Key takeaways
- Standard block runs at about 1.125 blocks per square foot of wall.
- Titan America's rule of thumb is 3 bags of mortar per 100 block.
- Retaining and load-bearing walls need a proper design, not just a block count.
Enter the wall's length and height, plus any door and window area to subtract. The count assumes standard blocks that lay up at 8 × 16 inches, including the mortar joint.
How the calculator works
- Wall area: length × height − openings
- Blocks: area × 1.125 × (1 + allowance %), rounded up
- Courses: height in inches ÷ 8, rounded up
- Mortar: blocks × 3 ÷ 100, rounded up
The 1.125 figure follows from the block size. A block laid at 8 × 16 inches covers 128 square inches, or 0.889 square feet, and 1 ÷ 0.889 is 1.125. Titan America's estimating guide uses the same rate for its standard block, in 4, 8, 10 and 12-inch widths, and describes its figures as rules of thumb.
Worked example
A wall 20 feet long and 4 feet high, with a 5% allowance:
- Area: 20 × 4 = 80 square feet
- Blocks: 80 × 1.125 × 1.05 = 94.5, which rounds up to 95 blocks
- Layout: 6 courses of about 15 blocks
- Mortar: 95 × 3 ÷ 100 = 2.85, which rounds up to 3 bags
Mortar and sand
The 3-bags-per-100 rule is from Titan America's guide, which also lists about 1 cubic yard of sand per 7 bags of mortar. That points to bagged mortar mixed with sand. Pre-mixed mortar is a different product, so check its coverage on the bag.
Filling the block cores
If your design calls for filled cores, Titan America's guide lists about 0.25 cubic feet per block for a two-core 8 × 8 × 16 block. Filling all 95 blocks in our example would take about 23.75 cubic feet, or about 0.88 cubic yards. Many walls fill only some cores, so follow your design.
When a block count isn't enough
Block walls can hold back soil or carry a building. Retaining walls, foundations and load-bearing walls need the right footing, reinforcement and design for your location. Check with your building department or an engineer before you build one.
Blocks by wall size
For each 10 feet of wall, before any extra allowance:
| Wall height | Courses | Blocks |
|---|---|---|
| 2 ft | 3 | 23 |
| 4 ft | 6 | 45 |
| 6 ft | 9 | 68 |
| 8 ft | 12 | 90 |
Plan your wall height in 8-inch steps where you can, as it saves cutting block.
Frequently asked questions
How many blocks are in a 100-square-foot wall?
About 113 standard blocks, using 1.125 per square foot, before any extra allowance.
How many blocks in one course?
Divide the wall length in inches by 16. A 20-foot wall is 240 inches, so about 15 blocks a course. Corners and half blocks can change this slightly.
How much mortar do I need for 100 blocks?
Titan America's rule of thumb is 3 bags per 100 blocks, used with sand. Other sources and mortar products give different figures, so check with your supplier.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- Titan America: Block Estimating Guide (block rate, mortar, sand and core fill rules of thumb)