CMU Calculator: Block Count & Mortar Bags

Estimate blocks and mortar for a concrete masonry (CMU) wall. Enter wall length and height, pick a block size, set the mortar joint, and subtract openings. Choose a waste allowance and get: courses, blocks per course, opening deductions, total blocks to order, and estimated mortar bags (Type S) for 60-lb / 80-lb or your own yields. Notes, formulas, and FAQs included.

Units Metric (m, mm) US Customary (ft, in)

Wall length

Wall height

Block (face) size

We count modules (block + one mortar joint) across and up.

Mortar joint thickness

Typical: 10 mm (metric) or 3/8 in (US).

Waste allowance

Many projects use 3–7% to cover cuts/breakage.

Subtract openings (optional)

Enter up to 5 rectangular openings to subtract (clear width × height).

#	Width m	Height m
1
2
3
4
5

We subtract the nearest whole modules covering each opening.

Corner / end allowance (optional)

Exposed wall ends

Adds ~2 corner/end units per course per exposed end.

Mortar bag yields (editable)

Blocks laid per bag (Type S)

80 lb: 60 lb:

Defaults from common Type S data sheets. Edit to match your brand.

Approx. wet mortar volume per bag

80 lb: ft³ 60 lb: ft³

Used to show approximate mortar volume. Edit per product sheet.

Results

Module geometry

Module (face) height: —

Module (face) length: —

Courses: 0

Blocks per course (before openings): 0

Block counts

Gross blocks (no openings): 0

Opening deduction: 0

Corner/end allowance: 0

Total blocks (net): 0

Blocks with waste: 0

Mortar bags (Type S)

80 lb bags: 0

60 lb bags: 0

Approx. wet mortar: 0.00 ft³ • 0.000 m³

How to use this CMU calculator

Select metric or US units.
Enter wall length and height.
Choose a block size (module) and joint thickness. (Module = block actual size + one joint.)
Optional: add openings and corner/end allowance.
Set a waste allowance. Review courses, blocks to order, and mortar bags. Round up for ordering.

Formulas & assumptions

Module dimensions: H_mod = H_actual + t_joint, L_mod = L_actual + t_joint (face dimensions).
Courses: n_courses = ⌈ H_wall / H_mod ⌉.
Blocks per course: bpc = ⌈ L_wall / L_mod ⌉.
Gross blocks: B_gross = n_courses × bpc.
Openings (deduction by modules): for each opening, n_open = ⌈ W / L_mod ⌉ × ⌈ H / H_mod ⌉. Sum and subtract.
Corner/end allowance: B_corner ≈ 2 × n_courses × n_{exposed ends}.
Net & waste: B_net = max(B_gross − B_open + B_corner, 0); B_order = ⌈ B_net × (1 + waste%) ⌉.
Mortar bags (Type S): user-editable yields. Defaults: 80 lb ≈ 13 blocks per bag, 60 lb ≈ 10 blocks per bag. Bags = ⌈B_net / yield⌉. Approx. wet volume uses per-bag ft³ inputs.
Scope: Planning estimate. Does not include bond beams, lintel units, specials, grout, reinforcement, or movement joints. Follow drawings & local code.

References

Rule-of-thumb block density: ~1 ⅛ blocks/ft² for 8×16 CMU. Block calculator guide.
Mortar estimating: ~3 bags per 100 standard blocks (rule-of-thumb). Masonry estimating guide (PDF).
QUIKRETE® Mason Mix Type S: 80 lb bag lays up to ~13 standard 8×8×16 blocks; 60 lb ≈ 10. Data sheet and retailer spec.
Wet mortar yield per 80 lb bag varies by product; many list ~0.7–0.72 ft³. SPEC MIX yield, retailer coverage.

FAQs

Should I count by area instead?

Area-based rules like “1.125 blocks/ft²” are fine for quick estimates. This calculator counts courses and modules so it handles openings and partials more realistically.

Does this include grout for filled cores?

No—this is mortar between units. For grout (core fill), use a concrete/grout volume calculator sized to your cell pattern and rebar schedule.

Do specialty units change counts?

Bond beam, lintel, jamb, and corner units can change waste and counts. This tool adds a simple corner/end allowance; adjust as needed for specials.