How many blocks per square foot?
A standard concrete block is sold as 8 × 8 × 16 inches, but it really measures 7⅝ × 7⅝ × 15⅝ in. The missing ⅜ inch is the mortar joint, so each block plus its joint covers exactly 8 × 16 = 128 square inches of wall.
8 × 16 in face: 144 ÷ 128 = 1.125 blocks per sq ft
Blocks = wall area × 1.125, plus extra for breakage
Wall thickness doesn't change the count. An 8-inch, 10-inch or 12-inch block still has an 8 × 16 face, so it's still 1.125 per square foot.
Worked example
A garden wall 20 ft long and 4 ft high is 80 sq ft. 80 × 1.125 = 90 blocks. Adding 5% for breakage gives 94.5, so buy 95 blocks. At about 13 blocks per 80 lb bag of mortar mix, that's 95 ÷ 13 = 7.3, so 8 bags.
Common block sizes
| Nominal size | Actual size | Blocks per sq ft |
|---|---|---|
| 8 × 8 × 16 in | 7⅝ × 7⅝ × 15⅝ in | 1.125 |
| 6 × 8 × 16 in | 5⅝ × 7⅝ × 15⅝ in | 1.125 |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in | 1.125 |
| 4 × 8 × 16 in | 3⅝ × 7⅝ × 15⅝ in | 1.125 |
Mortar and grout
An 80 lb bag of premixed mortar mix lays about 13 standard blocks (QUIKRETE). Contractors mixing their own often use about 3 bags of masonry cement per 100 blocks, plus sand. If the cores are filled with grout, a two-core 8 × 8 × 16 block holds about 0.25 cubic feet, so 100 blocks take about 25 cubic feet, just under a cubic yard.
Limitations
- Corners and ends: half blocks and corner blocks are counted as whole blocks here; a supplier can split the order by type.
- Footing: the wall needs a footing below it. Use the footing calculator for its concrete.
- Structural walls: load-bearing and retaining walls need rebar, grout and a design that meets your local building code.