Calculate the number of concrete masonry units (CMU blocks) for a wall. Enter dimensions to get block count with waste factor.
Concrete masonry units (CMUs), commonly called cinder blocks or concrete blocks, are the building blocks of foundations, retaining walls, commercial buildings, and residential structures worldwide. Accurately estimating the number of blocks before construction begins prevents costly delays and excess material storage.
This calculator determines how many standard CMU blocks you need based on wall length, wall height, and block size. The standard 8×8×16 block covers 0.89 square feet of wall surface (including mortar joints), meaning you need about 1.125 blocks per square foot of wall.
The calculator also accounts for waste (typically 5–10% for breakage and cutting) and can estimate the mortar bags needed for the project.
Understanding this metric in quantitative terms allows construction professionals to compare design alternatives, evaluate cost-effectiveness, and select the optimal approach for each project. Accurate calculation of this value helps construction professionals plan projects more effectively, reduce material waste, and ensure compliance with building codes and industry standards.
Block masonry projects require precise ordering. Running short means stopping mid-wall and waiting for delivery. Over-ordering wastes money and leaves pallets of unused blocks to deal with. This tool takes the guesswork out of block estimation. Accurate figures enable contractors to prepare competitive bids with confidence, reducing the risk of underestimating costs or overcommitting on project timelines and deliverables.
Wall area = Length × Height Net area = Wall area − Openings area Blocks = Net area × 1.125 (for 8×8×16) With waste: Blocks × (1 + waste %) Courses = Height in inches ÷ 8
Result: 378 blocks
Wall area = 40 × 8 = 320 ft². Blocks = 320 × 1.125 = 360. With 5% waste: 378 blocks. That's 12 courses of 8" blocks. Mortar: approximately 12 bags of 80-lb Type S mortar.
Stretcher blocks (standard hollow) are the workhorse. Corner blocks have one flat end for exposed corners. Bond beam blocks have knock-out webs to accept horizontal rebar. Lintel blocks have a U-shape for reinforced headers above openings. Half blocks are 8×8×8 for ending courses.
Running bond (each course offset by half a block) is standard. Stack bond (blocks aligned vertically) is decorative but structurally weaker and requires additional horizontal reinforcement.
Subtract the opening area from wall area, but add blocks for the lintel course above the opening and for the jamb blocks on each side. A rough rule: subtract 80% of the opening area (the remaining 20% accounts for extra blocks at jambs and lintels).
For standard 8×8×16 blocks with 3/8" mortar joints: 1.125 blocks per square foot. The nominal 8×16 face size (actual 7.625×15.625) plus mortar joints equals a module of approximately 0.89 ft².
The 8×8×16 (nominal) is by far the most common, with actual dimensions of 7.625×7.625×15.625 inches. For loadbearing walls or retaining walls, 10" or 12" wide blocks are used for greater strength.
One 80-lb bag of mortar mix typically sets 25–30 standard blocks. A 320 ft² wall using 360 blocks needs about 12–15 bags. The exact amount depends on mortar joint thickness and mason technique.
Type S mortar is standard for below-grade and structural walls. Type N is used for above-grade non-loadbearing walls. Type M is for severe below-grade conditions. Most block walls use Type S.
Building codes require vertical rebar in block walls in most areas. Typical spacing is #5 rebar at 48" on center vertically, with horizontal bond beams at the top and every 4 feet of height. Check your local code.
Most building codes limit un-engineered block walls to 4 feet for retaining walls and 6–8 feet for screen/fence walls. Taller walls, loadbearing walls, and walls subject to soil pressure require an engineer's design.