Why Wall Thickness Does Not Change the CMU Block Count
Updated: 9/24/2026
The block calculator counts concrete masonry units from wall area and the block’s nominal face dimensions. That can raise a practical buyer question: if a wall is thicker, should it need more blocks? For the estimate covered here, the answer is no. CMHA TEK 04-02A, retrieved 2026-09-08, states literally that its estimate is “independent of the concrete masonry unit width.” That is the source’s own statement, not an inference I am adding to it.
The reason becomes clear when the calculation is written out. The count is based on the rectangular face that is visible in the wall: nominal length multiplied by nominal height. Wall thickness is a different dimension, extending through the wall rather than across its face. Because the count formula never uses that through-wall dimension, changing it does not change the number of units covering the same net wall area. Thickness belongs to a separate structural question, including structural load-bearing calculations; this calculator does not perform those calculations.
The literal source statement
CMHA TEK 04-02A, “Estimating Concrete Masonry Materials,” gives an estimate of “119 units per 100 ft2” with “a 5 percent allowance for waste and breakage.” The same CMHA TEK 04-02A estimate is described as “independent of the concrete masonry unit width.” The width wording is important because it answers the question directly: the source does not say that the estimate changes with unit width. It says the estimate is independent of it.
The calculator’s boundaries repeat that exact point. Its source-backed explanation says the estimate is independent of the concrete masonry unit width, and its limits identify CMHA TEK 04-02A as the source for the quoted statement. So the absence of a wall-thickness field is deliberate. A thickness input would suggest that width changes this face-area count, even though the cited estimate says the opposite.
That does not make thickness irrelevant to every masonry decision. It means only that thickness is not an input to this particular material-count calculation. The calculator is estimating blocks, courses, masonry cement bags, and masonry sand for a wall. It is not sizing a structural assembly or checking structural load-bearing capacity. Keep those questions separate when using the result.
Count the face, not the wall depth
CMHA/NCMA TEK 02-01B, retrieved 2026-09-08, “Typical Sizes and Shapes of Concrete Masonry Units,” gives the typical nominal face dimensions as 8 in. by 16 in. CMHA TEK 05-12, retrieved 2026-09-08, “Modular Layout of Concrete Masonry,” clarifies the order as “(height by length) 8 by 16 in.” These are face dimensions: one dimension is the nominal height of the visible unit and the other is its nominal length.
The calculator converts that face rectangle into a square-foot rate. Its formula is 144 ÷ (nominal length × nominal height) per square foot of net wall area. The 144 is the square inches in one square foot as used by the calculator’s formula; the two dimensions in the denominator are the nominal 16 in. length and nominal 8 in. height from CMHA/NCMA TEK 02-01B and CMHA TEK 05-12. The resulting rate for that example is 1.125 blocks per square foot.
Notice what is missing from the formula: wall width. There is no multiplication by wall thickness, no division by wall thickness, and no thickness correction after the result is rounded. The formula tiles block faces across the net wall face. If the same net face area is built with a wider unit but the same nominal face length and height, this count still sees the same face coverage. If the construction question changes to structural capacity, that is outside the calculator’s calculation.
The calculator also subtracts door and window area before applying the waste allowance. That is why the relevant surface is called net wall area: it is the wall area remaining after openings are removed. Courses are a separate result, based on wall height and the block’s nominal height. CMHA TEK 05-12 supplies the standard module as “8 in. vertically and horizontally,” and the calculator uses the nominal block height for the course figure. Neither the block count formula nor that course formula introduces wall thickness.
Purchase blocks versus blocks actually laid
The calculator deliberately reports a purchase quantity that includes waste and breakage, while mortar is based on the smaller number of blocks actually laid. CMHA TEK 04-02A is the source for the “5 percent allowance for waste and breakage.” The calculator applies that allowance to the net-area block count to determine how many blocks are needed to purchase.
The blocks actually laid are calculated without the allowance and then rounded up to whole blocks. Broken or damaged blocks are never mortared in, so they do not belong in the mortar calculation. The calculator therefore uses the actually-laid figure for masonry cement bags and masonry sand, not the larger purchase figure. This is why the two block numbers must not be treated as interchangeable when ordering mortar materials.
Worked example: a 20 ft by 8 ft wall
Use the calculator’s example inputs: a 20 ft × 8 ft wall, no door or window openings, a 16 × 8 in. block, and a 5% waste allowance. The source file’s example gives the following path through the calculation.
First, the net area is 20 × 8 = 160 sq ft because there are no openings. The block face rate is 144 ÷ (16 × 8) = 1.125 blocks per sq ft. Those dimensions and the face-area formula are the calculator’s values, with the nominal face dimensions sourced to CMHA/NCMA TEK 02-01B and CMHA TEK 05-12.
For the amount that will actually be laid, multiply 160 × 1.125 and round up: 180 blocks. For the amount to purchase, apply the 5% waste allowance before rounding: round up(160 × 1.125 × 1.05) = 189 blocks. The 189 figure is therefore the buying figure, while 180 is the figure representing sound blocks that are actually mortared into the wall. The 5% allowance is the calculator’s stated default from CMHA TEK 04-02A.
Courses use the wall height and nominal block height: round up(8 × 12 ÷ 8) = 12 courses. The 12-inch conversion and the course calculation are the example’s stated arithmetic; the calculator explains that courses are based on wall height alone, and CMHA TEK 05-12 supplies the 8 in. vertical module.
Now calculate mortar from 180, not 189. The calculator uses CMHA TEK 04-02A’s yield of “approximately 30 hollow units” for a 70-lb bag of masonry cement with face shell bedding. Therefore, masonry cement bags are round up(180 ÷ 30) = 6 bags. The same source gives 1 ton, or 2,000 lb, of masonry sand for every 8 bags; the calculator expresses that relationship as 250 lb per bag. Thus, masonry sand is 6 × 250 = 1,500 lb.
The example shows why the labels matter. You purchase 189 blocks because the estimate includes the source-backed waste and breakage allowance. You mortar 180 blocks because that is the rounded count of units that actually get laid. The six bags and 1,500 lb of sand follow the 180-block figure, since damaged units are not mortared in. The wall’s thickness does not enter any of these face-area, course, or mortar steps.
What the calculator does and does not answer
For a single wall, the calculator’s scope is clear: net wall area, block face dimensions, waste allowance, courses, and mortar materials. It covers hollow concrete masonry units and the masonry-cement path described by CMHA TEK 04-02A. Its source notes also say that a 3/8 in. mortar joint is already built into the block’s nominal dimensions rather than being a separate changeable input.
That scope explains the answer to the buyer question. The block count is a face-coverage estimate, and CMHA TEK 04-02A literally calls its estimate “independent of the concrete masonry unit width.” Use the block calculator to work through the net area and nominal face dimensions. Treat any structural load-bearing calculation as a separate task, because wall thickness is not modeled by this calculator and cannot change the count it produces.