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Ancient Roman units of length or distance

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This page describes an obsolete unit.

The unit described on this page was in use prior to modern methods of precision measurement. It was based on a standard which is no longer available, and which is not capable of being calibrated against modern measurement units. Therefore, the equivalence to modern SI units or to current United States customary units can only be considered approximate.



The ancient Romans were sophisticated engineers, and consequently had well-defined units of measuring lengths and distances, and their relationships to each other are well known. However, no actual Roman standards are definitively known by the present day, so that the only way of determining the length of any Roman unit would be to measure something in modern terms whose length was given by the Romans in their units. The Romans were in the habit of putting mileposts on their roads, and these roads have, at least in some cases, survived to the present day, and thus a measurement of the distances they indicate (though requiring statistical treatment to allow for the inaccuracies of the Romans' own measurements) can be used to calculate the length of one unit, the Roman mile. (In fact, although the term Roman mile, or simply mile when the reference to Ancient Roman standards is obvious, is often used to translate the Latin mille passuum, it is not at all clear that one can consider this to be an actual Roman unit of length or distance. The Latin phrase simply means “1000 paces,” so that a distance given as “septem milia passuum,” though usually translated as “seven miles,” might with more accuracy be translated as “seven thousand paces.”) The measurements of the distances indicated by Roman mileposts give the best estimate of the Roman mile as 1472 m = 1609.799 yd = 0.915 mi[1], and this distance will be adopted on this wiki as the basis for interpreting all other Ancient Roman units of length or distance, as their relation to the mile is known.

Table of equivalencesEdit

Name of unit Value in terms of smaller units Value in terms of the meter Value in terms of United States customary measure
digitus 0.0184 0.7244 in.
uncia 1⅓ digitī 0.024533 0.9659 in.
palmus 3 unciae 0.0736 2.8976 in. = 0.2415 ft
pēs 4 palmī 0.2944 11.5906 in. = 0.9659 ft
palmipēs 1¼ pedēs 0.368 1.2073 ft = 0.4024 yd
cubitus 11/5 palmipedēs 0.4416 1.4488 ft = 0.4829 yd
gradus 1⅔ cubitī 0.736 2.4147 ft = 0.8049 yd
passus 2 gradūs 1.472 1.6098 yd = 0.0009 mi
decempeda 2 passūs 2.944 3.22 yd = 0.002 mi
actus 12 decempedae 35.328 38.635 yd = 0.022 mi
mile (mille passuum) 41⅔ actūs 1472 1609.799 yd = 0.915 mi

Additional relationshipsEdit

It may be noted that, while this table shows each unit in terms of the next smaller one, it is probably more convenient to think of the palmipēs as 5 palmī and the cubitus as 6 palmī than the equivalences shown in the table; similarly, the gradus can better be thought of as 2 palmipedēs, and the Roman mile was certainly thought of as 1000 passūs, as the name actually indicated.

ReferencesEdit

  1. Cardarelli, François (1998). Scientific Unit Conversion. Berlin, Heidelberg, New York: Springer-Verlag. ISBN 3-540-76022-9. 

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