Concrete masonry units (CMU) are the backbone of commercial construction, retaining walls, and fire-rated assemblies. Estimating a CMU job comes down to three quantities: blocks per square foot of wall, mortar per block, and grout fill for reinforced cells. Get these right and your material order matches the job. Get them wrong and you either run short on a Friday afternoon or return pallets of unused block.
This guide covers the standard estimating factors, the adjustments for bond beams, lintels, and openings, and the waste allowances that keep the job moving without over-ordering. The numbers here apply to standard 8 × 8 × 16 inch (nominal) CMU, which is by far the most common size in the field.
Blocks Per Square Foot
A standard 8 × 16 inch CMU with a 3/8-inch mortar joint covers 0.889 square feet of wall face (8.375 × 16.375 inches actual module). This means 1.125 blocks per square foot of wall area. For a wall that is 40 feet long and 10 feet high (400 square feet), you need 450 blocks before accounting for waste and openings.
Deduct openings by calculating their area and subtracting the corresponding block count. A 3-foot by 7-foot door opening removes 21 square feet, or about 24 blocks. However, you need extra blocks for the lintel course above the opening (bond beam blocks or U-blocks filled with rebar and grout). Add 5–10 % for waste, breakage, and cutting. A 7 % waste factor is standard for experienced crews; 10 % for less experienced crews or walls with many corners and openings.
CMU Block Estimator
Estimate CMU block count, mortar, grout, and rebar for concrete masonry walls.
Mortar Volume Estimating
Mortar volume depends on joint thickness, block face dimensions, and bed joint coverage. For standard 3/8-inch joints on 8 × 16 blocks with face-shell bedding (mortar on the outer shells only, not the cross webs), you need roughly 8.5 cubic feet of mortar per 100 blocks. With full mortar bedding (mortar across the entire top face including webs), the volume increases to about 11 cubic feet per 100 blocks.
A standard bag of masonry cement (Type S or Type N) mixed with the correct sand proportion yields about 4.5 cubic feet of mortar. So for face-shell bedding: approximately 2 bags of masonry cement per 100 blocks. For full bedding: about 2.5 bags per 100 blocks. Add the corresponding sand quantity, typically 3 cubic feet of sand per bag of masonry cement for Type S mortar (1:3 ratio by volume).
Grout Fill for Reinforced Cells
Reinforced CMU walls require grout in the cells containing vertical rebar. An 8-inch CMU block has a net cell volume of about 0.039 cubic feet per cell (two cells per block). If every cell is grouted (solid grouting), you need roughly 0.078 cubic feet of grout per block, or about 7.8 cubic feet per 100 blocks. If only reinforced cells are grouted (partial grouting at, say, 32 inches on center), the volume is about half that.
Grout is not mortar. It is a fluid concrete mix designed to flow into and fill the cells completely. Fine grout (with sand aggregate) is used for cells under 2 inches wide, while coarse grout (with pea gravel) is used for larger cells. Grout must be consolidated by puddling or vibrating to eliminate voids. Bond beams require additional grout to fill the U-block or knocked-out web channel across the full wall length at that course.
CMU Block Estimator
Estimate CMU block count, mortar, grout, and rebar for concrete masonry walls.
Bond Beams and Lintels
Bond beams are horizontal reinforced courses that tie the wall together and distribute loads. Most structural CMU walls have bond beams at the top course, at floor and roof bearing elevations, and every 4 feet of wall height (48 inches on center). Bond beam blocks have knocked-out webs or are purpose-made U-blocks that allow continuous horizontal rebar and grout through the course.
Lintels span openings (doors, windows, louvers) and carry the wall load above. A CMU lintel is typically one or two courses of bond beam block extending at least 8 inches (one block length) past each side of the opening. The lintel rebar and grout quantity must be calculated separately from the field wall. For openings wider than 6 feet, a steel angle or precast lintel may replace CMU bond beam lintels depending on the structural design.