CMU Block Sizes: Nominal vs Actual Dimensions Chart
Understanding concrete block dimensions is essential for calculating accurate material takeoffs. Compare nominal vs. actual sizes for standard 4", 6", 8", 10", and 12" CMUs.
In concrete masonry construction, blocks are identified by three nominal dimensions: Width (thickness) × Height × Length. While thickness determines the wall depth and load-bearing strength, only the face dimensions (height × length) determine how many blocks you need to cover a wall surface.
Standard CMU Dimensions Chart
| Nominal Size (W × H × L) | Actual Size (Inches) | Face Dimensions | Blocks per sq ft | Approx. Weight |
|---|---|---|---|---|
| 4 × 8 × 16 in | 3⅝ × 7⅝ × 15⅝ in | 16 × 8 in | 1.125 | 18–22 lbs |
| 6 × 8 × 16 in | 5⅝ × 7⅝ × 15⅝ in | 16 × 8 in | 1.125 | 26–30 lbs |
| 8 × 8 × 16 in (Standard) | 7⅝ × 7⅝ × 15⅝ in | 16 × 8 in | 1.125 | 34–38 lbs |
| 10 × 8 × 16 in | 9⅝ × 7⅝ × 15⅝ in | 16 × 8 in | 1.125 | 42–46 lbs |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in | 16 × 8 in | 1.125 | 50–55 lbs |
Nominal vs Actual Dimensions: The ⅜" Rule
Why are physical blocks smaller than their advertised size? In modular masonry, blocks are designed to be laid with a standard ⅜-inch (0.375") mortar joint. The nominal dimension includes the block plus one mortar joint. A standard 8 × 8 × 16 block actually measures 7⅝ × 7⅝ × 15⅝ inches. When the ⅜-inch mortar joint is added on the bed and head joints, the installed unit occupies an exact 8 × 16 inch space in the wall.
| Nominal product | Face used | Blocks per ft² |
|---|---|---|
| 4 × 8 × 16 in | 16 × 8 in | 1.125 |
| 6 × 8 × 16 in | 16 × 8 in | 1.125 |
| 8 × 8 × 16 in | 16 × 8 in | 1.125 |
Metric is a separate module
A 400 × 200 mm face covers 0.08 m², giving 12.5 blocks per m² before waste. It is not the same size as a 16 × 8 inch face, which converts to 406.4 × 203.2 mm. Switching measurement units preserves your existing dimensions in the calculator; selecting a different block preset deliberately changes the module.
What causes an overcount?
Using the smaller physical dimensions as though every joint had zero width reduces the coverage per block and increases the estimate. Choosing a thicker block with the same face should not change an area count. Check the installed face with your supplier before changing the Custom fields.
For a 20 × 8 ft wall using a 16 × 8 in installed face, the result is 180 full-block equivalents before waste and 198 blocks with an illustrative 10% allowance.
Reference: CMHA on concrete masonry unit dimensions. Examples and diagrams are our own; CMHA does not endorse this tool.