CMU Basics | Dimensions and Sizes
Three terms are used in referring to dimensions: specified, actual, and nominal.
Specified dimensions are those specified for the manufacture of masonry units or the construction of masonry. Calculations are based on specified dimension.
Actual dimensions are the measured dimensions of the unit. UBC and ASTM Standards allow the actual dimensions a permissible variation from the specified dimension. Refer to individual product sections for their respective tolerances.
Nominal dimensions are those used in stating unit size. They are equal to the specified dimensions plus the thickness of the mortar joint. The nominal dimensions compensate for a 3/8 inch (10 mm) joint for both precision and split face.
For Example:
|
Precision |
| Nominal Dimensions |
|
English (in.) |
8 x 4 x 16 |
Metric (mm) |
203 x 102 x 406 |
| Specified Dimensions |
|
English (in.) |
7 5/8 x 3 5/8 x 15 5/8 |
Metric (mm) |
194 x 92 x 397 |
In expressing the size of a cmu it is necessary to state dimensions in proper order - width, height, and length (WHL). For example, an 8 inch wide, 16 inch long, 4 inch high unit is termed an 8x4xl6. Reversing any two dimensions will describe a completely different unit. Using the same example stated as 4x8xl6 would indicate a unit 4 inches wide and 8 inches high (not 8 inches wide and 4 inches high).
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