The units of measurement of ancient Rome were generally consistent and well documented.
Length
The basic unit of Roman linear measurement was the (plural: ) 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 valuein English feet MetricequivalentFoot on the statue of Cossutius0.967 Foot on the monument of Statilius0.972 Foot of Villalpandus, derived from Congius of Vespasian0.986
William Smith (1851) gives a value of 0.9708 English feet, or about 295.9 mm. An accepted modern value is 296 mm. That foot is also called the pes monetalis to distinguish it from the pes Drusianus (about 333 or 335 mm) sometimes used in some provinces, particularly Germania Inferior.
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 Englishname Equalto Metricequivalent Imperialequivalent Notes digitus finger pes18.5 mm 0.728 in 0.0607 ft unciapollex inchthumb pes24.6 mm 0.971 in 0.0809 ft palmus (minor) palm pes74 mm 0.243 ft palmus maior palm length ("greater palm") pes222 mm 0.728 ft in late times pes (plural: pedes) (Roman) foot 1 pes296 mm 0.971 ft sometimes distinguished as the pes monetalis palmipes foot and a palm pedes370 mm 1.214 ft cubitum cubit pedes444 mm 1.456 ft graduspes sestertius step pedes0.74 m 2.427 ft passus pace 5 pedes1.48 m 4.854 ft decempedapertica perch 10 pedes2.96 m 9.708 ft path, track 120 pedes35.5 m 116.496 ft 24 passus or 12 decembeda stadium stade 625 pedes185 m 607.14 ft 600 Greek feet or 125 passus or milleEquivalent to the English cable (600 feet) or furlong ( mile) mille passusmille passuum (Roman) mile 5,000 pedes1.48 km 4,854 ft 0.919 mi 1000 passus or 8 stadia leugaleuca (Gallic) league 7,500 pedes2.22 km 7,281 ft 1.379 mi 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 ) and in the name of the Nubian land of Triacontaschoenus between the First and Second Cataracts on the Nile (where it had a value closer to ).
Area
The ordinary units of measurement of area were: + Ancient Roman units of area Roman unit Englishname Equalto Metricequivalent Imperialequivalent Description pes quadratus square foot 1 pes qu. 0.0876 m 0.943 sq ft scrupulum or decempeda quadrata 100 pedes qu. 8.76 m 94.3 sq ft the square of the standard 10-foot measuring rod actus simplex 480 pedes qu. 42.1 m 453 sq ft 4 × 120 pedes uncia 2,400 pedes qu. 210 m 2,260 sq ft clima 3,600 pedes qu. 315 m 3,390 sq ft 60 × 60 pedes actus quadratus or acnua 14,400 pedes qu. 1,262 m 13,600 sq ft also called arpennis in Gaul jugerum 28,800 pedes qu. 2,523 m 27,200 sq ft 0.623 acres heredium 2 jugera 5,047 m 54,300 sq ft 1.248 acres centuria 200 jugera 50.5 ha 125 acres formerly 100 jugera saltus 800 jugera 201.9 ha 499 acres modius 16 ha 40 acres Medieval Latin, plural modii 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 ) 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 Romanunit Romansquare feet Fractionof jugerum Metricequivalent Imperialequivalent Description dimidium scrupulum 50 4.38 m 47.1 sq ft scrupulum 100 8.76 m 94.3 sq ft duo scrupula 200 17.5 m 188 sq ft sextula 400 35.0 m 377 sq ft sicilicus 600 52.6 m 566 sq ft semiuncia 1,200 105 m 1,130 sq ft uncia 2,400 210 m 2,260 sq ft sextans 4,800 421 m 4,530 sq ft quadrans 7,200 631 m 6,790 sq ft triens 9,600 841 m 9,050 sq ft quincunx 12,000 1,051 m 11,310 sq ft semis 14,400 1,262 m 15,380 sq ft = actus quadratus septunx 16,800 1,472 m 15,840 sq ft bes 19,200 1,682 m 18,100 sq ft dodrans 21,600 1,893 m 20,380 sq ft dextans 24,000 2,103 m 22,640 sq ft deunx 26,400 2,313 m 24,900 sq ft jugerum 28,800 1 2,523 m 27,160 sq ft Except where noted, based on Millar (1745). Metric equivalents are approximate, converted at 1 pes = 296 mm.
Volume
Both liquid and dry volume measurements were based on the . The sextarius was defined as of a cubic (Roman foot), known as an . Using the value for the Roman foot, an amphora quadrantal can be computed at approximately , so a sextarius (by the same method) would theoretically measure , which is about 95% of an imperial pint ().
Archaeologically, however, the evidence is not as precise. No two surviving vessels measure an identical volume, and scholarly opinion on the actual volume ranges between .
The core volume units are: amphora quadrantal (Roman jar) – one cubic pes (Roman foot) congius – a half-pes cube (thus amphora quadrantal) sextarius – literally of a congius
Liquid measure + Ancient Roman liquid measures Roman unit Equal to Metric Imperial US fluid ligula congius 11.4 mL 0.401 fl oz 0.385 fl oz cyathus congius 45 mL 1.58 fl oz 1.52 fl oz acetabulum congius 68 mL 2.39 fl oz 2.30 fl oz quartarius congius 136 mL 4.79 fl oz 4.61 fl oz or cotyla congius 273 mL 9.61 fl oz 9.23 fl oz sextarius congius 546 mL 19.22 fl oz0.961 pt 18.47 fl oz1.153 pt congius 1 congius 3.27 L 5.75 pt0.719 gal 3.46 qt0.864 gal urna 4 congii 13.1 L 2.88 gal 3.46 gal amphora quadrantal 8 congii 26.2 L 5.76 gal 6.92 gal culeus 160 congii 524 L 115.3 gal 138.4 gal Except where noted, based on Smith (1851).Modern equivalents are approximate.
Dry measure + Ancient Roman dry measures Roman unit Equal to Metric Imperial US dry ligula congius 11.4 ml 0.401 fl oz 0.0207 pt cyathus congius 45 ml 1.58 fl oz 0.082 pt acetabulum congius 68 ml 2.39 fl oz 0.124 pt quartarius congius 136 ml 4.79 fl oz 0.247 pt hemina or cotyla congius 273 ml 9.61 fl oz 0.496 pt sextarius congius 546 ml 19.22 fl oz0.961 pt 0.991 pt semimodius congii 4.36 L 0.96 gal 0.99 gal modius congii 8.73 L 1.92 gal 1.98 gal modius castrensis 4 congii 12.93 LDominic Rathbone, "Earnings and Costs: Living Standards and the Roman Economy (First to Third Centuries AD), p. 301, in Alan Bowman and Andrew Wilson, Quantifying the Roman Economy: Methods and Problems. 2.84 gal 2.94 gal Except where noted, based on Smith (1851).Modern 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 with 5076 grains or 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 131 BC Lex Papiria...
The divisions of the libra were: +Uncial divisions of the libra Roman unit Englishname Equalto Metricequivalent Imperialequivalent Description uncia Roman ounce libra 27.4 g 0.967 oz "a twelfth" sescuncia or sescunx libra 41.1 g 1.45 oz "one and one-half twelfths" sextans libra 54.8 g 1.93 oz "a sixth" quadransteruncius libra 82.2 g 2.90 oz "a fourth" "triple twelfth" triens libra 109.6 g 3.87 oz "a third" quincunx libra 137.0 g 4.83 oz "five-twelfths". semis or semissis libra 164.5 g 5.80 oz "a half" septunx libra 191.9 g 6.77 oz "seven-twelfths"bes or bessis libra 219.3 g 7.74 oz "two [parts] of an as" dodrans libra 246.7 g 8.70 oz "less a fourth" dextans libra 274.1 g 9.67 oz "less a sixth" deunx libra 301.5 g 10.64 oz "less a twelfth" libra Roman poundlibra 328.9 g 11.60 oz 0.725 lb "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 Englishname Equalto Metricequivalent Imperialequivalent Description siliqua carat uncia 0.19 g 2.9 gr 0.0067 oz "carob seed"The Greek (kerátion) obolus obolus. uncia 0.57 g 8.8 gr 0.020 oz "obol", from the Greek word for "metal spit" scrupulum scruple uncia 1.14 g 17.6 gr 0.040 oz "small pebble". semisextula or dimidia sextula uncia 2.28 g 35.2 gr 0.080 oz "half-sixth", "little sixth" sextula sextula uncia 4.57 g 70.5 gr 0.161 oz "little sixth". sicilicus or siciliquus uncia 6.85 g 106 gr 0.242 oz "little sickle" duella uncia 9.14 g 141 gr 0.322 oz "little double [sixths]" semuncia half-ouncesemuncia uncia 13.7 g 211 gr 0.483 oz "half-twelfth". uncia Roman ounce 27.4 g 423 gr 0.967 oz "a twelfth" 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 AD 1, leap years occurred at irregular intervals. Starting in AD 4, 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. (As consuls' terms latterly ran from January to December, this eventually caused January, rather than March, to be considered the start of the 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, based on the supposed birth year of Jesus.
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 (i.e. 97 leap years in every 400) to more closely approximate the length of the tropical year.
Weeks The Romans grouped days into an eight-day cycle called the , with every eighth day being a market day.
Independent of the , astrologers kept a seven-day cycle called a hebdomas 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, Jupiter-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 begins at sunset. Christians followed the Jewish seven-day week, except that they commonly called the first day of the week the , or the Lord's day. In 321, Constantine the Great gave his subjects every Sunday off, thus cementing the seven-day week into Roman civil society.
Hours