Many units of measurement that are still in use today date back to the Romans. The British Pound, for example, derives from the Roman term Libra, which measured a pound of silver, and the £ sign still bears semblance to the L that the Romans used to signify a Libra.
As the Roman Empire expanded, a standardised or partially standardised measurement system was necessary to ensure that measurements across the empire could be easily managed and monitored.
Ancient Roman units of measurement were not created in a vacuum and were themselves derivative. Much of the Roman measurement system was based on ancient Greek systems, which were also heavily influenced by the Mesopotamian system. However, the Roman measurement systems were very well documented and consistent, which meant they outlasted and surpassed many of the systems that came before them.
We are going to take a look at ancient Roman units of measurement, what they were, what they are equivalent to, and whether we still use them at all today.
The Romans had sophisticated and consistent units of measurement. However, because many of the units were defined relative to one another or specific groupings (such as the area of a legion of soldiers), historians have had to approximate many of them, and there is still debate about the veracity of some of those approximations.
For this reason, many of the modern equivalencies we reference in this article should be taken as estimations rather than concrete conversions.
So let's now jump in and find out what units the Romans used to measure lengths.
Ancient Roman units of length
The standard Roman unit of length was called the "pes," - plural, "pedes" - which means foot. This was then later termed the "Roman foot."
In the 17th century, an Englishman called John Greaves visited Rome and investigated the true size of the Roman foot. He found that the foot on the statue of the great Roman architect Cossutius was the perfect measurement of the Roman foot.
The Roman foot could be divided into 16 digits or fingers, or 12 inches.
The Romans also developed the "mille," which was 1000 passus. A "passus," in turn, was 5 Roman feet. The modern mile derives from the mille and is only about 200 feet (the modern unit) from the original measurement.
| Unit | Roman equivalent | Modern equivalent (approx) | Notes |
| Mille (mille passus) | 1000 passus5,000 Roman feet | 4,854 modern ft. or 1,480 meters. | The Roman mile is the measurement the modern mile derives from. |
| Stadium (stadia) | 125 passus625 Roman feet | 607 ft. or 185 meters | Used as a measurement at sea. |
| Actus | 120 Roman feet | 116.5 ft. or 35.5 meters | Used for land surveying. An actus roughly translated as the distance oxen would plough before being turned. |
| Passus | 1/1000 mille or 2 gradus5 Roman feet. | 4.85 ft. or 1.48 meters | Roughly the 'pace' step of a single legionary. |
| Gradus (gradii) | 2.5 Roman feet0.5 passus | 2.43 ft. or .74 meters | Sometimes conceived as 1/2000th of a Roman mile. |
| Pes (pedes) | 12 unciae | 11.65 inches or 29.6 cm | Also known as a Roman foot. |
| Uncia (unciae) | Base unit | 0.97 inches or 24.6mm | Also known as a Roman inch. |
Ancient Roman units of weight
Ancient Roman weights were largely based on factors of 12. This was for division and multiplication purposes, as the three creases on each of the four fingers add up to make 12, which made working in factors of 12 simple.
Some of the names of the units of weight were also used as the names of coins during the Roman era. This is because weight was often used to determine the value of metals, such as silver, that were used as currency.
As we saw earlier, the modern British Pound (£) derives from the Roman unit "Libra," and so too does the modern pound weight (lb.).
Libra means balance and refers to the weighing scales used to balance the weights of metals as they were measured. This is why, to this day, the star sign Libra is depicted by a pair of balancing scales.
| Unit | Roman equivalent | Modern equivalent (approx) | Notes |
| Libra | 12 uncia | 3/4 pound or 336 grams. | Libra means balance, which refers to the balancing scales used to measure silver. The abbreviation for the modern pound weight (lb.) and the British pound (£) have roots directly to the Roman unit. |
| As | 6 uncia | 6 ounces or 168 grams. | Another base unit of the Roman coinage system. |
| Uncia | Base Unit | 1 ounce or 28 grams. | The modern ounce is derived from the Roman uncia. |
Ancient Roman units of volume
The standard ancient Roman unit of volume measurement was the sextarius. The sextarius is thought to have been roughly 0.646 litres.
The sextarius was used to measure both liquid and dry measurements. As with most of the Roman units of measurement, estimates of its exact size remain imprecise as no two surviving vessels measure to an equal volume.
| Unit | Roman equivalent | Modern equivalent (approx) | Notes |
| Amphora (amphorae) | 2 urnae | 6.8 gallons or 25.79 litres | Named after the large containers used to transport wine and other liquids on ships. |
| Urna (urnae) | 4 congii | 3.4 gallons or 12.8 litres | An urna is the root of the modern word "urn." |
| Modius (modii) | 16 sextarii | 2.4 gallons or 8.7 liters. For dry goods, roughly equal to a peck or bushel | Generally used to refer to dry goods, especially grain. |
| Congius (congii) | 12 heminae | 0.85 gallons or 3.2 litres. For dry goods, roughly 10 pounds | A unit of liquid measurement. 1/8 of an Amphora. |
| Sextarius (sextarii) | 2 heminae | 0.646 litres | The standard unit of liquid measurement. 1/6 of a Congius. |
| Hemina (heminae) | 24 ligulae | 0.27 litres | A unit of liquid measurement. 1/2 Sextarius. |
| Quartarius (quartarii) | 12 ligulae | 0.28 pints or 13 centilitres | 1/4 Sextarius |
| Ligula (ligulae) | Base unit | 0.34 fluid oz. or 1.14 centilitres | The smallest unit of Roman liquid measurement. |
Ancient Roman units of time
Although the Ancient Roman units of time were markedly different from ours, many of the divisions they developed and used form the basis of hourly, daily, and calendrical systems that we have today.
Hours
The Roman split daytimes into twelve horae (from which we get the word "hours") that started at sunrise and ended at sunset. The night was divided into four watches. A watch was a military unit that determined the period in which one soldier or one group of soldiers would keep watch whilst the others slept.
Because the divides were split relative to sunrise and sunset, the duration of horaes and watches varied depending on the season.
Roman astrologers divided the full solar day into 24 equal horaes, but this was not the timekeeping practice used by most common Romans.
Roman astrologers also had divisions of horaes that we derive many of our modern terms from to this day. For example, a minuta was equal to 1/60th of a day (24 minutes), and a secunda was equal to 1/3600th of a day (24 seconds).
Weeks
The Romans had eight-day weeks that were called the nundinae. Every eighth day was a market day.
Again, outside of the common practice, astrologers observed a seven-day week cycle, which they called a hebdomas.
In the hebdomas, each day was named after one of the seven classical planets, and we still use many of them today: Saturn-day, Sun-day, Moon-day, Mars-day, Mercury-day, Jupiter-day, and Venus-day.
It wasn't until the year 321 AD that Constantine the Great gave the people every Sunday off in honor of the sun. Thus began the common use of the seven-day week with one day of rest.
Years
Roman years ended in December and started in March, with January and February forming a winter period that was not specified by days. Hence, September was the seventh month (sept), October was the eighth (oct), November the ninth (nov), and December the tenth (dec).
This system was then replaced by the Julian calendar, named after Julius Ceasar, which demarcated years within 365 days and leap years within 366 days.
Roman years were rarely referred to in terms of numbers. Instead, they were named after the consuls for that year. When a year number was required, the Roman used the number of years since the founding of Rome. It wasn't until the Middle Ages that the modern system of the Anno Domini (AD) count was adopted.
The Romans are well known for their engineering, military, and technological advancements, but they also had highly developed and clearly defined measurement units that lay the foundations of many of their other successes. As the Roman Empire grew, so did the need for a sophisticated measurement system that could be used across every corner of its land.
Although the exact size of most of the Roman units of measurement is still hotly debated amongst historians, many of their names and what they represent form the basis of the measurement systems we use today.
