32
gills
.
[d]
However, whereas the US gill is divided into four US fluid ounces, the imperial gill is divided into five imperial fluid ounces. So whilst the imperial gallon, quart, pint and gill are about 20% larger than are their US fluid measure counterparts, the fluid ounce is about 4% smaller.
[e]
One
avoirdupois ounce
of water has an approximate volume of one imperial fluid ounce at 62 °F (16.67 °C).
[f]
This convenient fluid-ounce-to-avoirdupois-ounce relation does not exist in the US system.
One noticeable comparison between the imperial system and the US system is between some Canadian and American beer bottles. Many Canadian brewers package beer in a 12-imperial-fluid-ounce bottles, which are 341 mL each. American brewers package their beer in 12-US-fluid-ounce bottles, which are 355 mL each. As a result, 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, and when exported to the US, they are labelled as 12 fl oz.
Comparison of current imperial, US and metric volume measures
Notes:
- Approximate values are denoted with ?
- Exact values are denoted with ≡
- Definitions are marked in
bold
|
Unit name
|
Imperial measures
|
US fluid measures
|
US dry measures
|
Metric measures
|
fluid ounces
|
Imperial fluid ounce
(fl oz)
|
≡
1 imp fl oz
|
?
0.960
759
940
40
US fl oz
?
0.947
102
083
33
US fl oz
(food)
|
|
≡
28.413
0625
mL
≡
0.028
413
0625
L
|
US fluid ounce
(customary)
(fl oz)
|
?
1.040
842
730
79
imp fl oz
|
≡
1 US fl oz
≡
0.985
784
318
75
US fl oz
(food)
|
|
≡
29.573
529
5625
mL
≡
0.029
573
529
5625
L
|
US fluid ounce
(food nutrition labelling)
(fl oz) (food)
|
?
1.055
852
391
84
imp fl oz
|
?
1.014
420
681
06
US fl oz
≡
1 US fl oz
(food)
|
|
≡
30 mL
≡
0.03 L
|
pints
|
Imperial pint
(pt)
|
≡
20 imp fl oz
≡
1 imp pt
≡
0.5 imp qt
≡
0.125 imp gal
|
?
19.215
198
8081
US fl oz
?
18.942
041
6667
US fl oz
(food)
?
1.200
949
925
50
US pt
?
0.150
118
740
69
US gal
|
?
1.032
056
743
49
US dry pt
?
0.129
007
092
94
US dry gal
|
≡
568.261
25
mL
≡
0.568
261
25
L
|
US liquid pint
(pt)
|
?
16.653
483
6926
imp fl oz
?
0.832
674
184
63
imp pt
?
0.104
084
273
08
imp gal
|
≡
16 US fl oz
≡
15.772
5491
US fl oz
(food)
≡
1 US pt
≡
0.5 US qt
≡
0.125 US gal
|
?
0.859
367
007
375
US dry pt
?
0.107
420
875
922
US dry gal
|
≡
473.176
473
mL
≡
0.473
176
473
L
|
US dry pint
(pt)
|
?
19.378
779
4384
imp fl oz
?
0.968
938
971
92
imp pt
?
0.121
117
371
49
imp gal
|
?
18.618
354
9784
US fl oz
?
18.353
682
3786
US fl oz
(food)
?
1.163
647
186
15
US pt
?
0.145
455
898
27
US gal
|
≡
1 US dry pt
≡ 0.5 US dry qt
≡ 0.125 US dry gal
|
≡
550.610
471
3575
mL
≡
0.550
610
471
3575
L
|
quarts
|
Imperial quart
(qt)
|
≡
40 imp fl oz
≡
1 imp qt
≡
2 imp pt
≡
0.25 imp gal
|
?
38.430
397
6162
US fl oz
?
37.884
083
3333
US fl oz
(food)
?
1.200
949
925
50
US qt
?
0.300
237
481
38
US gal
|
?
1.032
056
743
49
US dry qt
?
0.258
014
185
87
US dry gal
|
≡
1
136
.5225 mL
≡
1.136
5225
L
|
US liquid quart
(qt)
|
?
33.306
967
3852
imp fl oz
?
0.832
674
184
63
imp qt
?
0.208
168
546
16
imp gal
|
≡
32 US fl oz
≡
31.545
0982
US fl oz
(food)
≡
2 US pt
≡
1 US qt
≡
0.25 US gal
|
?
0.859
367
007
375
US dry qt
?
0.214
841
751
844
US dry gal
|
≡
946.352
946
mL
≡
0.946
352
946
L
|
US dry quart
(qt)
|
?
38.757
558
8768
imp fl oz
?
0.968
938
971
92
imp qt
?
0.242
234
742
98
imp gal
|
?
37.236
709
9567
US fl oz
?
36.707
364
7572
US fl oz
(food)
?
1.163
647
186
15
US qt
?
0.290
911
796
54
US gal
|
≡
1 US dry qt
≡ 2 US dry pt
≡ 0.25 US dry gal
|
≡
1
101
.220
942
715
mL
≡
1.101
220
942
715
L
|
gallons
|
Imperial gallon
(gal)
|
≡
160 imp fl oz
≡
4 imp qt
≡
1 imp gal
|
?
153.721
590
465
US fl oz
?
151.536
333
333
US fl oz
(food)
?
1.200
949
925
50
US gal
|
?
4.128
226
973
95
US dry qt
?
1.032
056
743
49
US dry gal
|
≡
4
546
.09 mL
≡
4.546
09
L
|
US liquid gallon
(gal)
|
?
133.227
869
541
imp fl oz
?
3.330
696
738
52
imp qt
?
0.832
674
184
63
imp gal
|
≡
128 US fl oz
≡
126.180
3928
US fl oz
(food)
≡
4 US qt
≡
1 US gal
≡
231 cu in
|
?
3.437
468
029
501
US dry qt
?
0.859
367
007
375
US dry gal
|
≡
3
785
.411
784
mL
≡
3.785
411
784
L
|
US dry gallon
(gal)
|
?
155.030
235
507
imp fl oz
?
3.875
755
887
68
imp qt
?
0.968
938
971
92
imp gal
|
?
148.946
839
827
US fl oz
?
146.829
459
029
US fl oz
(food)
?
4.654
588
7446
US qt
?
1.163
647
186
15
US gal
|
≡
4 US dry qt
≡
1 US dry gal
≡
268.8025 cu in
|
≡
4
404
.883
770
86
mL
≡
4.404
883
770
86
L
|
metric
|
litre
(l or L or dm
3
)
|
?
35.195
079
7279
imp fl oz
?
0.879
876
993
20
imp qt
?
0.219
969
248
30
imp gal
|
?
33.814
022
7018
US fl oz
?
33.333
333
3333
US fl oz
(food)
?
1.056
688
209
43
US qt
?
0.264
172
052
36
US gal
|
?
0.908
082
984
27
US dry qt
?
0.227
020
746
07
US dry gal
|
≡
1000 mL
≡
1 L
|
Length
[
edit
]
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
[
further explanation needed
]
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.
[1]
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
.
[
further explanation needed
]
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
[
edit
]
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.
One important difference is the widespread use in Britain of the
stone
of 14 pounds (
6.350
293
18
kg
) for body weight; 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 arose when Britain abolished the
troy pound
(
373.241
7216
g
) on 1 January 1879, leaving only the
troy ounce
(
31.103
4768
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.
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.
The imperial system has a hundredweight, defined as eight stone of 14 lb each, or 112 lb (
50.802
345
44
kg
), whereas a US hundredweight is 100 lb (
45.359
237
kg
). In both systems, 20 hundredweights make a
ton
. In the US, the terms
long ton
(
2240 lb
,
1
016
.046
9088
kg
) and
short ton
(
2000 lb
;
907.184
74
kg
) are used. The metric ton is the name used for the
tonne
(
1000 kg
,
2
204
.622
62
lb
), which is about 1.6% less than the long 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
[
edit
]
Notes
[
edit
]
- ^
Wine gallons, however, continued to be used for tax purposes in the UK until the late 1990s.
- ^
The water was to be weighed in air with brass weights with the barometer standing at 30
inches of mercury
(102 kPa) at a temperature of 62
°F
(17 °C). In 1963 these conditions were redefined such that the water was to have a density of
0.998
859
g/mL
and to be weighed in air of density
0.001
217
g/mL
against weights of density 8.136 g/mL.
- ^
Originally defined as the volume of a cylinder
18
+
1
⁄
2
inches (470 mm) in diameter and 8 inches (200 mm) deep, the Winchester bushel was redefined in the US as
2
150
.42
cubic inches.
- ^
The gill is no longer in common use.
- ^
The now rarely used
apothecaries' system
of fluid measures further divides the fluid ounce into 8
fluid drams
or 480
minims
. Also in the imperial system there is a fluid scruple of 20 minims which is absent from the US customary system. Like the fluid ounce the dram and minim are about 4% smaller in the imperial system.
- ^
160 imperial fluid ounces is equivalent to one imperial gallon, which is the approximate volume of 10 pounds or 160 avoirdupois ounces of water at 62 °F.
References
[
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]
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Length
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Area
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Volume
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Speed
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Mass
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Pressure
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Related systems
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Length
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Area
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Volume
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Cooking
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Wood
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Other
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Speed
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Mass
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Force
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Pressure
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Other units and measures
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Related systems
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Current
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Background
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Historic
| Metric
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Europe
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Asia
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Africa
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Ancient
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