SI Metric Practice

SI Metric Practice

The Metric Conversion Act of 1975 commits the conversion of the United States to the International System of Units, universally abbreviated SI (from Le Système International d’Unités). The ever increasing use of the SI units requires the establishment of metric documentation practices to which this section is devoted.

Reference Documents

    ISO 1000: 1992 – SI units and recommendations for the use of their multiples and of certain other units.
    ISO 31:1992 – Quantities and units.
    ASTM E380 – Standard for Metric Practice.
    DOD-STD-1476 – Metric System, Application in New Design.
    IEEE 268 – IEEE Standard for Metric Practice.

SI Metric Units


  1. Base units
  2. Supplementary units
  3. Derived units

SI Drawing Practice

The units of measure to be used on drawings are the SI units and their multiples and submultiples. The most common ones are listed below. Included are conversions rounded to three significant digits.

Metric Threads

Metric screw threads are designated by the letter M followed by the diameter in millimeters, the letter x, and the pitch in millimeters.

Preferred Metric Sizes

This section defines the preferred metric sizes for engineering applications. Basic feature sizes should be selected where possible for the first choice sizes shown in the table.

Preferred Metric Limits and Fits

This section describes the ISO system of limits and fits for mating parts and identifies the designation symbols used to define specific dimensional limits on engineering drawings, material, stock, etc. It also identifies the preferred basic sizes (first and second choices), the preferred tolerance zones (first, second and third choices), definitions of related terms and the preferred ISO limits and fits.

General terms such as "hole" and "shaft" can also be taken as referring to the space containing or contained by two parallel faces of any part, such as the width of a slot (keyway), the thickness of a key, etc. This section deals mostly with how to display and interpret the ISO system of limits and fits, tolerance zones of basic sizes, both external and internal, and fundamental deviations for basic sizes. The derived formulas can be more thoroughly explained in ANSI B4.2.

General Tolerances for Metric Dimensioned Products

This section deals with how to specify the general tolerances for metric dimensions without tolerance designation.

The fine and medium tolerance series are mostly applicable to machined parts, and the coarse series show tolerances for use on all types of dimensions.