METRIC DESIGN GUIDE
Specifications
Millimeters (mm)
SI specifications have used mm for almost all measurements, even large ones. Use of mm
is consistent with dimensions in major codes, such as the National Building Code (Building
Officials and Code Administrators International, Inc.) and the National Electric Code
(National Fire Protection Association).
Use of mm leads to integers for all building dimensions and nearly all building product
dimensions, so use of the decimal point is almost completely eliminated. Even if some
large dimensions seem to have many digits there still will usually be fewer pencil or CAD
strokes than conventional English Dimensioning
Meters (m)
Meters have been used where large, round metric sizes are meant or where it is already
customary, such as in surveying.
Example: "Contractor will be provided an area of 5 by 20 meters for storage of
materials."
Centimeters (cm)
Centimeters are typically not used in U.S. specifications. This is consistent with the
recommendations of AIA and the American Society of Testing Materials (ASTM).
Centimeters are not used in major codes.
Use of centimeters leads to extensive usage of decimal points and confusion to new
readers. Whole millimeters are being used for specification measurements, unless extreme
precision is being indicated. A credit card is about 1 mm thick.
Example 1 - Mortar Joint Thickness. If a 3/8-inch mortar joint between brick is
needed, this would convert to 9.525 mm. Whole mm are used. Specify 10 mm
joint thickness.
Example 2 - Stainless Steel Thickness. Bath accessories are commonly made from
22-gage (0.034-inch) thick stainless steel. Exact conversion is 0.8 636 mm. This is
PBS-PQ260
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