How this calculator works
A standard 8×8×16 block covers 128 sq in of wall face once you include the 3/8 in mortar joint, which works out to 1.125 blocks per square foot. Mortar is estimated at about three bags per 100 blocks, a common industry rule of thumb.
Net area = Length × Height − Openings
Blocks = round up( Net area × 1.125 × (1 + Waste%/100) )
Mortar = round up( Blocks ÷ 100 × 3 )
Assumptions and tips
- Nominal 8×8×16 in blocks laid with 3/8 in mortar joints.
- Mortar figure is a planning estimate; joint style, bag size and waste change it.
- Corner, bond-beam and half blocks are counted as standard blocks — order the special shapes your plans call for.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Wall length | 40 ft |
| Wall height | 8 ft |
| Openings to subtract | 0 sq ft |
| Waste allowance | 5 % |
| Result | Value |
|---|---|
| Blocks needed | 378 blocks |
| Mortar | 12 bags |
| Net wall area | 320 sq ft |
| Courses (8 in each) | 12 courses |
| Blocks per course | 30 blocks |
Step by step:
- Net area = 40 × 8 − 0 = 320 sq ft
- Blocks = 320 × 1.125 × 1.05 → 378
- Mortar = 378 ÷ 100 × 3 = 11.34 → 12 bags
Blocks for common wall sizes
8×8×16 block, 5% waste, no openings.
| Wall (length × height) | Blocks needed | Mortar |
|---|---|---|
| 10 × 4 ft | 48 blocks | 2 bags |
| 20 × 4 ft | 95 blocks | 3 bags |
| 20 × 8 ft | 189 blocks | 6 bags |
| 40 × 8 ft | 378 blocks | 12 bags |
| 100 × 8 ft | 945 blocks | 29 bags |
Common mistakes to avoid
- Using the actual block size (7 5/8 × 15 5/8 in) without the mortar joint — the nominal 8×16 already includes it.
- Forgetting special shapes such as corner and bond-beam blocks.
Frequently asked questions
How many concrete blocks per square foot?
About 1.125 standard 8×8×16 blocks, or 112.5 blocks per 100 sq ft of wall.
How much mortar do I need for 100 blocks?
A common estimate is about 3 bags of masonry mortar mix per 100 blocks.