Metrified Imperial System System Length Area Volume Capacity

Nowadays all Imperial units are defined in terms of their metric equivalents, but mostly with quite odd factors. This choice was made to ensure largest possible continuity, but over time there have been several proposals to round off the values, which is thought to ease transition towards the metric system.

1 System

One of the earliest still known and most extensive proposals of a metrified Imperial system is from the early 1820s and is attributed to William Huskisson, but it wasn't embodied into the 1824 reformation of the Imperial system at all.

1.1 Length

Metrified Imperial length units
calibre		= 1/100 inch	= 250 µm = ¼ mm
point	= 1/6 line	= 1/75 inch	= 333 1/3 µm = 1/3 mm
line	= 10/125 inch	= ca. 1/12 inch	= 2 mm
barleycorn		= 1/3 inch	= 8 1/3 mm
digit		= ¾ inch	= 18¾ mm = 1.875 cm
finger		= 7/8 inch	= 21.875 mm = 2.1875 cm
inch			= 25 mm = 2½ cm
stick		= 2 inch	= 50 mm = 5 cm
nail	= 3 digit = ¼ span	= 2¼ inch	= 56¼ mm = 5.625 cm
palm		= 3 inch	= 75 mm = 7.5 cm
hand	= 1 palm + 1 inch	= 4 inch	= 100 mm = 1 dm
shaftment		= 6 inch	= 150 mm = 1.5 dm
link	= 1/25 rod	= 8 inch	= 200 mm = 2 dm
span	= 3 palm	= 9 inch	= 225 mm = 2.25 dm
foot		= 12 inch	= 300 mm = 3 dm
cubitFor the unit of information, see qubit Cubit is the name for the ancient Egyptian and Sumerian units of measure. These Cubits are among the first recorded units of length used by an ancient people. From around 1950 BC, the copper bar cubit of Nippur defin	= 1½ foot	= 18 inch	= 450 mm = 4.5 dm
pacePace may refer to: Pace, the rate of speed at which movement occurs Pace, a unit of length equal to 3 feet; defined as 91. 44 centimeters; originally taken to be the average length of a stride Pace University, in New York state, USA Pace, a horse gait Pac		= 32 inch	= 800 mm = 8 dm
yardThis article is about the unit of measure known as the yard''. For other definitions, see Yard (disambiguation). A yard (abbr. yd is an imperial unit of length, defined as 3 feet or 36 inches, which is exactly 0. 9144 metres, presuming international inche	= 3 foot	= 36 inch	= 900 mm = 9 dm
ell	= 20 nail	= 45 inch	= 1.125 mFor other uses of "metre" and "meter", see Metre (disambiguation). The metre is the basic unit of length in the International System of Units (SI: Systeme International d'Unites). It is defined as the length of the path travelled by light in absolute vacu
gradeA grade may refer to many different concepts, including: in various contexts: Each item in a (generally ordered and finite) collection of symbols or designators used as a particular grade system to distinguish and rank corresponding groups, where distinct	= 2 pace	= 64 inch	= 1.6 m
fathomThis page deals with fathom, a unit of measure. For information about the comic book title and character created by Michael Turner, see Fathom (comics). A fathom is a unit of measure equal to 2 yards or 6 feet, or 1. 8288 metres. It was originally defined	= 6 foot	= 72 inch	= 1.8 m
rodThe word Rod may have one of the following meanings. A rod is a straight and slender stick; a wand; hence, any slender bar, as of wood or metal (applied to various purposes). Rods are light-sensitive cells in the retina of the eye. See rod cell. The rod i (pole, perch)	= 25 link	= 200 inch	= 5 m
rope	= 20 foot	= 240 inch	= 6 m
chain	= 4 rod	= 800 inch	= 20 m = 2 dam
cable length	= 100 fathom	= 7.2e3 inch	= 180 m = 1.8 hm
furlong	= 10 chain	= 8e3 inch	= 200 m = 2 hm
mile	= 8 furlong = 2 000 pace	= 64e3 inch	= 1.6 km
nautical mile	= 10 cable = 2 000 yard	= 72e3 inch	= 1.8 km
league	= 3 mile	= 192e3 inch	= 4.8 km
sea league	= 3 nautical mile	= 216e3 inch	= 5.4 km

There's also a version with a nautical mile of 1.85 km, which is much closer to 1' of the circumference of Earth and its current value of 1.852 km. The mile is about ¼ mrad Earth, while a kilometre was at that time beleived to be 0.01 gon Earth. The pace was originally 2½ foot, i.e. 30 inches or 7.5 dm, but with 32 inches or 8 dm a mile retains both its definitions: the old Roman one of being 1000 gradus and the English one of 8 furrows long. Also note, that a 1.8-km seamile is 2 000 yards or nine furlongs.

1.2 Area

For area measurement of course all length units can simply be squared, e.g. 1 foot² = 9 dm². Those are not explicitely listed here.

Metrified Imperial area units
board	= 1 inch × 1 foot	= 12 inch²	= 7 500 mm² = 75 cm²
rood	= 1 furlong × 1 rod = 40 rod²	= 1.6e6 inch²	= 1 000 m² = 10 a
acre	= 1 furlong × 1 chain = 160 rod²	= 6.4e6 inch²	= 4 000 m² = 40 a
yardland , virgate	= ¼ hide = 25 acre	= 160e6 inch²	= 125 000 m² = 12.5 ha
hide	= 100 acre	= 640e6 inch²	= 400 000 m² = 40 ha
barony	= 40 hide	= 25.6e9 inch²	= 16 km²

1.3 Volume, capacity

For volume measures of course all length units can simply be cubed, e.g. 1 foot³ = 27 dm³ = 27 l. Those are not explicitely listed here.

Metrified Imperial capacity units
perch	= 1 perch × 1½ foot × 1 foot	= 42.768e3 inch³	= 675 l = 6.75 hl
freight ton	= ca. 40 foot³	= 64e3 inch³	= 1 m³ = 1 kl
load	= 50 foot³	= 86.4e3 inch³	= 1.35 m³
register ton	= 100 foot³	= 172.8e3 inch³	= 2.7 m³
acre-foot	= 1 acre × 1 foot	= 19.2e6 inch³	= 12 000 m³ = 12 dam³

There are at least two gallon-based variants, one with a 4-litre gallon, the other with a 5-litre one. With 5 litre a gallon of water still weighs 10 pounds; note the correspondence between the volume and mass units which share a name.

Metrified Imperial capacity units
minim	= 1/20 scruple	= 1/240 inch³	= 65.104 1¯6 µl
scruple	= 1/3 drachm	= 1/12 inch³	= 1.302 08¯3 ml
dra(ch)m	= 1/8 ounce	= ¼ inch³	= 3.906 250 ml
teaspoon	= 1/3 tablespoon	= 1/3 inch³	= 5.208¯3 ml
tablespoon	= 1/16 cup	= 1 inch³	= 15.625 ml = 1.562 500 cl
ounce	= ¼ gill	= 2 inch³	= 31¼ ml = 3.125 cl
gill	= ¼ pint	= 8 inch³	= 125 ml = 1¼ dl
cup	= ½ pint = 8 ounce	= 16 inch³	= 250 ml = 2½ dl
pint	= ½ quart	= 32 inch³	= 500 ml = 5 dl
pint	= 4/5 quart	= 32 inch³	= 500 ml = 5 dl
quart	= ¼ gallon	= 64 inch³	= 1 l
quart	= ¼ gallon	= 80 inch³	= 1.25 l
gallon		= 256 inch³	= 4 l
gallon		= 320 inch³	= 5 l
peck	= 2 gallon	= 512 inch³	= 8 l
peck	= 2 gallon	= 640 inch³	= 10 l
kenning	= 2 peck	= 1 024 inch³	= 16 l = 1.6 dal
kenning	= 2 peck	= 1 280 inch³	= 20 l = 2 dal
bushel	= 2 kenning	= 2 048 inch³	= 32 l = 3.2 dal
bushel	= 2 kenning	= 2 560 inch³	= 40 l = 4 dal
quarter	= 8 bushel	= 4 096 inch³	= 256 l = 2.56 hl
quarter	= 8 bushel	= 5 120 inch³	= 320 l = 3.2 hl
firkin	= ½ kilderkin	= 2 560 inch³	= 40 l = 4 dal
rundlet	= 1/7 pipe	= 4 389 3/7 inch³	~= 68.57 l = 6.857 dal
kilderkin	= ½ tierce	= 5 120 inch³	= 80 l = 8 dal
barrel	= ½ hogshead	= 7 680 inch³	= 120 l = 1.2 hl
tierce	= 1/3 pipe = ½ puncheon	= 10 240 inch³	= 160 l = 1.6 hl
hogshead	= ½ pipe	= 15 360 inch³	= 240 l = 2.4 hl
puncheon (pon), tertian	= 1/3 tun	= 20 480 inch³	= 320 l = 3.2 hl
pipe	= ½ tun	= 30 720 inch³	= 480 l = 4.8 hl
tun	= 240 gallon	= 61 440 inch³	= 960 l = 9.6 hl
tun	= 192 gallon	= 61 440 inch³	= 960 l = 9.6 hl

1.4 Mass, weight

The Troy pound gets replaced by the libra, the abbreviation for the real pound has to change accordingly.

Metrified Imperial mass units
grain	= 1/20 scruple	= 1/7 680 pound	= 65.104 1¯6 mg = 6.510 41¯6 cg
carat		= 1/2 500 pound	= 200 mg = 2 dg
scruple	= 1/3 dram	= 1/384 pound	= 1.302 08¯3 g
pennyweight	= 1/20 ounce = 1/240 libra	= 1/320 pound	= 1.562 500 g
dram	= 1/8 ounce	= 1/128 pound	= 3.906 250 g
ounce	= 1/12 libra	= 1/16 pound	= 31.25 g = 3.125 dag
libra	= 5760 grain	= ¾ pound	= 375 g = 3¾ hg
pound	= 7680 grain		= 500 g = 5 hg
clove, nail	= ½ stone	= 6¼ pound	= 3.125 kg
stone	= ½ tod	= 12½ pound	= 6.25 kg
tod , quarter	= ¼ hundredweight	= 25 pound	= 12.5 kg
hundredweight		= 100 pound	= 50 kg
wey	= 1½ hundredweight	= 150 pound	= 75 kg
sack	= 2 wey	= 300 pound	= 150 kg
ton, tonne	= 20 hundredweight	= 2 000 pound	= 1 Mg = 1 t
last	= 12 sack	= 3 600 pound	= 1.8 Mg = 1.8 t

1.5 Derived units

The knot is acknowledged as 1 nautical mile per hour.

Weight and mass are often confused and it is not surprising, that in many proposals there is no explict mention of (gravitational) force, but the pound-force is still 1 pound times Gee, see also kilopond. The poundal (1 lb.·ft./s²) equals 0.15 N with the above redefinitions of pound and foot.

Likewise you get the units of pressure, e.g. pound-force/inch² or inch mercury. For energy or heat, a British thermal unit is almost exactly 1 kJ already. The units of power are rather new, but it makes sense to round a horse power to 750 W.

1.6 Kitchen

There are other metrified values for cup (240 ml or 250 ml), tablespoon (15 ml) and teaspoon (5 ml) in use today, independently from all the other units.

2 Isolated

Besides redefinitions of the whole system, which try to keep the values close and the system immanent factors intact, there were and are simple approximations of Imperial units in metric.

2.1 Length

point (352 7/9 µm): 250 µm = ¼ mm, 350 µm, 375 µm = 3/8 mm
inch (25,4 mm): 2.5 cm = ¼ dm, 3 cm
foot (12 inch = 0.3048 m): 2.5 dm = ¼ m, 3 dm, 3.¯3 dm = 1/3 m
yard (3 foot = 0.9144 m): 0.9 m, 1 m
fathom (6 foot = 1.8288 m): 1.8 m, 1.85 m, 1.852 m, 2 m
rod, pole, perch (192 inch = 5.0292 m): 5 m
chain (4 rod = 20.1168 m): 20 m
cable (1/10 sea mile): 180 m, 185 m, 185.2 m, 200 m
furlong (10 chain = 201.168 m): 200 m
mile (8 furlong = 1.609344 km): 1.5 km, 1.6 km, 1.609 km, 1.61 km, 2 km
nautical mile (6080 foot = 1.853184 km): 1.8 km, 1.85 km, 1.852 km, 1.853 km, 1.9 km, 2 km; 1 equatorial minute, 1 meridian minute

2.2 Area

acre (4046.8564224 m²): 40 a, 40.5 a, 50 a = ½ ha

2.3 Volume, capacity

scruple (1/3 drachm ≅ 1.184 ml): 1 ml, 1.2 ml, 1.25 ml = 1¼ ml
drachm/dram (1/8 ounce ≅ 3.552 ml | 3.697 ml): 3.5 ml = 3½ ml, 3.7 ml, 3.75 ml = 3¾ ml
ounce (1/5 gill ≅ 28.413 ml | ¼ gill ≅ 29.574 ml): 25 ml, 27.5 ml, 28 ml, 28.5 ml, 30 ml
gill (¼ pint ≅ 142.065 | 118.294 ml): 100 ml, 120 ml, 140 ml, 150 ml
pint (¼ gallon ≅ 568.26 ml | 473.176 ml | 550.61 ml): 475 ml = 4¾ dl, 500 ml = ½ l, 550 ml = 5½ dl, 600 ml
quart (½ gallon ≅ 1.137 l | 0.946 l | 1.101 l): 1 l, 1.1 l
gallon (≅ 4.546 l | 3.785 l | 4.405 l): 3.8 l, 4 l, 4.4 l, 4.5 l, 5 l
(petrol) barrel (35 gallon ≅ 159,113 l | 42 gallon ≅ 158,987 l): 159 l, 160 l
peck (2 gallon ≅ 9.092 l | 8.810 l): 8.8 l, 9 l, 10 l
bushel (4 peck ≅ 36.368 l | 35.239 l): 35 l, 36 l, 40 l
quarter/hogshead (8 bushel ≅ 290.9 l | 281.9 l): 250 l ≅ volume of ¼ t water, 256 l, 280 l, 290 l, 300 l

teaspoon: 5 ml
tablespoon: 15 ml
cup: 225 ml, 240 ml, 250 ml = ¼ l

2.4 Mass, weight

grain (64.79891 mg): 50 mg
pennyweight (24 grain ≅ 1,555 g): 1.5 g, 2 g = 10 carat
dram (1⁄16 ounce ≅ 1,772 g): 1.5 g, 1.75 g, 2 g
ounce (1⁄16 pound ≅ 28,35 g | 20 pennyweight ≅ 31.103 g): 25 g, 28.5 g, 30 g | 30 g, 33 1/3 g
pound (7000 grain ≅ 453,592 g | 5760 grain ≅ 373.242 g): 450 g, 500 g | 350 g, 375 g, 400 g
stone (14 pound ≅ 6.35 kg): 5 kg, 6.25 kg
quarter (¼ hundredweight ≅ 11.34 kg | 12.701 kg): 12.5 kg
hundredweight (100 pound ≅ 45.359 kg | 50.802 kg): 45 kg, 50 kg
ton (20 hundredweight ≅ 0.907 t | 1.016 t): 0.9 t, 1 t

3 See also

Informal conversion of common units

Systems of units

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