Concrete block calculator

A standard 8 by 16 inch block face plus its mortar joints occupies 128 square inches — eight-ninths of a square foot — which works out to 1.125 blocks per square foot. That constant is the whole calculation: a 20 by 8 foot wall is 160 square feet, so 180 blocks before waste, or 189 with the usual 5 percent.

Because blocks course out at 8 inches, wall heights that are multiples of 8 inches lay without cutting. An 8 foot wall is exactly 12 courses. Design to that grid where you can — every height that doesn't course out means cutting the top row, which is slow and looks it.

Mortar runs about 3 bags per 100 blocks, so that 189 block wall takes roughly 6 bags. Blocks use less mortar per square foot than brick simply because there are fewer joints in the same area.

The count assumes a plain running bond wall of full blocks. Real walls need shapes this does not separate out: half blocks to start alternate courses, bond beam blocks where a horizontal reinforced course runs, lintel blocks over openings, and corner or sash blocks with a finished end. Your supplier will substitute those out of the same total, but tell them what the wall is doing so the mix is right.

What this count does not include matters more than what it does. This is the block count for a single-wythe wall only. A structural wall also needs vertical rebar, grout in the filled cells, and a footing — and all three are sized by load, soil and local code, not by a calculator. Get those from an engineer or your building department before you start laying.