How to Calculate Concrete Blocks
Calculate wall face area in square feet (length times height). Multiply by the blocks-per-sq-ft factor: standard 8x8x16 CMU with 3/8 inch joints gives 1.125 blocks per sq ft. A 20x4 foot wall has 80 sq ft x 1.125 = 90 blocks. Add 5% waste for straight runs or 10% for walls with many corners and openings. For half-blocks at corners and wall ends, add 1-2 extra blocks per corner per course. Mortar is estimated at about 25 blocks per 60-lb bag of pre-mixed mortar.
For walls with openings (windows, doors), calculate the opening area and subtract it from the total, then add the blocks back for the jamb and sill. A 3x5 foot window eliminates about 17 blocks but adds 8-10 extra blocks for the jamb, header, and sill. The net effect is roughly a wash for small openings. For large openings like overhead doors, subtract the blocks but add significantly for the lintel beam above the opening. For the base material behind retaining walls, our gravel calculator helps estimate the drainage aggregate.
Block Wall Cost Breakdown
Standard 8x8x16 CMU blocks cost $1.50-2.50 each at most building supply stores. Colored, split-face, or scored blocks run $2.50-5.00 each. Mortar costs $6-12 per 60-lb bag. Rebar for vertical reinforcement runs $6-10 per 10-foot stick. Concrete for filling reinforced cores costs $100-150 per cubic yard (a 20-foot wall with rebar every 4 feet uses about 0.5 CY of fill concrete). Total material cost for a 20x4 foot reinforced wall: blocks $180-225, mortar $30-50, rebar $60-100, fill concrete $75, footing concrete $150-250. Materials total: $500-650.
Professional block laying costs $15-30 per sq ft of wall face, or $5-10 per block laid. A skilled mason lays 100-200 blocks per day depending on wall complexity. A 20x4 foot wall (80 sq ft, about 90 blocks) takes 1-2 days for a two-person crew at a labor cost of $800-1,600. Total installed cost for a reinforced, grouted, and surfaced block wall runs $25-50 per sq ft. For budget comparisons with other wall types, our brick calculator and fence calculator provide alternative material estimates.
Block Wall Construction Process
Start with a concrete footing at least twice the wall width, poured level and below frost depth. After the footing cures (7 days minimum), snap chalk lines for the wall location. Mix mortar to a workable consistency (holds its shape when you cut a furrow with a trowel). Lay the first course carefully — it determines the alignment of every course above. Set the corner blocks first, then string a mason line between corners as a guide for the intermediate blocks. Check level and plumb on every block in the first course.
For reinforced walls, place vertical rebar in the footing at the specified spacing (usually 4 feet OC) before the concrete sets. The rebar extends up through the block cores. As you lay each course, the blocks slide down over the protruding rebar. After every 3-4 courses, fill the reinforced cores with concrete grout (a soupy concrete mix that flows into the cores around the rebar). Install horizontal joint reinforcement (ladder wire or truss wire) every other course for crack control. Tool all mortar joints when thumbprint hard. After the wall is complete and cured, apply waterproofing coating on the below-grade face and install drainage aggregate behind retaining walls.
Retaining Wall Design with CMU
Concrete block retaining walls must resist the lateral pressure of the soil they hold back. For walls under 4 feet high, a standard 8-inch block wall with vertical rebar every 4 feet and filled cores provides adequate strength. For walls 4-8 feet high, use 12-inch blocks or engineer the wall with a reinforced concrete stem. All retaining walls need a gravel drainage column behind the wall and weep holes at the base to prevent hydrostatic pressure buildup. The footing must be below frost depth and at least twice the wall width.
The most critical design element is the base preparation. Excavate to below the frost line and pour a reinforced concrete footing with #4 rebar both ways. The wall is then built on this footing with vertical reinforcement aligned with the rebar in the footing. Backfill in 6-inch lifts with granular material (gravel or crushed stone), not the excavated clay soil. Install a perforated drain pipe at the base of the wall behind the gravel column, day-lighting to a lower elevation. Without proper drainage, even a well-built retaining wall will eventually fail from water pressure. For the soil volume to excavate, our excavation calculator provides accurate estimates.
Common Block Wall Mistakes
The most common mistake is an inadequate footing. A block wall is only as good as its foundation. Footings that are too narrow, too shallow, or not reinforced with rebar will settle and crack, causing the wall above to fail. The footing must be below frost depth, level, and at least twice the wall width. The second mistake is not filling reinforced cores with grout. Placing rebar in empty block cores provides no structural benefit — the rebar must be encased in concrete grout to develop its strength.
Poor mortar mixing causes weak joints. Mortar that is too dry crumbles and does not bond to the block. Mortar that is too wet runs out of the joints and shrinks excessively as it dries. The ideal consistency is like thick peanut butter. Using the wrong mortar type is another error: Type S is required for structural and below-grade walls, while Type N is only suitable for above-grade veneer. Finally, not installing drainage behind retaining walls is the most common cause of retaining wall failure. Water pressure behind the wall can exceed the wall's structural capacity, especially during heavy rain or spring thaw.
For related calculations, try the concrete slab to estimate materials for your project.