Engineering an Empire: Roman Units of Linear Measurement (Part 1 of 3).

By Pat Lowinger

From the Iberian Peninsula of modern Spain, to the isles of Britain, across the shores of North Africa and to the eastern borders of the Empire, the Romans were united not only by the Pax Romana, but also a very precise system of measurements which spread uniformity in construction, trade, cartography and science to all corners of the Empire.  

The Romans have long been hailed by modern scholars for their ability to undertaken great feats of engineering and construction throughout the span of the Empire.  Whether it is the famous aqueducts, vast roadways or the grand monuments which dotted the landscape, Roman engineers and labors relied upon a precise system of measurement in order to bring civilization to a barbaric world.


Marcus Vipsanius Agrippa (62 -12 BCE).  A key Roman general, statesman and cartographer during the reigns of Julius Caesar and Augustus, Agrippa is believed to have standardized all units of measurement throughout the Roman Empire.

The Romans devised fourteen commonly referenced units for linear measurement (length).  The most basic unit was the pes (plural pedes) or Roman foot.  In terms of modern reference, the Roman foot measured 0.97 feet (296 mm).[1] One of the problems facing historians is finding those points of reference (objects) which the Romans themselves defined as being equivalent to one Roman foot.  While this may seem rather easy, there are numerous examples of objects purported as being the equivalent of one foot but varying slightly (albeit subtly) in length from other similar objects.[2] One of the most common objects used for reference were brass or wooden rods measuring 10 feet in length (decempeda).  Other commonly used measurements were the gradus (step) which measured 2.5 pedes and the passus which measured 5 pedes.  For measurements of less than one pedes the Romans commonly used the digitus (1/16 of a pedes), uncia (1/12 of a pedes) and palmus (1/4 of a pedes).  When it came to longer distances, the Roman mile (mille passus) was simply 1000 passus or 5000 pedes.  Again, despite its name, the Roman mile was not equal to its later Imperial counterpart, being only 0.92 miles (1.48 km).

An Army of Engineers

One of the more fascinating features of the Roman military was the inclusion of a cadre of trained engineers (architecti) within each legion.  These men were not civilians attached to the legion, but instead were legionary soldiers often recruited for their knowledge in arithmetic and geometry.  When in the field or on campaign the duty of these engineers was threefold.  The first was to oversee the construction of camps, buildings and bridges.  The second was to construct and maintain the legions siege engines (ballista, onagers) and towers as well as the construction of earthen siege ramps and investments (counter fortifications).  The third and possibly least glamorous duty of a Roman engineer was to measure the distance which the army had traveled, construct roads and map major geographical features of the Empire’s expanding territories.  Roman milliarium  (milestones) are a testament to the precision of Roman road engineers.[3]


The aqueduct (bridge) at Segovia in Roman Iberia (Spain) completed c. 100 AD

The Roman legions were the greatest military force of their day.  But their greatness was not only limited to their ability to make war.  The Roman army was a vital component in the construction of much of the Empire’s infrastructure.  One obvious example are the hundreds of aqueducts built during Rome’s history.  The aqueduct was a complex system which typically combined tunneling, piping and bridge building of monumental proportions.  The legions provided a ready source of trained labors and engineers which was combined with local artisans, laborers and slaves. Depending on the purpose and scale, projects could take years and sometimes decades to complete.  As a result, legions which had fought on the frontier were often tasked in the building of infrastructure necessary to promote Roman lifestyles as well as encouraging the relocation of Roman civilians to these areas.

The Odometer

As previously mentioned one of the most prolific activities of Roman military engineers was the construction and measurement of roads.  But how where accurate measurements made over long distances?  The repeated use of a measuring stick would have been ludicrous and although the use of a known length of rope may have seemed plausible the Romans needed a more rapid, accurate and reliable method of measuring a distance traveled by Rome’s armies.


Reconstruction of a typical Roman Odometer.  Not shown is the container in which the small stones would fall located under the central horizontal gear.

The answer as we now know it came down from the hands of a Roman engineer, named Marcus Vitruvius Pollio (Vitruvius).  During the 1st century BCE Vitruvius wrote extensively on the topics of engineering and architecture in his 10 volume work, De Architectura, in which he details a ‘mechanical cart for measuring distances’ – the odometer.  While most historians believe the concept of the odometer originated with the Greek inventor Archimedes of Syracuse during the 3rd century BCE, it is unknown if Archimedes had ever constructed the device or had only conceptualized it.[4] Although it appears Vitruvius was basing his invention (in whole or in part) upon the concepts of Archimedes, widespread utilization of the odometer helped reshape the Roman Empire.  No longer would the distances between to far removed places be given in crude estimations, instead, precise measurements (in mille or miles) were available not only to the military but also traders and travelers throughout the Empire.

The true genius of the odometer is its simplicity and reliability.  Basically, the odometer was cart which had mechanical gears which turned as the wheels of the cart (cartwheels) moved along the ground.  As the cartwheel completed a revolution (or a known number of multiple revolutions), a gear would trigger the release of a small round stone (pea-sized) which dropped into a bucket located under the ‘gear-box.’  Since the length of one revolution (or more) of the cartwheel could be easily measured and one stone had been placed in the bucket for each complete revolution of the cartwheel, the rest became a simple matter of mathematics.  So if for example at cartwheel made a complete revolution every five feet (passus) and deposited  one stone per revolution, if 120 stones where in the bucket, a distance of 600 feet (0.6 miles) had been traveled.  If instead one stone in the bucket equaled four revolutions (1:4) of the cartwheel, the same 120 stones whole have indicated a distance of 2400 feet (2.4 miles).  It was simply a matter of knowing the ratio of cartwheel revolutions to stones deposited by the odometers gear mechanism or what we commonly refer today as ‘gear-ratio.’

Part 2 will examine the units of measuring wet and dry goods and well as the effect that standardization of these measurements had on the economy of the Roman Empire.  


[1] Values of Roman lengths and measurements were taken from the 8th Chapter (Roman Weights and Measures) of Heinrich Glarean’s Books: The Intellectual World of a Sixteenth-Century Musical Humanist. Cambridge University Press, 2013.

[2] Several ‘colossal’ monuments were purported to have had measurements incorporated into their bases of a ‘true Roman foot’ which themselves show variation of 0.1 and 0.5%.

[3] It has been estimated that the Romans constructed approximately 400,000 km of roads throughout the Empire, of which 20-25% were stone-paved.   For more information see the works of Robert James Forbes- Studies in Ancient Technology (mult. volumes).

[4] Archimedes original work on the odometer is unknown to us but Plutarch does comment on Archimedes’ extensive work in the area of gears and gear-ratios.  Diagrams of many of Archimedes’ inventions were kept in the Great Library of Alexandria and subsequently lost to history.




About Patrick Lowinger

Patrick Lowinger holds a M.A. in Ancient and Classical History from American Military University (AMU) and B.S. in Microbiology (1993) from California State University, Long Beach.
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