Block Calculator — How Many Concrete Blocks

The rule of thumb every mason uses is 1.125 standard blocks per square foot of wall face. That number already accounts for the 3/8 inch mortar joint, which is why a nominal 8 × 8 × 16 block is actually manufactured at 7-5/8 × 7-5/8 × 15-5/8 inches. Measure your wall in feet, multiply length by height, then multiply the area by 1.125 and round up.

For a wall 40 feet long and 8 feet high, the area is 320 square feet, so 320 × 1.125 = 360 blocks. Subtract the openings — a 3 ft × 7 ft doorway removes 21 square feet, or about 24 blocks — then add waste back on top. Five percent is enough for a straight run; use 8 to 10% if the wall has multiple corners, returns or a stepped footing, because corner blocks and half blocks generate more cuts.

Mortar consumption for standard CMU runs about 3 bags of premixed type S mortar per 100 blocks laid, which works out to roughly 1 cubic foot of mortar per 30 blocks. If you are mixing from raw materials rather than premix, budget one bag of masonry cement plus about 18 shovels of damp sand per 25 to 30 blocks. Cold, dry or windy conditions increase consumption because the mix stiffens faster and more gets thrown away.

Reinforcement depends on what the wall is doing. A free-standing garden wall may need nothing more than horizontal joint reinforcement every other course. A retaining wall or a load-bearing foundation wall needs vertical #4 or #5 rebar in grouted cores at 32 to 48 inches on center, plus bond beam courses. Grouting the cores takes roughly 0.9 cubic feet of grout per block core in an 8 inch block, so a partially grouted wall can add a surprising volume of concrete to your order.

Material cost for a plain 8 inch CMU wall lands between $2.50 and $4.00 per square foot in 2026, with the block itself at roughly $1.85 each in most of the country. Installed cost, which includes footing, reinforcement, mortar and mason labor, runs $12 to $22 per square foot depending on region, wall height and whether the finished face needs to be split-face, ground-face or painted.

The variables that move the number most are footing depth and access. A wall in a frost-prone northern state needs a footing below the frost line, which can be 42 to 48 inches deep, and that excavation and concrete often costs more than the block above it. Sites that a mixer or forklift cannot reach add hand-hauling labor, which is billed by the hour and quickly outruns the material bill.