The count of blocks in a wall comes from the wall’s face, not from its length. Each block takes up its own face plus a share of the mortar joints around it, so the question is how many of those pieces fill the area you need to cover.
Net wall area = length × height − openings
Block module = (face length + joint) × (face height + joint)
Blocks = net wall area ÷ module, × (1 + allowance), rounded up
A 20 × 8 ft wall
The wall face is 160 sq ft. A standard nominal 8 × 8 × 16 block has a nominal face of 8 by 16 in. Its actual face is 3/8 in smaller each way, and the 3/8 in joint brings the installed module back to 16 × 8 in, so the planner works from a module of 16 in × 8 in (face + joint). That gives 180 blocks, and with the 5% allowance the list says 189 blocks. The worked table below shows the actual face as 15.63 × 7.63 in because the planner rounds to two decimals; the exact figures are 15⅝ and 7⅝.
Worked example 20 × 8 ft wall, standard 8 × 8 × 16 block, 3/8 in joints, no openings
| Material | Calculated | Buy |
|---|---|---|
| Concrete blocks15.63 in × 7.63 in face, 0.38 in joints, 5% extra | 180 blocks | 189 blocks |
Take off the openings
A door or window opening is not filled with block, so it comes off the wall area first. With a 16 sq ft opening the net wall is 144 sq ft, and the count falls to 171 blocks.
Worked example The same wall with a 16 sq ft opening
| Material | Calculated | Buy |
|---|---|---|
| Concrete blocks15.63 in × 7.63 in face, 0.38 in joints, 5% extra | 162 blocks | 171 blocks |
An opening as large as the wall is rejected, because it would leave nothing to build.
The joint changes the module
The planner starts from a 3/8 in joint, the figure the standard block sizes are specified for. A different joint changes the module, so it changes the count. The same plain wall:
| Mortar joint | Blocks before allowance | Blocks to buy |
|---|---|---|
| 1/4 in | 184.3 | 194 blocks |
| 3/8 in | 180 | 189 blocks |
| 1/2 in | 175.9 | 185 blocks |
Enter the joint your wall will actually use. The blocks-per-square-foot figure behind these numbers has its own page.
This example assumes
- A standard nominal 8 × 8 × 16 block, entered as its actual face plus the joint
- 3/8 in joints
- 5% allowance for cuts and breakage, an editable planning default
What this is not
This is a count of blocks from the wall face. It is not a wall design: nothing here says what reinforcement, footing or wall height a project needs, and corners, special shapes and mortar are separate. Brick is estimated the same way, so how many bricks for a wall shows the parallel method.
What changes the estimate?
- Block size: actual face, not the nominal name
- Joint width
- Openings subtracted from the wall area
- The allowance for cuts and breakage