CMU count, mortar bags & cost for 8×8×16 block
Concrete blocks — also called concrete masonry units or CMUs — are the standard material for foundation walls, retaining walls, and many load-bearing structures. In the United States the most common block is the nominal 8x8x16 unit: 8 inches high, 8 inches wide, and 16 inches long. Because the nominal dimensions include one mortar joint (typically 3/8 inch), the actual block face is 7-5/8 x 7-5/8 x 15-5/8 inches, and it occupies exactly 8x16 = 128 square inches of wall face per unit.
That 128 square inch figure is why the standard rule of thumb exists: one 8x8x16 block covers 0.889 square feet of wall face, so a wall's area in square feet divided by 0.889 gives the block count — or more simply, 1.125 blocks per square foot of wall.
Blocks come in several widths (4, 6, 8, 10, and 12 inch) but the 8-inch unit is the default for foundations. Height and length stay 8x16 nominal across widths, so the face coverage per block is the same 0.889 sq ft regardless of width. Open-ended, bond-beam, and half-block shapes exist for corners and lintel courses.
Mortar estimation follows industry rules of thumb: standard 3/8 inch joints on 8x8x16 blocks consume roughly 12.5 cubic feet of mortar per 100 blocks when using a full mortar bed, which works out to about 7 bags of 80-lb premix mortar per 100 blocks in common contractor references. Yield varies with joint thickness and waste.
The table below shows nominal dimensions, actual dimensions, and face coverage for the common CMU sizes.
| Nominal size (in) | Actual size (in) | Face coverage (sq ft) | Blocks per sq ft |
|---|---|---|---|
| 8 x 8 x 16 | 7-5/8 x 7-5/8 x 15-5/8 | 0.889 | 1.125 |
| 6 x 8 x 16 | 5-5/8 x 7-5/8 x 15-5/8 | 0.889 | 1.125 |
| 4 x 8 x 16 | 3-5/8 x 7-5/8 x 15-5/8 | 0.889 | 1.125 |
| 12 x 8 x 16 | 11-5/8 x 7-5/8 x 15-5/8 | 0.889 | 1.125 |
| 8 x 8 x 8 (half) | 7-5/8 x 7-5/8 x 7-5/8 | 0.444 | 2.25 |
Measure wall area as length times height, then subtract door and window openings. Apply the calculator's waste factor (5–10% is typical) to cover breakage, cuts, and damaged units. For a standard single-wythe wall, multiply area in square feet by 1.125 to get blocks directly. Footings for block walls are typically concrete poured wider than the wall; use a concrete calculator for that volume separately.
Reinforcement matters as much as count: most building codes require vertical rebar in cores at specified spacing and grout filling at corners and openings, which changes mortar and grout quantities. Always follow your local building code and engineered plans for structural walls.
Unreinforced block walls work for short garden walls, but most structural walls need rebar and grout. Standard practice places vertical rebar in designated cores at spacing set by the engineer or code — commonly 24 to 48 inches on center in seismic regions — and those cores are then filled with grout, a high-strength concrete mix thin enough to pour. Grout-filled cores change material quantities: each filled 8-inch core consumes roughly 0.02 to 0.03 cubic yards of grout per foot of height, so tall reinforced walls need their own grout estimate.
Horizontal reinforcement usually means bond beams — U-shaped blocks threaded with rebar and filled with grout — at the top course and at openings. Ladder-type or truss-type joint reinforcement laid in the mortar bed every course or two controls cracking in long walls. These items don't change the block count but belong on the same material order.
Estimating blocks for a retaining wall follows the same face-area math for standard units, though segmental retaining wall (SRW) blocks — the interlocking kind sold for landscaping — have their own published coverage per block set by the manufacturer, since their heights and setbacks differ from a flat CMU wall. For tall retaining walls, engineering is not optional: setback, geogrid reinforcement layers, and drainage aggregate behind the wall are what keep it standing, and most codes require design input above roughly 4 feet of exposed height.
How many 8x8x16 blocks are in a square foot of wall?
One 8x8x16 block covers 0.889 square feet of wall face (128 sq in), so you need 1.125 blocks per square foot. A 100 sq ft wall needs about 113 blocks before waste.
Why are nominal and actual block sizes different?
The nominal size includes one 3/8 inch mortar joint, so the actual manufactured block is 3/8 inch smaller in each dimension: 7-5/8 x 7-5/8 x 15-5/8 for a nominal 8x8x16.
How much mortar do I need per 100 blocks?
A common industry estimate is about 12.5 cubic feet of mortar per 100 standard blocks with 3/8 inch joints, roughly 7 bags of 80-lb premix. Joint size, block style, and waste change this number.
What waste percentage should I add for blocks?
5% is typical for straightforward walls; use 8–10% for walls with many openings, corners, or cuts. The calculator lets you set the waste factor directly.
Does block width change the number of blocks I need?
No — width changes wall thickness and cost per block, but a 6-inch and a 12-inch block both cover the same 0.889 sq ft of wall face because height and length stay 8x16 nominal.