Ancient Roman units of measurement

Bronze modius measure (4th century AD) with inscription acknowledging Imperial regulation of weights and measures

The ancient Roman units of measurement were largely built on the Hellenic system, which in turn was built upon Egyptian and Mesopotamian influences. The Roman units were comparatively consistent and well documented.

Length

The basic unit of Roman linear measurement was the pes or Roman foot. Investigation of its relation to the English foot goes back at least to 1647, when John Greaves published his Discourse on the Romane foot. Greaves visited Rome in 1639, and measured, among other things, the foot measure on the tomb of Titus Statilius Aper, that on the statue of Cossutius formerly in the gardens of Angelo Colocci, the congius of Vespasian previously measured by Villalpandus, a number of brass measuring-rods found in the ruins of Rome, the paving-stones of the Pantheon and many other ancient Roman buildings, and the distance between the milestones on the Appian Way. He concluded that the Cossutian foot was the "true" Roman foot, and reported these values compared to the iron standard of the English foot in the Guildhall in London:

Values of the ancient Roman foot determined by Greaves in 1639

Source	Reported value in English feet	Metric equivalent
Foot on the statue of Cossutius	0.967	294.7 mm
Foot on the monument of Statilius	0.972	296.3 mm
Foot of Villalpandus, derived from Congius of Vespasian	0.986	300.5 mm
Metric equivalents are approximate.

Smith (1851) gives a value of 0.9708 English feet, or about 295.9 mm. An accepted modern value is 296 mm.

The Roman foot was sub-divided either like the Greek pous into 16 digiti or fingers; or into 12 unciae or inches. Frontinus writes in the 1st century AD that the digitus was used in Campania and most parts of Italy. The principal Roman units of length were:

Ancient Roman units of length

Roman unit	English name	Equal to	English equivalent	Metric equivalent	Notes
digitus	finger	​1⁄16 pes	0.728 in  0.0607 ft	18.5 mm
uncia pollex	inch thumb	​1⁄12 pes	0.971 in  0.0809 ft	24.6 mm
palmus	palm width	​1⁄4 pes	0.243 ft	74 mm
palmus major	palm length	​3⁄4 pes	0.728 ft	222 mm	in late times
pes	(Roman) foot	1 pes	0.971 ft	296 mm
palmipes		​1 1⁄4 pedes	1.214 ft	370 mm
cubitus	cubit	​1 1⁄2 pedes	1.456 ft	444 mm
gradus pes sestertius	step	​2 1⁄2 pedes	2.427 ft	0.74 m
passus	pace	5 pedes	4.854 ft	1.48 m
decempeda pertica	perch	10 pedes	9.708 ft	2.96 m
actus (length)		120 pedes	116.496 ft	35.5 m
stadium	stade	625 pedes	607.14 ft	185 m	600 Greek feet Equivalent to the English cable
mille passus mille passuum	(Roman) mile	5000 pedes	4854 ft  0.919 mi	1.48 km
leuga	(Gallic) league	7500 pedes	7281 ft  1.379 mi	2.22 km
Except where noted, based on Smith (1851). English and Metric equivalents are approximate, converted at 1 pes = 0.9708 English feet and 296 mm respectively.

Other units include the schoenus (from the Greek for "rush rope") used for the distances in Isidore of Charax's Parthian Stations (where it had a value around 5 km) and in the name of the Nubian land of Triakontaschoenus between the First and Second Cataracts on the Nile (where it had a value closer to 10.5 km).

Area

The ordinary units of measurement of area were:

Ancient Roman units of area

Roman unit	English name	Equal to	Metric equivalent	Description
pes quadratus	square foot	1 pes qu.	0.0876 m2
scrupulum or decempeda quadrata		100 pedes qu.	8.76 m2	the square of the standard 10-foot measuring rod
actus simplex		480 pedes qu.	42.1 m2	4 × 120 pedes
uncia		2400 pedes qu.	210 m2
clima		3600 pedes qu.	315 m2	60 × 60 pedes
actus quadratus or acnua		14400 pedes qu.	1262 m2	also called arpennis in Gaul
jugerum		28800 pedes qu.	2523 m2
heredium		2 jugera	5047 m2
centuria		200 jugera	50.5 ha	formerly 100 jugera
saltus		800 jugera	201.9 ha
Except where noted, based on Smith (1851). Metric equivalents are approximate, converted at 1 pes = 296 mm.

Other units of area described by Columella in his De Re Rustica include the porca of 180 × 30 Roman feet (about 473 m²) used in Hispania Baetica and the Gallic candetum or cadetum of 100 feet in the city or 150 in the country. Columella also gives uncial divisions of the jugerum, tabulated by the anonymous translator of the 1745 Millar edition as follows:

Uncial divisions of the jugerum

Roman unit	Roman square feet	Fraction of jugerum	Metric equivalent	Description
dimidium scrupulum	50	​1⁄576	4.38 m2
scrupulum	100	​1⁄288	8.76 m2
duo scrupula	200	​1⁄144	17.5 m2
sextula	400	​1⁄72	35.0 m2
sicilicus	600	​1⁄48	52.6 m2
semiuncia	1200	​1⁄24	105 m2
uncia	2400	​1⁄12	210 m2
sextans	4800	​1⁄6	421 m2
quadrans	7200	​1⁄4	631 m2
triens	9600	​1⁄3	841 m2
quincunx	12000	​5⁄12	1051 m2
semis	14400	​1⁄2	1262 m2	= actus quadratus
septunx	16800	​7⁄12	1472 m2
bes	19200	​2⁄3	1682 m2
dodrans	21600	​3⁄4	1893 m2
dextans	24000	​5⁄6	2103 m2
deunx	26400	​11⁄12	2313 m2
jugerum	28800	1	2523 m2
Except where noted, based on Millar (1745). Metric equivalents are approximate, converted at 1 pes = 296 mm.

Volume

Both liquid and dry measures were based on the sextarius. As no two surviving examples are identical, scholarly opinion ranges from 0.53 l to 0.58 l. Cardarelli gives a value 0.54928 l. A 1952 estimate for its value in Pliny the Elder's Natural History estimated it as 500 ml.

Since the Romans themselves defined the sextarius as ​¹⁄₄₈ of an amphora quadrantal, and the amphora quadrantal as one cubic foot, assuming a value of 296 mm for the Roman foot yields a theoretical value for the sextarius of about 540.3 ml, which falls comfortably within the accepted range.

The core volume units are:

• amphora quadrantal (Roman jar) – one cubic pes (Roman foot)
• congius – a half-pes cube (thus ​¹⁄₈ amphora quadrantal)
• sextarius – literally ​¹⁄₆, of a congius

Liquid measures

Ancient Roman liquid measures

Roman unit	Equal to	Metric
ligula	​1⁄288 congius	11.4 ml
cyathus	​1⁄72 congius	45 ml
acetabulum	​1⁄48 congius	68 ml
quartarius	​1⁄24 congius	136 ml
hemina or cotyla	​1⁄12 congius	273 ml
sextarius	​1⁄6 congius	546 ml
congius	1 congius	3.27 l
urna	4 congii	13.1 l
amphora quadrantal	8 congii	26.2 l
culeus	160 congii	524 l
Except where noted, based on Smith (1851). Metric equivalents are approximate.

Dry measures

Ancient Roman dry measures

Roman unit	Equal to	Metric
ligula	​1⁄288 congius	11.4 ml
cyathus	​1⁄72 congius	45 ml
acetabulum	​1⁄48 congius	68 ml
quartarius	​1⁄24 congius	136 ml
hemina or cotyla	​1⁄12 congius	273 ml
sextarius	​1⁄6 congius	546 ml
semimodius	​1 1⁄3 congii	4.36 l
modius	​2 2⁄3 congii	8.73 l (or 8.62 l)
modius castrensis	4 congii	(12.93 l?)
Except where noted, based on Smith (1851). Metric equivalents are approximate.

Weight

The units of weight or mass were mostly based on factors of 12. Several of the unit names were also the names of coins during the Roman Republic and had the same fractional value of a larger base unit: libra for weight and as for coin. Modern estimates of the libra range from 322 to 329 grams (11.4 to 11.6 oz) with 5076 grains or 328.9 grams (11.60 oz) an accepted figure. The as was reduced from 12 ounces to 2 after the First Punic War, to 1 during the Second Punic War, and to half an ounce by the 191 BC Lex Papiria.

The divisions of the libra were:

Uncial divisions of the libra

Roman unit	English name	Equal to	Metric equivalent	Description
uncia	Roman ounce	​1⁄12 libra	27.4 g	lit. "a twelfth"
sescuncia or sescunx		​1⁄8 libra	41.1 g	lit. "​1 1⁄2 twelfths"
sextans		​1⁄6 libra	54.8 g	lit. "a sixth"
quadrans teruncius		​1⁄4 libra	82.2 g	lit. "a fourth" lit. "triple twelfth"
triens		​1⁄3 libra	109.6 g	lit. "a third"
quincunx		​5⁄12 libra	137.0 g	lit. "five-twelfths"
semis or semissis		​1⁄2 libra	164.5 g	lit. "a half"
septunx		​7⁄12 libra	191.9 g	lit. "seven-twelfths"
bes or bessis		​2⁄3 libra	219.3 g	lit. "two [parts] of an as"
dodrans		​3⁄4 libra	246.7 g	lit. "less a fourth"
dextans		​5⁄6 libra	274.1 g	lit. "less a sixth"
deunx		​11⁄12 libra	301.5 g	lit. "less a twelfth"
libra	Roman pound libra		328.9 g	lit. "balance"
Except where noted, based on Smith (1851). Metric equivalents are approximate, converted at 1 libra = 328.9 g .

The subdivisions of the uncia were:

Subdivisions of the uncia

Roman unit	English name	Equal to	Metric equivalent	Description
siliqua	carat	​1⁄144 uncia	0.19 g	lit. "carob seed" The Greek κεράτιον (kerátion)
obolus	obolus	​1⁄48 uncia	0.57 g	lit. "obol", from the Greek word for "metal spit"
scrupulum	scruple	​1⁄24 uncia	1.14 g	lit. "small pebble"
semisextula		​1⁄12 uncia	2.28 g	lit. "half-little sixth"
sextula	sextula	​1⁄6 uncia	4.57 g	lit. "little sixth"
sicilicus siciliquus		​1⁄4 uncia	6.85 g	lit. "little sickle"
duella		​1⁄3 uncia	9.14 g	lit. "little double [sixths]"
semuncia	half-ounce semuncia	​1⁄2 uncia	13.7 g	lit. "half-twelfth"
uncia	Roman ounce		27.4 g
Except where noted, based on Smith (1851). Metric equivalents are approximate, converted at 1 libra = 328.9 g .

Time

Years

The complicated Roman calendar was replaced by the Julian calendar in 45 BC. In the Julian calendar, an ordinary year is 365 days long, and a leap year is 366 days long. Between 45 BC and 1 AD, leap years occurred at irregular intervals. Starting in the year 4 AD, leap years occurred regularly every four years. Year numbers were rarely used; rather, the year was specified by naming the Roman consuls for that year. When a year number was required, the Greek Olympiads were used, or the count of years since the founding of Rome, "ab urbe condita" in 753 BC. In the middle ages, the year numbering was changed to the Anno Domini count.

The calendar used in most of the modern world, the Gregorian calendar, differs from the Julian calendar in that it skips three leap years every four centuries to more closely approximate the length of the tropical year.

Weeks

The Romans grouped days into an eight-day cycle called a nundina, with every eighth day being a market day.

Independent of the nundinae, astrologers kept a seven-day cycle called a hebdomada where each day corresponded to one of the seven classical planets, with the first day of the week being Saturn-day, followed by Sun-day, Moon-day, Mars-day, Mercury-day, Jove-day, and lastly Venus-day. Each astrological day was reckoned to begin at sunrise. The Jews also used a seven-day week, which began Saturday evening. The seventh day of the week they called Sabbath; the other days they numbered rather than named, except for Friday, which could be called either the Parasceve or the sixth day. Each Jewish day was reckoned to begin at sunset. Christians followed the Jewish seven-day week, except that they commonly called the first day of the week the Dominica, or the Lord's day. In 321 Constantine the Great gave his subjects every Sunday off in honor of his family's tutelary deity, the Unconquered Sun, thus cementing the seven-day week into Roman civil society.

Hours

Main article: Roman timekeeping

The Romans divided the daytime into twelve horae or hours starting in the morning and ending in the evening. The night was divided into four watches. The duration of these hours varied with seasons; in the winter, when the daylight period was shorter, its 12 hours were correspondingly shorter and its four watches were correspondingly longer.

Astrologers divided the solar day into 24 equal hours, and these astrological hours became the basis for medieval clocks and our modern 24-hour mean solar day.

Although the division of hours into minutes and seconds did not occur until the middle ages, ancient astrologers had a minuta equal to ​¹⁄₆₀ of a day (24 modern minutes), and a secunda equal to ​¹⁄₃₆₀₀ of a day (24 modern seconds).

Unicode

Main article: Ancient Symbols (Unicode block)

Ancient Roman units of measurement were added to the Unicode Standard in April, 2008 with the release of version 5.1.

The Unicode block for ancient Roman units of measurement, called Ancient Symbols, is U+10190–U+101CF, i.e. in the upper plane:

Ancient Symbols Official Unicode Consortium code chart (PDF)

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

U+1019x

𐆐

𐆑

𐆒

𐆓

𐆔

𐆕

𐆖

𐆗

𐆘

𐆙

𐆚

𐆛

U+101Ax

𐆠

U+101Bx

U+101Cx

Notes 1.^ As of Unicode version 8.0 2.^ Grey areas indicate non-assigned code points

See also

• Greek units
• Byzantine units
• History of measurement

External links

• Proposal to Add Ancient Roman Weights and Monetary Signs to UCS (Universal Character Set)