Why block counting is easier than brick counting
Concrete masonry units are built around a 4 in module, and that makes the arithmetic clean. A standard unit is 15⅝ × 7⅝ in on the face, and the 3/8 in mortar joint brings it to a nominal 16 × 8 in. Sixteen times eight is 128 square inches, and 144 ÷ 128 = 1.125 blocks per square foot of wall face — the number every block estimator carries in their head. A half-height unit, 4 in nominal face height, doubles it to 2.25.
Notice that the nominal width of the unit — 4, 6, 8, 10 or 12 in — has no effect at all on the count. A 12 in wall takes exactly as many blocks per square foot as a 4 in wall. Width changes the weight, the price, the mortar in a full-bedded joint and, above all, the grout volume, because a wider unit has bigger cells.
Units themselves are specified by ASTM C90, which sets dimensional tolerances, minimum face-shell and web thicknesses, and strength and absorption limits. The mortar is specified by ASTM C270, and how the mortar is placed — face-shell or full bedding — is governed by TMS 602, the specification that accompanies the TMS 402 masonry code.
Mortar and grout are two different materials doing two different jobs
Mortar bonds units together. It goes in the joints only, it is mixed stiff enough to hold a unit up, and on hollow units it normally sits on the two face shells alone. That is face-shell bedding, and TMS 602 permits it for hollow units except at the starting course on the foundation and around cells that will be grouted, both of which get a full bed. Because face-shell bedding covers only 2 × 1.25 = 2.5 in of the 7⅝ in unit width on an 8 in block, it uses roughly a third of the mortar that full bedding does.
Grout is a fluid, high-slump concrete that fills the cells around reinforcement, bonding the bar to the masonry so the wall acts as a reinforced element. It is not mortar and it is not ordinary structural concrete — its slump is deliberately high, typically 8 to 11 in, so that it flows and consolidates in a narrow cell. Grout volume dwarfs mortar volume on any reinforced wall: on a solid-grouted 8 in wall, the grout is roughly twenty times the face-shell mortar.
Cells in a 16 in unit are on 8 in centres, so the fraction of cells you fill follows directly from the bar spacing on the structural drawings: bars at 32 in on centre fill one cell in four, bars at 48 in fill one in six. Set the grout percentage from the drawings, not from a guess — the difference between 25% and 100% on a 40 ft basement wall is several cubic yards.
Worked example: a 40 ft × 8 ft wall of 8 in block with one door
A 40 ft long, 8 ft high wall of nominal 8 × 8 × 16 block, with one 3 ft × 7 ft door, laid face-shell bedded with 3/8 in joints, bars at 32 in on centre, and a 5% waste allowance.
- Cell dimensions. S = 15.625 + 0.375 = 16.000 in. C = 7.625 + 0.375 = 8.000 in.
- Block per square foot. 144 ÷ (16 × 8) = 144 ÷ 128 = 1.125.
- Areas. Gross = 40 × 8 = 320 ft². Door = 3 × 7 = 21 ft². Net = 299 ft².
- Block before waste. 1.125 × 299 = 336.4 block.
- Order. 336.4 × 1.05 = 353.2 → 354 block.
- Courses. 96 in ÷ 8 in = 12 courses exactly, which is why block walls are dimensioned in 8 in steps. Blocks per course = 480 in ÷ 16 in = 30.
- Mortar. Cell face area 16 × 8 = 128 in²; unit face 15.625 × 7.625 = 119.14 in²; joint area = 8.86 in². Bedded depth = 2 × 1.25 = 2.5 in. Per block: 8.86 × 2.5 ÷ 1728 = 0.01282 ft³. For 336.4 block plus 5%: 4.53 ft³, or 8 bags of pre-blended mortar at 0.60 ft³ per 80 lb bag.
- Grout. Void area per unit = 15.625 × 7.625 − [2 × 1.25 × 15.625 + 3 × 1.25 × 5.125] = 119.14 − 58.28 = 60.86 in². Cell volume = 60.86 × 7.625 ÷ 1728 = 0.2685 ft³ per block. At 25% of cells: 0.2685 × 336.4 × 0.25 = 22.6 ft³ = 0.84 yd³.
Order the grout at a full cubic yard: ready-mix suppliers charge short-load fees, and grout you run out of mid-lift is a cold joint in a structural element.
How to read the result and where to check it
Check the course count first. A block wall should come out on a whole number of 8 in courses, because that is what the module is for. If your height leaves a remainder, the answer is almost never to rip the top course — block cuts badly along its length and a ripped course looks like what it is. Change the footing elevation, add a bond beam of a different depth, or use a course of half-height units.
Then reconcile block per course against the plan. Forty feet at 16 in per unit is 30 block per course, and 30 × 12 courses = 360 gross, against 336 net after the door. If those two routes disagree by more than a few units, you have mismeasured a run or double-counted a corner: at a corner, the two walls share the block, so measure along the centreline of the wall, not the outside face of both legs.
Grout is the number to be most careful with. The cell volume here is modelled from a standard two-core unit with face shells at the ASTM C90 minimum and webs of the same thickness; plants vary, lightweight units differ from normal weight, and some plants make units with deliberately larger cells for reinforcement. Net-area figures are published on every producer's data sheet, and on a job with more than a few yards of grout it is worth using theirs. Add grout waste separately — cells are never quite as clean as geometry says.
Block quantities and grout by unit size
| Nominal size | Block per 100 ft² | Face-shell mortar (ft³) | Full-bed mortar (ft³) | Grout, all cells (yd³) |
|---|---|---|---|---|
| 4 × 8 × 16 | 112.5 | 0.87 | 2.09 | 0.52 |
| 6 × 8 × 16 | 112.5 | 1.15 | 3.24 | 0.84 |
| 8 × 8 × 16 | 112.5 | 1.44 | 4.40 | 1.12 |
| 10 × 8 × 16 | 112.5 | 1.44 | 5.55 | 1.56 |
| 12 × 8 × 16 | 112.5 | 1.44 | 6.70 | 1.99 |
| 8 × 4 × 16 (half-height) | 225.0 | 2.40 | 7.31 | 1.06 |
Mortar and grout are geometric quantities before waste. Face-shell depths use the ASTM C90 minimum: 0.75 in for 4 in units, 1.00 in for 6 in, and 1.25 in for 8 in and wider.
Cinder block and concrete block are not the same thing
"Cinder block" describes an older unit made with coal cinder aggregate. What is sold today is a concrete masonry unit made with sand, gravel or expanded lightweight aggregate, and it is specified by ASTM C90 with a minimum net-area compressive strength. The distinction matters when you are matching an existing wall: units from the cinder era are lighter, weaker and dimensionally less consistent, and mortar type and anchor selection for repair work should reflect that. If the wall is structural, get the existing unit strength assessed rather than assuming it matches new stock.
Mistakes that wreck a block takeoff
- Measuring corners twice. At every outside corner the two walls share units. Measure along the centreline and the overlap disappears by itself.
- Deducting every small opening. Cutting around a small window generates offcuts that roughly cancel the deduction. Take out doors and large windows only.
- Forgetting bond beams, lintels and pilasters. Bond-beam and lintel units cost more, come out of a separate count, and are solid-grouted regardless of the wall's grout spacing.
- Using face-shell mortar for the whole wall. The starting course and every grouted cell get a full bed under TMS 602, so real mortar sits between the two figures this calculator reports.
- Ordering grout by the mortar rule of thumb. They are different materials at different volumes; grout on a solid-filled 8 in wall is over a cubic yard per 100 ft².
- Assuming a wall is fine without reinforcement. Bar size and spacing come from the structural drawings and from TMS 402 — never from a grout table. Lay out the bars with the rebar spacing calculator once an engineer has specified them.
What this calculator does not decide
This is a quantity tool, not a design tool. It will not tell you whether an 8 in wall is thick enough to retain four feet of soil, what bar size the wall needs, how far apart control joints belong, or whether your mortar type suits the exposure. Those questions live in TMS 402/602 and in the local building code, and on anything retaining soil or carrying load they belong to a licensed engineer. Where a wall retains earth, start with the retaining wall block calculator for segmental systems and get a design for anything over the height your code allows without one.
Everything below the block is a separate takeoff. The footing that carries the wall goes through the concrete footing calculator, a slab poured inside it through the concrete slab calculator, and the mortar itself — if you are site-mixing masonry cement, lime and sand rather than buying pre-blended bags — through the mortar mix calculator. For a brick veneer in front of the block, the brick calculator uses the same net-area logic.
One practical note on ordering: block is heavy and it is priced and delivered by the cube or the truckload. An 8 in normal-weight unit runs roughly 35 lb, so 354 block is about six tons on the ground. Plan the drop points before the truck arrives — moving block twice is the most expensive mistake on a small masonry job.