The ancient Roman units of measurement were built on the Hellenic system with Egyptian, Hebrew, and Mesopotamian influences. The Roman units were comparatively consistent and well documented.
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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:[1]
| 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.[2] An accepted modern value is 296 mm.[3]
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 first century AD that the digitus was used in Campania and most parts of Italy.[4] The principal Roman units of length were:
| Roman unit | English name | Equal to | Metric equivalent | Description |
|---|---|---|---|---|
| digitus | finger | 1⁄16 pes | 18.5 mm | |
| uncia or pollex | inch or thumb | 1⁄12 pes | 24.6 mm | |
| palmus | palm width | 1⁄4 pes | 74 mm | |
| palmus major | palm length | 3⁄4 pes | 222 mm | in late times |
| pes | foot | 296 mm | ||
| palmipes | 11⁄4 pedes | 370 mm | ||
| cubitus | cubit | 11⁄2 pedes | 444 mm | |
| gradus or pes sestertius | step | 21⁄2 pedes | 0.74 m | |
| passus | (double) pace | 5 pedes | 1.48 m | |
| decempeda or pertica | 10 pedes | 2.96 m | ||
| actus (in length) | 120 pedes | 35.5 m | ||
| mille passuum or milliarium[5][6][7] | mile | 5000 pedes | 1.48 km | |
| Gallic leuga | league | 7500 pedes | 2.22 km | |
| Except where noted, based on Smith (1851).[2] Metric equivalents are approximate, converted at 1 pes = 296 mm. | ||||
The ordinary units of measurement of area were:
| Roman unit | English name | Equal to | Metric equivalent | Description |
|---|---|---|---|---|
| pes quadratus | square foot | 0.0876 m² | ||
| scrupulum or decempeda quadrata | 100 pedes | 8.76 m² | the square of the standard 10-foot measuring rod | |
| actus simplex | 480 pedes | 42.1 m² | 4 × 120 pedes[8] | |
| uncia | 2400 pedes | 210 m² | ||
| clima | 3600 pedes | 315 m² | 60 × 60 pedes[8] | |
| actus quadratus or acnua | 14400 pedes | 1262 m² | also called arpennis in Gaul[8] | |
| jugerum | 28800 pedes | 2523 m² | ||
| heredium | 2 jugera | 5047 m² | ||
| centuria | 200 jugera | 50.5 ha | formerly 100 jugera[8] | |
| saltus | 800 jugera | 201.9 ha | ||
| Except where noted, based on Smith (1851).[2] 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:
| 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[2] |
| 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).[8] Metric equivalents are approximate, converted at 1 pes = 296 mm. | ||||
Both liquid and dry measures were based on the sextarius. As no two surviving examples are identical, scholarly opinion ranges from 0.53 l[9] to 0.58 l.[10] Cardarelli gives a value 0.54928 l.[11]
Since the Romans themselves defined the sextarius as 1/48th 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 Roman jar, so-called "amphora quadrantal" is the cubic foot. The congius is a half-foot cubed.
| Roman unit | English name | Equal to | Metric equivalent | Description |
|---|---|---|---|---|
| ligula | 1⁄48 sextarius | 11.4 ml | ||
| cyathus | 1⁄12 sextarius | 45 ml | ||
| acetabulum | 1⁄8 sextarius | 68 ml | ||
| quartarius | 1⁄4 sextarius | 136 ml | ||
| hemina or cotyla | 1⁄2 sextarius | 273 ml | ||
| sextarius | 546 ml | a sixth of a congius | ||
| congius | 6 sextarii | 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).[2] Metric equivalents are approximate. | ||||
| Roman unit | English name | Equal to | Metric equivalent | Description |
|---|---|---|---|---|
| ligula | 1⁄48 sextarius | 11.4 ml | ||
| cyathus | 1⁄12 sextarius | 45 ml | ||
| acetabulum | 1⁄8 sextarius | 68 ml | ||
| quartarius | 1⁄4 sextarius | 136 ml | ||
| hemina or cotyla | 1⁄2 sextarius | 273 ml | ||
| sextarius | 546 ml | a sixth of a congius | ||
| semimodius | 8 sextarii | 4.36 l | ||
| modius | 16 sextarii | 8.73 l | ||
| Except where noted, based on Smith (1851).[2] Metric equivalents are approximate. | ||||
The units of weight (in the everyday sense of the word; technically, "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.[3][10][12]
The uncial divisions of the as or libra were:
| Roman unit | English name | Equal to | Metric equivalent | Description |
|---|---|---|---|---|
| uncia | Roman ounce | 1⁄12 libra | 27.4 g | |
| sescuncia or sescunx | 1⁄8 libra | 41.1 g | ||
| sextans | 1⁄6 libra | 54.8 g | ||
| quadrans or teruncius | 1⁄4 libra | 82.2 g | ||
| triens | 1⁄3 libra | 109.6 g | ||
| quincunx | 5⁄12 libra | 137.0 g | ||
| semis or semissis | 1⁄2 libra | 164.5 g | ||
| septunx | 7⁄12 libra | 191.9 g | ||
| bes or bessis | 2⁄3 libra | 219.3 g | ||
| dodrans | 3⁄4 libra | 246.7 g | ||
| dextans | 5⁄6 libra | 274.1 g | ||
| deunx | 11⁄12 libra | 301.5 g | ||
| as or libra | Roman pound | 328.9 g | ||
| Except where noted, based on Smith (1851).[2] Metric equivalents are approximate, converted at 1 libra = 328.9 g . | ||||
The subdivisions of the uncia were:
| Roman unit | English name | Equal to | Metric equivalent | Description |
|---|---|---|---|---|
| siliqua | 1⁄144 uncia | 0.19 g | ||
| obolus | 1⁄48 uncia | 0.57 g | ||
| scrupulum | 1⁄24 uncia | 1.14 g | ||
| semisextula | 1⁄12 uncia | 2.28 g | ||
| sextula | 1⁄6 uncia | 4.57 g | ||
| sicilicius | 1⁄4 uncia | 6.85 g | ||
| duella | 1⁄3 uncia | 9.14 g | ||
| semuncia | 1⁄2 uncia | 13.7 g | ||
| uncia | Roman ounce | 27.4 g | ||
| Except where noted, based on Smith (1851).[2] Metric equivalents are approximate, converted at 1 libra = 328.9 g . | ||||
The complicated Roman calendar was replaced by the Julian calendar in 45 BC. In the Julian calendar, an ordinary year is 365 days long, 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.
Our currently used 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.
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 the 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.
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 a 60th of a day, and a secunda equal to one 3600th of a day.
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