A 24 × 24 ft block garage with 8 ft walls, a 16 × 7 ft overhead door, a walk-in door and two windows has 612 square feet of block wall. Laid in standard 8 × 16 in units that is 648 blocks plus 76 bond beam units for the top course, with 5 % for breakage. The mortar for them is 25 bags of masonry cement and about 3 tons of sand; grouting the cells with #4 bars at 48 inches and the bond beam takes about 2.4 cubic yards of grout. The calculator above works this out for any block wall, from a garden wall to a house foundation.
How the calculator works
Enter the total wall length and height, list the windows and doors, and add gables or interior walls if there are any. Then choose the reinforcement, the grout and the mortar you will buy. Feet and inches can be mixed freely: 8' 4" is read exactly, and the result follows the market in the header.
Counting the blocks
Block walls are estimated by face area, not by volume. A standard concrete masonry unit is 8 × 16 in nominal: the block itself is 7⅝ × 15⅝ in and the ⅜ in mortar joint makes up the rest. Each block therefore covers 8 × 16 = 128 square inches, or 8/9 of a square foot, and a wall takes 1.125 blocks per square foot. Half-high 4 × 16 in units take 2.25 per square foot. The width of the block, 4, 6, 8, 10 or 12 in, does not change the count; it changes the weight, the mortar a little, and the grout a lot.
The net wall area is the length times the height, plus each gable as a triangle (width × height ÷ 2), minus every window and door. NCMA TEK 04-02A adds about 5 % for waste and breakage, which turns 112.5 blocks per 100 ft² into 119. Raise it for walls with many corners, pilasters or cut blocks.
Bond beam and interior walls
The top course of a reinforced wall is usually a bond beam: U-shaped bond beam units filled with grout around horizontal bars. Those units are counted separately, one per 16 inches of wall, and taken out of the standard block count. Interior walls get their own count with their own block width, minus 3' × 6' 8" door openings.
Mortar
Mortar is the material most often misestimated, so the calculator uses the masons' batch figures from TEK 04-02A Table 2, which already include waste and assume face-shell bedding, the normal way to lay hollow block:
| Mortar you buy | One batch | Blocks laid |
|---|---|---|
| Mortar mix (preblended) | one 80 lb bag | about 16 |
| Masonry cement + sand | 8 bags of 70 lb + 1 ton of sand | about 240 |
| Portland cement + lime + sand, Type S | 1 bag of cement, ½ ft³ of lime, 4½ ft³ of sand | about 46 |
For the garage that means 724 blocks in all, about three masonry-cement batches: 25 bags and 3 tons of sand. In bags of premixed mortar it would be 46 bags, which is why bagged mix is used for small jobs and masonry cement for anything larger.
Reinforcement and grout
Joint reinforcement, ladder or truss wire, goes in the bed joints. Every second course (16 in on center) is the usual spacing; the calculator places it in every N-th joint below the top and, if you tick it, under each window with an extension past both sides of the opening. It comes in 10 ft pieces lapped 6 in.
Vertical bars go in grouted cells at the spacing you choose, plus one each side of every opening. Each bar is the wall height plus a 48 bar-diameter lap with the dowels from the footing: a #4 bar laps 24 in, so an 8 ft wall takes 10 ft bars, two from every 20 ft stock bar.
Grout in the vertical cells follows TEK 04-02A Table 3. An 8 in wall grouted solid takes 36.1 cubic feet per 100 ft² of wall; with cells grouted only where the bars are, every 48 in, it takes a sixth of that, about 6 ft³ per 100 ft². The bond beam takes one 80 lb bag of grout (0.66 ft³) for every 2 ft of 8 in beam. The total is shown in 80 lb bags and in cubic yards, because for more than a yard or so it is cheaper to order grout by the truck.
| Wall | Solid grouted, ft³ per 100 ft² | Cells every 48 in, ft³ per 100 ft² | Bond beam, ft per 80 lb bag |
|---|---|---|---|
| 6 in | 25.6 | 4.3 | 2.7 |
| 8 in | 36.1 | 6.1 | 2.0 |
| 10 in | 47.0 | 7.9 | 1.8* |
| 12 in | 58.9 | 9.9 | 1.6 |
*10 in bond beams are not in Table 5; the value is taken halfway between 8 and 12 in. Walls of 4 in block are not grouted at all: the cells are too small to fill and consolidate properly, so a reinforced wall needs 6 in block or wider.
Common mistakes
- Dividing by the block size. A 15⅝ × 7⅝ in block covers 8/9 ft² only together with its joint. Using the actual size overcounts by about 5 %.
- Forgetting the bond beam units. They are a different product, ordered separately, and they are taken out of the standard count.
- Ordering mortar by theory. The volume of the joints is small; the volume masons actually use is not. Batch figures include the mortar that falls and the mortar mixed but not used.
- Grouting with concrete. Grout is a fluid mix with small aggregate made to fill cells; ordinary concrete leaves voids.
For the footing under the wall, use the Concrete Calculator and the Rebar Calculator. Structural drawings govern bar size and spacing, grout and bond beams wherever there are any; this calculator turns them into a materials list.