What Is Metric?

A metric unit, when pertaining to length, is a system of measurement devised to eliminate the monotonous task of converting different units of length. For example, when adding up lengths in Imperial units, you would potentially have to convert Miles, Yards, Feet, Inches, and fractions of an inch before getting a total length sum.

In Metric, it is a simple matter of moving a decimal point. When measuring length in Metric, the terminologies used are: Millimeters (mm), Centimeters (cm), Decimeters (dm), Meters (m) and Kilometers (km), and converting from one metric unit to another is a simple matter of multiplying or dividing in multiples of 10 (e.g. 1m = 1,000mm or 100cm or 10dm).

In the Engineering and Architecture industries, only Meters and Millimeters are typically used, and this approach also applies to CadCARDs. There are only a small number of professions, such as meteorology and the fashion industry, that use Centimeters (and generally do not use AutoCAD), and practically no industry has ever used Decimeters.

Also, when Metric is being converted from Imperial units, it is common practice (with a few exceptions) to use the "soft conversion" method where Metric values are either rounded down or rounded up as needed. For example, there are 25.4mm in 1 inch, but the common method would be to express it simply as 25mm = 1 inch. Or, instead of saying 6" = 152.4mm, we would simply say 150mm (i.e. 6×25=150). [Note: if a project is done exclusively with Metric units, the soft conversion is generally used, but if Metric is to work in conjunction or as a continuation of an existing project originally done in Imperial units, then the more precise "hard" conversion will be used]

Sometimes Metric values are rounded up such as when using plotted Text Heights. On an Architectural drawing, a standard plotted text height of 3/32" would precisely translate to 2.38mm. There are no Drafting scales precise enough to measure the .38, so the text height would simply be rounded up to 2.50mm. In the old-style method of board drafting, there were technical pens and Metric lettering guides available using text heights such as 2.50mm.

However, it is common practice to use the precise conversion when dealing with ANSI or Architectural sheet sizes when converting to metric. This is to accommodate large rolls or single sheets of Imperial sized paper when used for plotting hard-copy drawings. The hard conversion also takes into account how AutoCAD translates Metric for its plotting parameters, i.e., a standard Architectural 24"x36" "D" size sheet would be precisely be converted to 609.6mm x 914.4mm.

"Millimeters" and "Meters" are the units used in Engineering and Architecture, and AutoCAD uses this practice as well in both the drawing and plotting environments.

Typically, the Units in AutoCAD are usually set to "Decimal" with a Precision set to 0 when drawing in Millimeters, and a precision of 0.000 when using Meters (or whatever precision the company or client standards dictate). Therefore, when dimensioning a detail in Millimeters, the dimension value may say "5000", whereas in a Meter drawing, the dimension value will say "5.000".

In typical AutoCAD practice, Millimeters are generally used for setting up title-sheets in the Layout environment (a.k.a. Paperspace) as well as drafting details and cross-sections (i.e. 1 AutoCAD unit = 1mm). Drawings done in Meters are generally reserved for large civil type Plan and Profile drawings and large floor plans (i.e. 1 AutoCAD unit = 1m).