CMU and cinder block walls
CMU block calculator
A 20 ft × 8 ft wall with a 21 ft² door has 139 ft² of net wall and needs about 165 standard 8 × 16 in. concrete blocks including 5% waste, plus 6 bags of masonry cement for mortar. Enter your own wall size and openings below, choose the block face, and add pallet size or prices for pallets and cost. Last updated October 12, 2026.
Concrete block estimate
Measurement system: Imperial. Planning estimate from the wall dimensions and block size below.
Count blocks for your wall
Enter the wall size, subtract openings, and choose the block face.
Subtract door and window areas from the wall area. Diagram not to scale.
Net blocks plus your selected waste, rounded up.
WALL AND BLOCK BREAKDOWN
MORTAR (NCMA TEK 04-02A)
Counts use the nominal block face and mortar rules of thumb from published masonry guidance. Pallet size, unit size and mortar yield vary by supplier and product. It does not size foundations, reinforcement or grout.
The formula
Net wall area divided by the face area of one block
Measure the wall length and height, multiply them for the gross area, and subtract every door and window. Divide what remains by the face area of one block, then add a waste allowance and round up to whole blocks. A nominal 8 × 16 in. block covers 8 × 16 ÷ 144 = 8/9 ft² of wall, so a square foot of wall needs 1.125 blocks.
Nominal sizes are the layout module, which includes the mortar joint, so the counts do not need a separate joint allowance. This is the method in NCMA’s TEK 04-02A, which uses a 5% waste allowance. Larger or more complicated walls, with many corners and returns, can need more than the simple count.
Worked examples
Five wall estimates, calculated step by step
- A 20 ft × 8 ft wall with a 3 ft × 7 ft door. Gross area 20 × 8 = 160 ft², less 21 ft² = 139 ft². Blocks = 139 ÷ (8/9) = 156.375. With 5% waste: 164.19, rounded up to 165 blocks.
- The published benchmark: 100 ft² with no openings. 100 ÷ (8/9) = 112.5 blocks; with 5% waste that rounds up to 119, matching the 119 units per 100 ft² in TEK 04-02A. The mortar rule of thumb in TEK 03-11 is 8.5 ft³ for the same 100 ft².
- The first wall in half-high 4 × 16 in. blocks. A half-high block covers 4/9 ft², so the same 139 ft² needs 312.8 blocks, or 329 with 5% waste: twice the count of the standard block.
- A metric wall, 6 m × 2.4 m with a 2 m² opening. 14.4 − 2 = 12.40 m² = 133.47 ft². Standard 8 × 16 in. blocks: 150.2 before waste, 158 with 5%.
- Pallets and cost, using hypothetical figures. If a pallet holds 90 blocks (hypothetical, ask your supplier), 165 blocks is 2 pallets. At a hypothetical 2.50 per block, 165 blocks cost 412.50, and 6 mortar bags at a hypothetical 12.00 each add 72.00. These prices are not market quotes.
These are quantity calculations from the dimensions shown. Wall design, reinforcement and grouting come from your plans.
Blocks and mortar by wall area
| Net wall area (ft²) | Blocks before waste | Blocks with 5% | 70 lb masonry cement bags | Masonry sand (tons) |
|---|---|---|---|---|
| 100 | 112.5 | 119 | 4 | 0.50 |
| 200 | 225.0 | 237 | 8 | 1.00 |
| 500 | 562.5 | 591 | 19 | 2.38 |
| 1,000 | 1,125.0 | 1,182 | 38 | 4.75 |
Block face sizes and blocks per square foot
- 8 × 16 in112.5 blocks
- 4 × 16 in225.0 blocks
- 12 × 16 in75.0 blocks
- 8 × 18 in100.0 blocks
- 8 × 24 in75.0 blocks
| Nominal face (in) | Face area (ft²) | Blocks per ft² | Blocks per 100 ft², before waste |
|---|---|---|---|
| 8 × 16 | 0.889 | 1.125 | 112.5 |
| 4 × 16 | 0.444 | 2.250 | 225.0 |
| 12 × 16 | 1.333 | 0.750 | 75.0 |
| 8 × 18 | 1.000 | 1.000 | 100.0 |
| 8 × 24 | 1.333 | 0.750 | 75.0 |
The widths available are 4, 6, 8, 10 and 12 in. for standard units. Width does not change the count, because the count depends only on the face. It changes the weight of the wall and the mortar and grout details.
Mortar for block walls
For face shell bedding, NCMA’s TEK 04-02A says one 70 lb bag of masonry cement lays about 30 hollow blocks, with 1 ton of masonry sand per 8 bags. For portland cement–lime mortar, one 94 lb bag of portland cement lays about 62 blocks, using about half a 50 lb bag of hydrated lime and 4¼ ft³ of sand. In the 20 ft × 8 ft wall above, masonry cement needs 6 bags and 0.75 tons of sand; portland cement–lime needs 3 cement bags, 2 lime bags and 12.75 ft³ of sand.
The sand figure is unrounded. TEK 04-02A suggests adding half a ton of waste when more than 3 tons are needed and rounding smaller amounts up. A second rule of thumb, in TEK 03-11, gives about 8.5 ft³ of mortar per 100 ft² of wall with 3/8 in. joints, face shell bedding and 10% waste. Preblended mortar is sold in 60 to 80 lb bags, so follow its label for coverage.
What this estimate leaves out
The calculator counts field blocks for a straight wall. It does not include grout, reinforcing steel, special units, the footing, flashing, control joints or labor. Grout volume depends on unit density, which cells are filled and how it is placed, and TEK 04-02A has tables for it. Treat the count as a planning number and confirm it with your supplier or an estimator for large or complex walls.
Common questions
Concrete block questions
How many concrete blocks do I need for a wall?
Divide the net wall area (wall area minus doors and windows) by the face area of one block, then add waste. A nominal 8 × 16 in. block covers 8/9 ft², so each square foot needs 1.125 blocks. A 20 ft × 8 ft wall with a 21 ft² door has 139 ft² of net wall, needs 156.38 blocks, and 165 with 5% waste.
How many CMU blocks are in 100 square feet?
112.5 blocks before waste, because 100 ÷ (8/9) = 112.5. NCMA’s TEK 03-11 rounds this to about 113 units per 100 ft², and TEK 04-02A lists 119 units per 100 ft² once a 5% waste allowance is included. This calculator gives 119 for the same wall at 5% waste, since it rounds the order up to whole blocks.
How many bags of mortar per block?
NCMA’s TEK 04-02A says one 70 lb bag of masonry cement lays about 30 hollow blocks, about 0.033 bags per block, and one 94 lb bag of portland cement lays about 62. For the 156.38 blocks in the 20 ft × 8 ft wall that is 6 bags of masonry cement, or 3 bags of portland cement with lime. These are rules of thumb for face shell bedding.
How many cinder blocks are on a pallet?
It depends on the block size and the producer. Pallet counts are not a standard figure, so enter the number from your supplier’s quote in the optional pallet field and the calculator divides your order into whole pallets.
What is the difference between a cinder block and a CMU?
“Cinder block” is the everyday name for what suppliers sell as concrete block or concrete masonry units (CMU). The calculation is the same: wall area divided by the face area of one unit. Check which unit your supplier is quoting, because sizes and weights differ.
Does this include grout, rebar or special blocks?
No. Grout volume depends on the unit, which cells are filled and how the grout is placed, so use the tables in NCMA’s TEK 04-02A for a grouted wall. Corner, bond beam, lintel and pilaster units are counted separately and subtracted from the field total.
How do I subtract windows and doors?
Multiply each opening’s width by its height, add them up, and enter the total as openings. A 3 ft × 7 ft door is 21 ft². Removing it from the 160 ft² wall above cuts the count from 189 to 165 blocks.
Are the sizes nominal or actual?
Nominal. The nominal size is the module used for layout, so an 8 × 16 in. unit is counted as 8/9 ft² of wall including its mortar joint. The unit actually delivered is slightly smaller. Counting from nominal face area is the method in NCMA’s TEK 04-02A.
Sources and limits
Technical notes (TEK) published by the Concrete Masonry & Hardscapes Association, read on October 12, 2026; the pages show no revision date: TEK 04-02A, Estimating Concrete Masonry Materials (units per area, 5% waste, mortar bag yields); TEK 03-11 (113 units and 8.5 ft³ of mortar per 100 ft²); TEK 02-02B (standard unit sizes).
Pallet quantities, unit weights and prices come from your supplier. For brick or a closer look at mortar, use the mortar calculator. For gravel under a wall or slab, use the gravel calculator. To price concrete you pour yourself, use the concrete cost calculator.
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