◆ Calculators ◆ Concrete Block Calculator: How Many CMU Blocks, Courses and Mortar Bags You Need
Concrete Block Calculator: How Many CMU Blocks, Courses and Mortar Bags You Need
Work out how many concrete blocks (CMU), courses and masonry cement bags a wall needs, with the CMHA/NCMA source figures spelled out.
Total area of doors and windows in this wall, in sq ft — subtracted before the waste allowance is applied. Leave at 0 if there are none.
CMHA/NCMA TEK 02-01B and TEK 05-12: concrete masonry units typically have nominal face dimensions of (height by length) 8 by 16 in. Change these only if your blocks use a different module.
CMHA/NCMA TEK 02-01B and TEK 05-12: concrete masonry units typically have nominal face dimensions of (height by length) 8 by 16 in. Change these only if your blocks use a different module.
CMHA TEK 04-02A recommends a 5 percent allowance for waste and breakage — already the source of the 5 % default.
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How the number is worked out
The steps from input to result
This calculator turns a wall area, minus any door and window openings, into the number of concrete blocks (CMU) you need to buy, how many courses (rows) the wall will be, and — separately — how many bags of masonry cement and how much sand it takes to mortar the blocks that actually get laid.
CMHA/NCMA TEK 02-01B, "Typical Sizes and Shapes of Concrete Masonry Units", states that concrete masonry units typically have nominal face dimensions of 8 in. (203 mm) by 16 in. (406 mm). CMHA TEK 05-12, "Modular Layout of Concrete Masonry", assigns which number is which: nominal face dimensions of (height by length) 8 by 16 in. (203 by 406 mm) — the first source names the two figures, the second says which one is the height. The same TEK 05-12 also states that standard concrete masonry modules are typically 8 in. (203 mm) vertically and horizontally, which is why the course count below uses the block's nominal height, not an assumed 8 in.
CMHA TEK 04-02A, "Estimating Concrete Masonry Materials", gives 119 units per 100 ft2 (9.29 m2) of wall area as its estimate, already built on a 5 percent allowance for waste and breakage — this calculator arrives at the same figure independently from the block's own nominal dimensions (144 ÷ (nominal length × nominal height), applied per square foot of net wall area), and the two ways agree once rounded, so the 5 % default matches the source's own estimate rather than being a separate guess.
The same source states that a 70-lb bag of masonry cement lays approximately 30 hollow units if face shell bedding is used, and that 1 ton (2,000 lb, 907 kg) of masonry sand is required for every 8 bags of masonry cement — this calculator uses those two figures for the mortar cement bag and sand figures. Waste and breakage (broken or damaged blocks) are never mortared in, so the mortar figures are based on the blocks actually laid, before the waste allowance is added — not on the larger purchase quantity above.
The source also states that the estimate is independent of the concrete masonry unit width, because it assumes face shell mortar bedding — that's why this calculator has no wall-thickness field: entering one would have no effect on the result, which would be misleading rather than helpful.
The formula
Written out, so you can check it by hand
Net area = max(0, wall length × wall height − door/window area). Blocks per sq ft = 144 ÷ (block nominal length × block nominal height). Blocks needed = round up(net area × blocks per sq ft × (1 + waste ÷ 100)). Blocks actually laid = round up(net area × blocks per sq ft), without waste. Courses (rows) = round up(wall height × 12 ÷ block nominal height). Masonry cement bags = round up(blocks actually laid ÷ 30); masonry sand = bags × 250 lb.
Worked example
Worked with the preset values
With a 20 ft × 8 ft wall, no openings, a 16 × 8 in. block and a 5 % waste allowance: the gross and net area are both 20 × 8 = 160 sq ft. Blocks per sq ft = 144 ÷ (16 × 8) = 144 ÷ 128 = 1.125, so the blocks actually laid = round up(160 × 1.125) = round up(180) = 180, and the blocks needed to purchase = round up(160 × 1.125 × 1.05) = round up(189) = 189. Courses = round up(8 × 12 ÷ 8) = round up(12) = 12. Mortar is figured from the 180 blocks actually laid, not the 189 with waste: masonry cement bags = round up(180 ÷ 30) = 6, and masonry sand = 6 × 250 = 1,500 lb.
What this calculator does not do
Named, not left out
- 01The existing mortar calculator states a different yield for the same 70-lb bag of masonry cement — approximately 46 units, not 30. Both figures are correctly sourced; they assume different laying methods (full mortar bed there, face shell bedding here), and this calculator does not smooth the difference away.
- 02The block count is independent of the concrete masonry unit width — CMHA TEK 04-02A states the estimate is "independent of the concrete masonry unit width", which is why there is no wall-thickness field here.
- 03The table "Minimum Face Shell and Web Requirements" in CMHA/NCMA TEK 02-01B is an image, not text, in the source PDF — no figure from that table was readable, so none is used in this calculator.
- 04No correction for half blocks or running-bond patterns at corners and ends is applied — none of the three sources checked gives a factor for it.
- 05Only hollow concrete masonry units are covered — CMHA TEK 04-02A states its figures for "hollow units"; no primarily sourced figure was found for solid units.
- 06Only the masonry cement path (70-lb bag) is covered for mortar — a separate Portland cement mix is not modeled here.
- 07A 3/8 in. mortar joint is already built into the block's nominal dimensions and is not a separate, changeable input.
- 08One wall per calculation — no multi-wythe or cavity wall construction is modeled.
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Updated: 9/8/2026