How to Calculate Concrete Blocks
CMU (concrete masonry unit) block calculation uses nominal dimensions — the actual block size plus the mortar joint. A standard 8×8×16 block is actually 7.625 × 7.625 × 15.625 inches; add a 3/8-inch mortar joint on each face and you get the nominal 8×8×16. All block calculations use nominal dimensions because that's what fills the wall.
Standard 8×16: 144 ÷ 128 = 1.125 blocks/sq ft
Total blocks = Net wall area (sq ft) × 1.125 × (1 + waste factor)
Block Types and When to Use Them
- Standard 8×8×16 (most common) — The workhorse of residential and commercial masonry. Used for foundations, retaining walls, garages, outbuildings, and interior non-load-bearing walls. Available everywhere, the cheapest per block.
- Half block 4×8×16 — Used for the first course in some applications, for window sills, and for cap courses. Usually paired with standard blocks in the same project.
- Square 8×8×8 — Used for corners and end conditions where a half-length block is needed. Often called a "half block" in the field (confusingly — it's half the length, not half the height).
- 6-inch block 6×8×16 — Narrower wall, used for non-load-bearing partitions and interior walls where a thinner wall section is acceptable or required by space constraints.
- 12-inch block 12×8×16 — Wider and heavier. Used for heavily loaded foundation walls and retaining walls where a standard 8-inch block would be undersized.
Mortar: Type S vs Type N
Type S mortar (high-strength, 1,800 PSI minimum) is the standard for below-grade applications — foundations, retaining walls, and any masonry in contact with soil or moisture. Type N mortar (medium-strength, 750 PSI) is used for above-grade exterior walls and interior non-structural applications. When in doubt, use Type S — it's more forgiving and appropriate for more conditions. The mortar estimate in this calculator uses Type S 70 lb bag coverage of approximately 33 blocks per bag.