The units of measurement of ancient Rome were generally consistent and well documented.

The basic unit of Roman linear measurement was the pes (plural: pedes) 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]

William Smith (1851) gives a value of 0.9708 English feet, or about 295.9 mm.^[2] An accepted modern value is 296 mm.^[3] 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.^[4][5]

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.^[6] The principal Roman units of length were:

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 or 3 miles)^[8][9] 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 10.5 km or 6+1⁄2 miles).^[10][11]

The ordinary units of measurement of area were:

Other units of area described by Columella in his De Re Rustica include the porca of 180 × 30 Roman feet (about 473 m² or 5,090 sq ft) used in Hispania Baetica and the Gallic candetum or cadetum of 100 feet^{[clarification needed]} 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:

Both liquid and dry volume measurements were based on the sextarius. The sextarius was defined as 1⁄48 of a cubic foot, known as an amphora quadrantal. Using the value 296 mm (11.7 in) for the Roman foot, an amphora quadrantal can be computed at approximately 25.9 L (6.8 US gal), so a sextarius (by the same method) would theoretically measure 540.3 ml (19.02 imp fl oz; 18.27 US fl oz), which is about 95% of an imperial pint (568.26125 ml).

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 500 ml (17 US fl oz)^[14] and 580 ml (20 US fl oz).^[15]

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

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 g (11.4 to 11.6 oz) with 5076 grains or 328.9 g (11.60 oz) an accepted figure.^[3][15][17] 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.^[18][19]

The divisions of the libra were:

The subdivisions of the uncia were:

A number of special symbols for Roman currency were added to the Unicode Standard version 5.1 (April 2008) as the Ancient Symbols block (U+10190–U+101CF, in the Supplementary Multilingual Plane ).

As mentioned above, the names for divisions of an as coin (originally one libra of bronze) were also used for divisions of a libra, and the symbols U+10190–U+10195 are likewise also symbols for weights:

• U+10190 (𐆐): Sextans
• U+10191 (𐆑): Uncia
• U+10192 (𐆒): Semuncia
• U+10193 (𐆓): Sextula
• U+10194 (𐆔): Semisextula
• U+10195 (𐆕): Siliqua

• Ancient Egyptian units
• Ancient Greek units
• Biblical and Talmudic units of measurement
• Byzantine units
• History of measurement