For one wall layer of conventional concrete masonry with a nominal 8 × 16-inch face module, 100 sq ft / (8/9 sq ft) = 112.5 block equivalents. Rounded up, that is 113; with the cited CMHA 5% waste/breakage estimate, it is 119.1 This area estimate is not a course layout or a structural design. Openings, special shapes and cuts need separate checks.
Nominal face module, not solid concrete face area
CMHA gives an exposed wall-area module of 8/9 sq ft for conventional units with nominal 8-inch height and 16-inch length, independent of their width.1 The face module is 8 × 16 / 144 = 0.888889 sq ft. Do not use the wall thickness as the exposed height or assume an actual measured block face has those exact nominal dimensions.
The area-estimate formula
Net wall area = gross wall face area − openings.
Block equivalents = net area × 1.125.
With chosen allowance = round up [net area × 1.125 × (1 + allowance % / 100)]. Keep fractional block equivalents until the final estimate is rounded. All tables here assume one wall layer and the conventional nominal face module.
CMHA includes 5% waste and breakage in its conventional-unit estimate, but cautions that non-modular layouts, returns and corners can significantly increase material needs.1 The 5% column is that estimating scenario, not a guarantee of enough blocks for every layout.
Block estimate by net wall area
No openings remain to be deducted from these areas. “Net rounded” is the rounded area estimate, not a verified full/half-block schedule. Special units must replace the corresponding standard units rather than simply being added to all of them.
| Net area (sq ft) | Block equivalents | Net rounded | With 5% |
|---|---|---|---|
| 32 | 36.0 | 36 | 38 |
| 40 | 45.0 | 45 | 48 |
| 72 | 81.0 | 81 | 86 |
| 80 | 90.0 | 90 | 95 |
| 100 | 112.5 | 113 | 119 |
| 120 | 135.0 | 135 | 142 |
| 139 | 156.375 | 157 | 165 |
| 180 | 202.5 | 203 | 213 |
| 320 | 360.0 | 360 | 378 |
Mortar: preserve the product’s “up to” wording
The cited QUIKRETE Mason Mix No. 1136 sheet says one 80-lb bag lays up to 13 standard 8 × 8 × 16-inch blocks.2 Dividing by 13 produces an estimate at that stated maximum coverage, not a guaranteed bag count or a universal mortar yield. CMHA’s separate preblended-mortar estimate uses about 16 conventional units per 80-lb bag under its stated assumptions.1 Do not average those figures or apply the general one to the named product.
CMHA notes that mortar quantities can vary with bedding, unit tolerances, construction conditions, batching and work stoppages.1 Confirm the selected mortar, joint/bedding plan and usable coverage. Stored spare blocks that are never laid do not consume mortar.
| Example units actually laid | 80-lb bags at up to 13 units/bag |
|---|---|
| 36 | 3 |
| 90 | 7 |
| 113 | 9 |
| 135 | 11 |
| 157 | 13 |
| 360 | 28 |
This mortar table uses integer units laid, not the block purchase allowance. Cuts and special shapes can change the takeoff. It excludes grout and footing concrete; those are different materials and quantities.
Three checked examples
1. 100 sq ft, no openings
Block equivalents = 100 × 1.125 = 112.5. Net rounded estimate = 113. With the CMHA 5% scenario: 112.5 × 1.05 = 118.125, rounded up to 119. If a verified layout actually lays 113 equivalent standard units, 113 / 13 = 8.692308, rounded to 9 bags at the Mason Mix sheet’s maximum coverage. Actual coverage can be lower.
2. 4 ft high × 20 ft long
Area = 80 sq ft. Block equivalents = 90. With 5%: 94.5, rounded up to 95. At 90 standard units laid, 90 / 13 = 6.923077, rounded to 7 bags at the product’s stated maximum coverage. Confirm the actual course and special-unit layout before ordering.
3. 8 × 20 ft with a 3 × 7-foot opening
Gross area = 160 sq ft; opening = 21 sq ft; net = 139 sq ft. Block equivalents = 156.375. With 5%: 164.19375, rounded up to 165. Net rounded estimate is 157; if a checked takeoff lays 157 equivalent standard units, 157 / 13 = 12.076923, rounded to 13 bags at maximum coverage. The opening’s lintel, jamb and cut-unit requirements are not designed by subtracting its area.
Count courses and special units before buying
An area estimate cannot tell you which block shapes fit each course. Check lengths and heights against the intended module, bonding pattern, joint dimensions, openings and returns. List corner, half, bond-beam, lintel or other specified units separately. CMHA says those units should be calculated separately and subtracted from the total conventional-unit count.1 Do not count a special unit twice.
The tables exclude reinforcement, grout, footing concrete, caps, flashing and other specified components. Wall height and length here are quantity examples, not approval for a freestanding, load-bearing or retaining wall. A masonry unit wall and a segmental retaining-wall system are not interchangeable.
FAQ
How many blocks per square foot?
For the conventional nominal 8 × 16-inch face module, 1.125 block equivalents per square foot, before allowance. Other nominal face sizes need their own area calculation.
Does block width change this face-area count?
CMHA says the conventional face-area method is independent of width.1 Width still matters for the specified wall, structural requirements and other quantities such as grout. It cannot be ignored for design.
Does 119 blocks cover any 100-sq-ft wall?
It is CMHA’s conventional single-layer area estimate including about 5% waste. A non-modular or detailed layout can need a different unit mix and allowance. Check the takeoff rather than treating it as a guarantee.
Can I fill block cells with mortar or bagged concrete?
This guide does not select a cell-fill material. Grout is estimated separately in CMHA’s note, with quantities affected by unit configuration and fill pattern.1 Follow the actual project specification.
Related tools
Square footage calculator | Concrete calculator for separately specified concrete quantities | Segmental retaining-wall block calculator for that different system
Sources
- CMHA: TEK 04-02A, Estimating Concrete Masonry Materials (nominal face module, allowance, special-unit replacement and variable mortar/grout estimates)
- QUIKRETE: Mason Mix No. 1136 (product-specific “up to 13” standard blocks per 80-lb bag)