Concrete Block and CMU Calculator

How Many Concrete Blocks a Wall Needs

Concrete masonry units, usually called CMUs or cinder blocks, are counted by wall face area rather than by volume. The standard block is nominally 8 in high by 8 in wide by 16 in long, and it actually measures 7.625 x 7.625 x 15.625 inches. The missing 3/8 inch on each dimension is the mortar joint. Once the block is laid, it occupies exactly its nominal 8 by 16 inch footprint on the face of the wall, which is what makes the counting simple.

A worked example

A garden retaining wall 24 feet long and 4 feet high has a face area of 96 square feet. Multiply by 1.125 and the wall needs 108 blocks. Add 5 percent for breakage, cuts and the occasional block that arrives chipped, and the order becomes 114 blocks. Blocks are usually sold by the cube, typically 90 to 180 units depending on the supplier and the block size, so round the order to whole cubes where it is close.

Deduct openings only if they are large. A single door or window opening in a block wall removes real block count, but the blocks around it need cutting, so the savings are smaller than the raw area suggests. A practical approach is to subtract the full opening area and then not take any waste deduction on the perimeter courses.

Bond Patterns

Almost every structural block wall is laid in running bond, with each course offset half a block from the one below, so the vertical joints land at the center of the block beneath. Running bond spreads load sideways through the wall and is what engineering tables and reinforcement details assume.

Stack bond, where the vertical joints line up in a continuous column, is a visual choice used on screen walls and some commercial facades. It has no natural interlock, so it needs horizontal joint reinforcement in every course or every other course, and it should not be used on a load bearing wall without an engineer specifying the reinforcement.

Whichever bond is used, the block count per square foot does not change, because the face dimensions are the same. What changes is the number of half blocks. Running bond needs one half block at each end of every other course. On a wall 4 feet high, which is 6 courses, that is 3 courses starting with a half block at each end, so 6 half blocks.

Rebar and Grouted Cells

A dry stacked or purely mortared block wall has very little resistance to bending, which is what matters for retaining walls, tall freestanding walls and anything carrying a roof. Vertical rebar set in grouted cells provides that resistance.

• Vertical bar. Number 4 (1/2 in) or number 5 (5/8 in) bar dropped into the hollow cells and lapped into dowels cast in the footing. Typical spacing is 32 to 48 inches on center, which lines up with the cells at every second or third block, but the spacing must come from the engineer or the prescriptive tables in local code.
• Grout. Only the cells containing rebar are filled, unless the design calls for a fully grouted wall. Coarse grout at an 8 to 11 inch slump is used so it flows around the bar and consolidates without vibration. Each cell in a standard block holds roughly 0.13 cubic feet of grout per course, so a cell filled through a 8 foot high wall takes about 1.6 cubic feet, and about 17 such cells make a cubic yard.
• Horizontal bar. Placed in bond beam blocks, which have a knocked out web, at the top of the wall and at any point the design requires. A bond beam course also goes under a floor or roof bearing point.
• Joint reinforcement. Ladder or truss type wire laid in the bed joint every second or third course controls shrinkage cracking and is cheap insurance on long walls.

Mortar

Allow about 3 bags of type S mortar per 100 blocks laid. That figure covers the bed and head joints at a standard 3/8 inch thickness with normal waste, and it assumes face shell bedding, which is the usual practice on hollow block. A wall of 500 blocks therefore needs about 15 bags.

Type S is the general purpose choice for block work below grade, for retaining walls and for anything carrying load, with a compressive strength around 1800 psi. Type N is softer and suits above grade, non load bearing work. Type M is the strongest and is used for deep foundation walls and heavy load conditions. Do not assume a stronger mortar is always better, because mortar is meant to be weaker than the block so that cracks track the joints rather than splitting the units.

Each bag of mortar mix needs roughly 2.5 to 3 cubic feet of masonry sand if the mortar is being mixed from separate cement and sand rather than a pre-blended bag. Check what the bag on the pallet actually is, since pre-blended mortar mix and plain masonry cement are packaged similarly.

Half Blocks, Corner Blocks and Specials

A block wall is not just stretcher units. Order the specials at the same time, because they are often a separate item with a longer lead time.

• Half blocks measure 7.625 in long and close out the alternate courses at wall ends and each side of an opening. Count 1 per course end that needs one, or cut a full block on site if the supplier does not stock them.
• Corner or single bullnose blocks have one finished end so the exposed face at a corner or a wall end is not a rough cut. Count 2 per course at every external corner, alternating direction each course.
• Bond beam blocks take horizontal rebar. Count the full length of any bond beam course and divide by 1.33 feet per block.
• Cap or solid top blocks finish the top course of a freestanding wall. Count the wall length in feet divided by 1.33.
• Lintel blocks span openings and are sized by the opening width plus bearing at each end, usually at least 8 inches per side.

Wall Size Reference

Block counts above are the neat figure with no waste. Add 5 percent on a simple wall and closer to 10 percent where there are many corners, openings and angled cuts.