How many blocks do I need?
Block sizes are nominal: an "8 × 8 × 16" block is really 7⅝ × 7⅝ × 15⅝ inches, and the 3/8 inch mortar joint makes up the difference. So a standard block covers 16 × 8 = 128 square inches of wall face.
Blocks = wall area (sq in) ÷ block face (sq in) × (1 + waste)
Example: a 20 ft wall, 4 ft high
The wall is 80 square feet, or 11,520 square inches. Each block covers 128, so you need 90 blocks: 6 courses of 15. With 5% extra, buy 95 blocks, about $261 at $2.75 each.
Mortar and reinforcement
Mortar needs vary with joint thickness and block type, so check the coverage on the bag. Walls that retain soil or exceed about 4 feet usually need rebar, grouted cores, a footing and a permit.
Frequently asked questions
How many cinder blocks are in a square foot?
1.125 standard 8 × 8 × 16 blocks per square foot of wall, or 9 blocks for every 8 square feet.
What's the difference between cinder block and concrete block?
Today almost all are concrete masonry units (CMU). True cinder blocks, made with coal ash, are rare. The calculation is the same.
Last reviewed: 2026-10-31