How this instrument works
Concrete masonry unit (CMU) walls are estimated much like brick walls: figure out the wall's total face area, then multiply by how many blocks it takes to cover one square foot of that face. A nominal 8x8x16 in CMU — actually 7-5/8 x 7-5/8 x 15-5/8 in to allow for a standard 3/8 in mortar joint on each side — covers about 0.889 sq ft of wall face, which works out to the widely used industry figure of 1.125 blocks per square foot.
That 1.125 figure specifically assumes the standard 8x8x16 in block size with 3/8 in mortar joints — other block sizes (4 in, 6 in, 12 in, or non-standard dimensions) cover different amounts of area per unit, so the coverage rate is left editable here rather than locked to one number.
As with brick, a waste allowance on top of the base block count accounts for cuts at corners, openings, and courses, plus breakage during handling — 10% is a common default for straightforward rectangular walls.
- Enter the wall's length and height in feet.
- Leave the blocks-per-sq-ft rate at the standard 8x8x16 in CMU default, or replace it for a different block size.
- Enter a waste/breakage allowance percentage — 10% is a common starting point.
- Read the wall area and the total blocks needed, rounded up to a whole block.
A 20 x 8 ft standard CMU wall
A contractor is pricing a 20 ft long, 8 ft tall CMU wall using standard 8x8x16 in blocks. Wall area = 20 x 8 = 160 sq ft. At the standard 1.125 blocks per sq ft: 160 x 1.125 = 180 base blocks. Applying a 10% waste allowance brings the raw figure to 198, and because of how the multiplication rounds in floating-point math, the calculator's ceiling step lands on 199 blocks to order — a good reminder to trust the calculator's exact output rather than hand-rounding the base figure yourself.
A larger 40 x 8 ft wall with a lower 5% waste allowance needs proportionally more blocks before waste, but a smaller buffer on top: area = 320 sq ft, base = 320 x 1.125 = 360 blocks, and with 5% waste, 360 x 1.05 = 378 blocks exactly — a clean round number in this case, unlike the first example.
Questions
Why isn't 1.125 blocks per square foot exactly right for every CMU wall?
That figure is specific to the standard 8x8x16 in block size with 3/8 in mortar joints. Walls built with 4 in, 6 in, 12 in, or other non-standard block sizes cover different amounts of wall face per unit, so check your specific block's dimensions and adjust the coverage rate accordingly for an accurate count.
How much waste allowance should I use for a CMU wall?
10% is a common default for a straightforward rectangular wall. Walls with more corners, openings, or courses that require partial-block cuts typically generate more waste, so consider a higher allowance for complex layouts — your mason can advise based on the specific design.
Does this include the mortar or any grout fill needed?
No, this counts blocks only. Mortar quantity is typically estimated separately based on joint dimensions, and if you're planning to fill some or all of the block cores with grout or concrete, use the companion concrete-block-fill calculator for that separate material estimate.
Does this account for door and window openings in the wall?
No — enter your wall's net length and height (excluding any openings) if you want an accurate count, since this calculator simply multiplies the length and height you provide by the coverage rate without subtracting anything automatically.