How it’s calculated
An 8×8×16 block is 7⅝ × 7⅝ × 15⅝ in; with a ⅜ in mortar joint each one fills a nominal 8 × 16 in = 128 in² (8/9 ft²) of wall. So a wall needs 1.125 blocks per square foot (112.5 per 100 ft²).
Example: a 20 ft × 4 ft wall = 80 ft² → 80 × 1.125 = 90 blocks. Add 5% waste → 94.5 → 95 blocks. At 13 blocks per 80 lb bag of mortar mix you need ⌈95 ÷ 13⌉ = 8 bags.
Corner, bond-beam and half blocks are counted separately in a detailed takeoff. Grout and reinforcement are not included.
Frequently asked questions
How many concrete blocks are in 100 square feet?
112.5 standard 8×8×16 blocks, or about 119 with the 5% allowance for waste and breakage used in CMHA (formerly NCMA) TEK 4-2A for waste and breakage.
How many blocks does a bag of mortar lay?
QUIKRETE states an 80 lb bag of Mortar Mix lays up to 13 standard 8×8×16 blocks. If you mix your own, CMHA (formerly NCMA) TEK 4-2A says one 70 lb bag of masonry cement, mixed with sand, lays about 30 hollow units with face-shell bedding.
What is the actual size of an 8x8x16 block?
7⅝ in × 7⅝ in × 15⅝ in. The ⅜ in mortar joint brings it to the 8 × 8 × 16 in nominal module.
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Sources
- TEK 4-2A: Estimating Concrete Masonry Materials — Concrete Masonry & Hardscapes Association (formerly NCMA)
- QUIKRETE Mortar Mix product data sheet (yield per bag) — QUIKRETE Companies
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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