

The passus (meaning pace, but understood as a double pace, i. e., measured from one foot hit the ground to where the same foot hit the ground again) was an ancient Roman unit of length or distance.
Value in terms of modern unitsEdit
It is not possible, in discussing ancient Roman standards, to distinguish base units from subsidiary units, because no actual Roman standards are definitively known by the present day. Therefore, 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, 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, leading to the treatment of the Roman mile as the base unit, and others as subsidiary units. Thus the passus can be thought of as a derived unit, equal to ^{1}/_{1000} of the Roman mile, and therefore equal to 1.472 m = 1.6098 yd = 0.0009 mi.