The figure is a square foot of wall divided by the area one block takes up, so it depends on which block size and which joint you mean. For the common case it comes out to a clean number.
Module area = (face length + joint) × (face height + joint)
Blocks per sq ft = 144 sq in ÷ module area in square inches
Nominal, specified and module
Three sizes get called “the size of a block”, and they are different:
| Name | Face (height × length) | What it is |
|---|---|---|
| Nominal | 8 × 16 in | The name and the layout grid |
| Specified (actual) | 7⅝ × 15⅝ in | The unit itself, 3/8 in under nominal |
| Installed module | 8 × 16 in | Actual face plus one 3/8 in joint |
CMHA states that typical nominal faces are 8 by 16 in, that actual dimensions run typically 3/8 in under nominal, and that the difference is what keeps the 8 in module with 3/8 in joints. The module is the number to divide by.
The arithmetic
The module is 8 × 16 = 128 square inches. A square foot is 144 square inches, so 1.125 blocks per square foot. The planner agrees: a 100 sq ft wall with no allowance needs 112.5 blocks, which is 1.125 per sq ft. (The table shows the actual face rounded to 15.63 × 7.63 in; the exact figures are 15⅝ and 7⅝.)
Worked example 100 sq ft wall, standard 8 × 8 × 16 block, 3/8 in joints, no allowance
| Material | Calculated | Buy |
|---|---|---|
| Concrete blocks15.63 in × 7.63 in face, 0.38 in joints, 0% extra | 112.5 blocks | 113 blocks |
Entering a nominal size as an actual one
Custom block fields ask for the actual face, because the planner adds the joint itself. If the nominal 8 × 16 is typed there instead, the joint is counted twice: the module becomes larger than the real one, and the count falls to 105 blocks for the same wall, about 7% too few. Use the standard size option, or enter the actual face.
Other joints
The figure holds for a 3/8 in joint. The same block with other joint widths, per 100 sq ft:
| Mortar joint | Blocks per 100 sq ft |
|---|---|
| 1/4 in | 115.2 |
| 3/8 in | 112.5 |
| 1/2 in | 109.9 |
This is a count per square foot of wall face. For a whole wall, with openings and an allowance, see how many concrete blocks you need for a wall.
What changes the estimate?
- The actual face of your block, not its nominal name
- The joint width
- Openings, which come off the area before you multiply
- An allowance for cuts and breakage, which is separate from this figure