Order the right number of blocks
Foundation walls, garages, retaining walls and garden walls are often built with concrete masonry units (CMU). This calculator gives you the number of blocks, courses, mortar bags, mortar volume, pallets and the block cost for a wall of any size.
The 1.125 blocks per square foot rule
A standard 8 × 8 × 16 in block has an actual size of 7⅝ × 7⅝ × 15⅝ in. Add a ⅜ in mortar joint and its face covers exactly 8 × 16 in, or 0.889 sq ft. That is why you need 1.125 blocks per square foot of wall. Blocks 4, 6, 10 and 12 in wide have the same face, so the count does not change with width.
Blocks = (length × height − openings) × 1.125 × (1 + waste)
Courses = wall height in inches ÷ 8, rounded up.
Mortar
With ⅜ in joints, plan on roughly 8.5 cu ft of mortar per 100 standard blocks. In premixed 80-lb bags of Type N or S mortar, that is about one bag for every 12–14 blocks. Wider blocks use proportionally more. If you mix from masonry cement and sand, one 70-lb bag of masonry cement with roughly 20 shovels of mason’s sand lays around 28 to 30 blocks.
Worked example: a 40 ft wall, 8 ft tall
One 3 × 7 ft door opening, standard 8 in block, 5% waste, $2.50 per block.
- Net area: 40 × 8 − 21 = 299 sq ft.
- Blocks: 299 × 1.125 × 1.05 ≈ 354 blocks.
- Courses: 96 in ÷ 8 = 12 courses.
- Mortar: about 30 cu ft, or 28 bags of 80-lb mortar mix.
- Pallets: about 4 at 90 blocks per pallet.
- Block cost: 354 × $2.50 ≈ $885.
Materials by wall length (8 ft tall, one door)
| Wall length | Net area | Blocks | Mortar bags | Block cost |
|---|---|---|---|---|
| 10 ft | 59 sq ft | 70 | 6 | $175 |
| 20 ft | 139 sq ft | 165 | 13 | $412 |
| 40 ft | 299 sq ft | 354 | 28 | $885 |
| 60 ft | 459 sq ft | 543 | 42 | $1,358 |
| 100 ft | 779 sq ft | 921 | 71 | $2,302 |
What else the wall needs
- Footing: a continuous reinforced concrete footing below the frost line; use our concrete calculator for the volume.
- Reinforcement: vertical rebar in grouted cores and horizontal joint reinforcement or bond beams, as required by your engineer or local code.
- Grout for reinforced cores: about 0.26 cu ft per core in an 8 in block, filled with fine or coarse grout or concrete.
- Lintels above openings and cap blocks on top of free-standing walls.
Building tips
Set the first course in a full mortar bed and make it perfectly level; every course after it depends on it. Use a story pole marked every 8 in to keep courses consistent, stagger head joints by half a block, and tool joints when the mortar is thumbprint hard. Check with your local building department before building retaining walls over about 4 ft or any load-bearing wall.