Ancient Roman units of length or distance
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The ancient Romans were sophisticated engineers, and consequently had welldefined 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  1^{1}/_{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
 ↑ Cardarelli, François (1998). Scientific Unit Conversion. Berlin, Heidelberg, New York: SpringerVerlag. ISBN 3540760229.