Concrete Block (CMU) Calculator
Block count, courses, mortar and grout for a concrete block wall, with the exact blocks-per-square-foot arithmetic shown.
Blocks, mortar and grout for a wall. The block count is exact arithmetic; the mortar is not, and this is explicit about which is which.
Quick answer: 8 × 16 blocks and mortar for a wall
| Net wall | Blocks +5 % | Blocks +10 % | 80 lb mortar bags |
|---|---|---|---|
| 100 ft² | 119 | 124 | 8 |
| 200 ft² | 237 | 248 | 16 |
| 400 ft² | 473 | 495 | 31 |
| 800 ft² | 945 | 990 | 62 |
| 1,200 ft² | 1,418 | 1,485 | 93 |
An 8 × 16 in. nominal block covers 128 in² with its joint, 1.125 blocks per ft². Mortar is TEK 4-2A's 6.25 preblended bags per 100 hollow units (face-shell bedding), counted on the +10 % blocks. Deduct openings before multiplying.
| Net wall area | — |
|---|---|
| Blocks per ft² | — |
| Blocks (with waste) | — |
| Courses | — |
| Mortar (approx) | — |
| Grout (approx) | — |
Table 3 counts vertical cells only, and says so. CMHA (formerly NCMA) TEK 4-2A prints the caveat in the text beside it: additional grout may be necessary for horizontally grouting discrete courses. Those courses are bond beams, drawn on the wall section, not in the plan note about bar spacing. Table 5 sizes one: an 80 lb bag of preblended grout fills 2.0 linear ft of 8 in. bond beam and yields 0.66 ft³, so 0.33 ft³ per foot. Over a 30 ft wall, 9.9 ft³. Every vertical cell in that same 30 × 8 ft wall at 48 in. o.c. is 14.6 ft³ (Table 3, 6.1 ft³ per 100 ft²). One bond beam is 40% of the pour. Add a second at the lintel line and the horizontal outweighs the vertical.
Every row of Table 3 is the same number. A hundred square feet of wall holds 25 ft of cell at 48 in. o.c., 150 ft at 8 in.: 6.1 ÷ 25 = 0.244, 36.1 ÷ 150 = 0.241. Rounding, not spacing. Call it 0.24 ft³ per foot of cell in an 8 in. wall — about 1.95 ft³ for one cell 8 ft tall, two-core units, 3% waste included. A rule of thumb near 1 ft³ per cell is describing half a cell.
A bag is not a unit. Same document, Table 2: one 70 lb bag of masonry cement lays about 30 hollow units face-shell bedded; one 80 lb bag of preblended mortar lays 16. The first is cement alone, with a ton of sand for every eight bags. The second has it already. Per 100 blocks that is 3.3 bags one way and 6.25 the other, and the familiar three bags per 100 is the masonry-cement number. Printed next to an 80 lb preblended bag, it buys half the mortar.
What I check on a job
Check the wall against the block first. The block count is area times 1.125, and in height that only holds when the wall is a whole number of courses. The course count beside it rounds up, so I compare the two. Take the page's default 30 ft wall and raise it to 8 ft 4 in. The page gives 296 blocks with 5% waste and 13 courses. But 100 in. of height is 12.5 courses of 8 in. block, so the top half course is either ripped from whole 8 in. units or laid as one course of the 4 × 16 unit the page offers. Ripped, thirteen courses of 22.5 blocks is 293 before breakage, leaving three blocks of the 5% for chipped corners and bad cuts. With the 4 in. unit, that course is priced and ordered as its own line. Either way I size from courses times blocks per course, then add the waste, and I ask whether the dimension can move a few inches to land on a full course.
Exact: a nominal 8×16 block occupies 8″ × 16″ = 128 in²
of wall face including its mortar joint, so one square foot (144 in²) takes 144 ÷ 128 =
1.125 blocks. Courses are wall height ÷ nominal block height. That arithmetic does not vary.
Approximate: mortar is estimated at about 6.25 bags of 80 lb preblended mortar per 100 blocks (TEK 4-2A Table 2, ~16 units per bag;
the often-quoted three bags per hundred is 70 lb masonry cement, which still needs its sand), and grout at
about 1.95 ft³ per grouted cell per 8 ft of height (Table 3, 0.244 ft³ per foot of cell). Both move with joint thickness, block
absorption, the mason's hand and the weather. Reinforcing steel, bond beams, lintels and cleanouts
are not counted here, and grout spacing is a structural decision that belongs to the engineer of
record, not to this page.
Worked example
A 30 × 8 ft wall in standard 8 × 16 block: how many blocks?
- Area = 240 ft².
- An 8 × 16 block with its joint covers 8/9 ft², so 1.125 blocks per ft².
- 240 × 1.125 = 270; with 5% waste, 284. Courses = 8 ft ÷ 8 in = 12.
284 blocks in 12 courses, with mortar by the bag. Grouting every 48 inches adds the grout volume the page prints.
8 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How many 8 × 16 blocks per square foot?
1.125 — a block with its joint covers 8/9 ft². A 30 × 8 ft wall is 270 blocks before waste.
How many blocks per course in an 8 ft wall?
12 courses of 8-inch block make 8 ft; each course is the wall length in inches ÷ 16.
How much grout does one cell take?
About 1.95 ft³ for a cell 8 ft tall in an 8 in. wall: TEK 4-2A Table 3 works out to 0.244 ft³ per foot of cell at any spacing, two-core units, 3% waste included. A rule of thumb near 1 ft³ per cell describes half a cell.
Is it three bags of mortar per 100 blocks?
Only for 70 lb masonry cement, which lays about 30 units a bag and still needs a ton of sand per eight bags. An 80 lb bag of preblended mortar lays 16, so 100 blocks take 6.25 bags (TEK 4-2A, Table 2).