The formula
Wall area = length × height − openings Per sq ft = 144 ÷ (16 × 8) = 1.125 standard blocks Blocks = wall area × 1.125 × (1 + extra %) Mortar = blocks ÷ about 13 per 80 lb bag
Example: a wall 30 ft long and 6 ft high is 180 square feet. At 1.125 blocks per square foot that is 202.5 blocks, or 213 with 5% extra, laid with about 17 bags of mortar.
Quick answers for common walls
| Wall | 8 × 8 × 16 blocks | 80 lb mortar bags |
|---|---|---|
| 10 ft long, 4 ft high | 48 | 4 |
| 20 ft long, 4 ft high | 95 | 8 |
| 20 ft long, 6 ft high | 142 | 11 |
| 40 ft long, 6 ft high | 284 | 22 |
| 50 ft long, 8 ft high | 473 | 37 |
Block sizes
An 8 × 8 × 16 block actually measures 7⅝ × 7⅝ × 15⅝ inches; with a 3/8 inch mortar joint it takes up 8 by 16 inches of wall. 12-inch-wide blocks have the same face, so the count is the same. Half-high blocks have a 4 × 16 inch face and need 2.25 per square foot.
If you fill the cores with concrete, use the concrete calculator for the grout.
Sources and assumptions
Mortar uses about 13 standard 8 × 8 × 16 blocks per 80 lb bag of mortar mix, as printed on common bagged mortar; change Blocks per 80 lb bag to match yours.
Questions
How many concrete blocks are in a square foot?
1.125 standard 8 × 8 × 16 blocks, or 9 blocks for every 8 square feet.
How many blocks does one bag of mortar lay?
An 80 lb bag of mortar mix lays about 13 standard blocks.
How many blocks do I need for a 10 ft by 4 ft wall?
45 blocks before extra, or 48 with 5% for breakage.
References and review
- Quikrete Mortar Mix: bag sizes and coverage (product page and data sheet)
- NIST Special Publication 811: unit conversion definitions
Units convert with exact definitions: 1 ft = 12 in = 0.3048 m, 1 yd³ = 27 ft³. Results round up to whole bags, sheets and pieces so you don’t come up short. Last reviewed October 19, 2026. If your bag, pallet or supplier lists a different figure, use theirs: change the matching field above.