Roman Inches to Millimeters (uncia to mm)

1 Roman inch (uncia) equals 24.6 millimeters exactly.

1 Roman inch (uncia) = 24.6 millimeters

How to convert Roman inches to millimeters

To convert Roman inches to millimeters, multiply the number of Roman inches by 24.6. This factor is exact. To go the other way, multiply millimeters by 0.0406504065 (rounded).

millimeters = Roman inches × 24.6

Example: 10 Roman inches = 246 millimeters.

Approximate value. Standards for the Roman inch (uncia) varied by place and period, so this converter uses the most commonly cited value. Treat results as estimates, not exact figures.

About these units

Roman Inch (uncia) (uncia)

Ancient RomanHistorical unit (no longer in standard use)Approximate value

One twelfth of a Roman foot, about 2.46 cm. Its name, uncia, is the root of the English words 'inch' and 'ounce'.

Twelve of these made up a Roman foot, and that fraction is the whole meaning of the name: uncia is Latin for a twelfth. The same word was applied to weight, where it gave the ounce as a twelfth of the Roman pound. Because the foot is about 296 mm, the uncia works out to roughly 24.6 mm, the value Wikipedia gives, though that article notes the figure lacks a citation. Any difference in the foot carries straight through to its inch.

Latin also had pollex, meaning thumb, as a synonym for the length. Old English borrowed uncia as ynce, and a regular sound change turned it into the modern word inch, which is a cousin of ounce. The English inch kept the idea of a twelfth of a foot but was later fixed by international agreement at exactly 25.4 mm, so it is about 3 percent larger than the Roman original.

1 Roman inch (uncia) = 0.0246 meters (approximately).

Sources: Wikipedia: Roman inch; Wikipedia: Inch; Wikipedia: Ancient Roman units of measurement

Millimeter (mm)

Metric (SI)Current unit

One thousandth of a meter. Used in engineering, manufacturing and rainfall measurement.

Carl Friedrich Gauss put the millimeter at the base of his 1832 proposal for absolute units, pairing it with the milligram and the second, although the centimeter-based CGS system later won out among scientists. The name combines the metric prefix milli-, from the Latin for one thousand, with the meter. Since 1983 a millimeter has been tied to the speed of light: it is the distance light covers in 1/299,792,458,000 of a second.

Millimeters are the language of fine mechanical work. Standard rulers are divided into them, digital calipers commonly read to 0.01 mm, and mechanical pencil leads come in 0.5 and 0.7 mm. Copy paper is about 0.1 mm thick, and a thin hair, at 0.02 to 0.04 mm across, is close to the smallest thing the naked eye can see. A wave with a frequency of 300 GHz has a wavelength of 1 mm. An inch is exactly 25.4 mm. One trap for readers in the United States is that mil there traditionally means a thousandth of an inch, not a millimeter.

1 millimeter = 0.001 meters (exactly).

Sources: Wikipedia: Millimetre; Wikipedia: Centimetre-gram-second system of units; NIST Guide to the SI, Appendix B.8: conversion factors

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