Hectometers to Millimeters (hm to mm)

1 hectometer equals 100,000 millimeters exactly.

1 hectometer = 100,000 millimeters

How to convert hectometers to millimeters

To convert hectometers to millimeters, multiply the number of hectometers by 100,000. This factor is exact. To go the other way, multiply millimeters by 0.00001.

millimeters = hectometers × 100,000

Example: 10 hectometers = 1,000,000 millimeters.

About these units

Hectometer (hm)

Metric (SI)Current unit

100 meters. An official SI unit, rarely used.

Hecto, the prefix for one hundred, dates to 1795 in Wikipedia's table of metric prefixes, which makes the hectometer (hm) a contemporary of the original French system. It equals 100 meters, one tenth of a kilometer, or about 328 feet. English speakers rarely use it, and the Wikipedia article flags even that claim as lacking a citation.

Its best-known appearance is hidden in another word. The hectare is a square hectometer, 10,000 square meters, so every land listing in hectares quietly rests on this length. A football pitch is about one hectometer long, and the Great Pyramid of Giza, at 137.7 m, stands about 1.377 hm high. Military writers have occasionally given target ranges in hectometers, with Wikipedia citing a 1927 Field Artillery Journal among its examples. The unit sits between the decameter at 10 m and the kilometer at 1,000 m.

1 hectometer = 100 meters (exactly).

Sources: Wikipedia: Hectometre; Wikipedia: Metric prefix; NIST: metric (SI) prefixes

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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