Concrete Block Wall Calculator
Calculate how many standard 8×8×16 concrete masonry units (CMU) you need for a block wall.
How to use this tool
- Enter the wall length and height in feet.
- Set a waste allowance of 5–10%.
- The result shows wall area and number of 8×8×16 CMU blocks needed.
Estimate how many concrete blocks (CMU) you need for a retaining wall or foundation.
Formula
Wall area = length (ft) × height (ft)
Blocks = ⌈ wall area × 1.125 × (1 + waste% ÷ 100) ⌉
The factor 1.125 represents approximately 1.125 standard 8×8×16 CMU blocks per square foot of wall face, including a ⅜ in mortar joint.
How it works
This calculator counts standard 8×8×16 inch concrete masonry units (CMU) for a single-course wall. Each block with a ⅜ inch mortar joint covers roughly 0.889 ft² of wall face, giving a coverage factor of 1.125 blocks per square foot. A user-defined waste percentage is applied to account for cuts, corners, and breakage, and the total is rounded up to the nearest whole block. The calculation assumes a single-wythe wall; pilasters, cores, or reinforcing schedules are not included.
Worked example
Worked example
- Wall area = 20 ft × 4 ft = 80 ft².
- Base blocks = 80 × 1.125 = 90 blocks.
- Apply 5% waste: 90 × 1.05 = 94.5 blocks.
- Round up: ⌈94.5⌉ = 95 blocks.
You need 95 concrete blocks for a 20 ft × 4 ft wall with a 5% waste allowance.
Key terms
- CMU (Concrete Masonry Unit)
- A standard hollow or solid concrete block; the most common size in the US is 8×8×16 inches (nominal), weighing 28–36 lb each.
- Coverage factor
- The number of blocks needed per square foot of wall face; for 8×8×16 CMU with a ⅜ in mortar joint it is approximately 1.125 blocks/ft².
- Mortar joint
- The bed of mortar between blocks, typically ⅜ inch thick; it slightly reduces the effective face area of each block and is built into the coverage factor.
- Single-wythe wall
- A wall that is only one block thick. Multi-wythe walls or walls with insulated cores require separate calculations.
Frequently asked questions
- How many concrete blocks per square foot?
- A standard 8×8×16 CMU block laid with a 3/8" mortar joint covers approximately 0.889 sq ft, so you need about 1.125 blocks per square foot.
- What is a CMU block?
- CMU stands for Concrete Masonry Unit. Standard blocks are 8 inches wide, 8 inches tall, and 16 inches long (nominal dimensions).