Case 1
- Wall area (sq ft)
- 100sq ft
- Blocks per square foot
- 1.125per sq ft
- Waste allowance
- 5%
119blocks
Open with these valuesConcrete Block Calculator
119blocks
Result: 119 blocksWall area times the blocks one square foot takes, times the waste factor, rounded up. A nominal 8 × 8 × 16 in unit shows 8/9 sq ft of face, so 1.125 blocks per sq ft: 100 sq ft of wall is 119 blocks with the 5 % allowance the standard builds in. Mortar runs about 30 blocks to a 70 lb bag.
The numbers at a glance
Held fixed: Wall area (sq ft) 100.0 sq ft, Blocks per square foot 1.125 per sq ft.
| Waste allowance (%) | Result (blocks) |
|---|---|
| 0 | 113 |
| 2 | 115 |
| 4 | 117 |
| 5Your value | 119 |
| 6 | 120 |
| 8 | 122 |
| 10 | 124 |
Worked examples
Case 2
- Wall area (sq ft)
- 200sq ft
- Blocks per square foot
- 1.125per sq ft
- Waste allowance
- 0%
225blocks
Open with these valuesCase 3
- Wall area (sq ft)
- 200sq ft
- Blocks per square foot
- 2.25per sq ft
- Waste allowance
- 0%
450blocks
Open with these valuesHow it's calculated
blocks = ⌈area × blocks per sq ft × (1 + waste)⌉
- StepEnter the net wall area in square feet — length × height, less doors and windows.
- StepSet the rate: 1.125 for a standard 8 × 8 × 16 in unit, 2.25 for a half-high one.
- StepSet the waste allowance; the standard's own table builds in about 5 %.
- ResultThe count rounds up to whole blocks. Mortar follows at roughly 30 blocks per 70 lb bag.
What this number means
For conventional units having nominal heights of 8 in. and nominal lengths of 16 in., the exposed surface area of a single unit in the wall is 8/9 ft2. Including a 5 percent allowance for waste and breakage, this translates to 119 units per 100 ft2 of wall area.
Where 1.125 blocks per square foot comes from
NCMA TEK 4-2A works it out from the face: a unit of nominal 8 in height and 16 in length shows 8/9 sq ft of wall, and the inverse of that is 1.125. The block's width does not enter — the standard calls the method independent of the unit width, so 6, 8, 10 and 12 in blocks all count the same.
The waste allowance here is published, not invented
The standard's own table already carries about 5 % for waste and breakage, which is why 100 sq ft comes out at 119 rather than 113. A masonry supplier's estimating guide gives the working range as 5 % to 10 % for breakage, spillage and errors.
Openings and special units come off the total
The standard says to subtract the area of windows, doors and other openings before you divide, and to count pilaster, corner and bond beam units separately and take them off the total. A wall with many returns and corners also runs over any rule of thumb.
Mortar is the most misestimated part
One 70 lb bag of masonry cement lays about 30 hollow units with face shell bedding, one 94 lb bag of portland cement about 62, and a ton of sand covers roughly 240 units. A supplier's guide puts the same job at three bags per hundred block.
Commonly misread
An 8 in block is wider than a 6 in one, so it needs its own rate.
The rate counts the face, not the width. Nominal 8 × 16 in faces give 1.125 blocks per sq ft whether the block is 6, 8, 10 or 12 in thick.
100 sq ft at 1.125 is 112.5, so I ordered 113 blocks.
That is the bare count with nothing broken. The standard's own figure for 100 sq ft is 119, because it carries about 5 % for waste.
I entered the wall volume, because the block is three-dimensional.
Single wythe walls are counted by face area, not volume — length × height. The block's width only matters for the mortar and grout.
Reference table
| Wall area, blocks per sq ft, waste | Blocks |
|---|---|
| 100, 1.125, 5 % | 119 |
| 200, 1.125, 0 % | 225 |
| 200, 1.125, 10 % | 248 |
| 200, 2.25, 0 % | 450 |
| 400, 1.125, 5 % | 473 |
Questions
How many concrete blocks do I need per square foot?
For the standard nominal 8 × 8 × 16 in unit with 3/8 in joints, 1.125 blocks per square foot. That is the inverse of the 8/9 sq ft of face the standard credits to one unit, and it holds for every block width.
How many blocks are in 100 square feet of wall?
NCMA TEK 4-2A puts it at 119 units per 100 sq ft, and that figure already includes about 5 % for waste and breakage. Without the allowance the bare count is 112.5, which rounds up to 113.
How much mortar does a block wall take?
One 70 lb bag of masonry cement lays roughly 30 hollow units with face shell bedding, so about three and a half bags per hundred block. A 94 lb bag of portland cement mixed with lime and sand lays about 62 units, and one ton of sand covers around 240.
How much waste should I add?
The 5 % default is the allowance the standard's table already carries. A masonry supplier's estimating guide gives 5 % to 10 % for breakage, spillage and errors — take the upper end for a wall with many corners, returns or cut units.
Do I subtract doors and windows?
Yes. The standard says to take the area of openings off the total wall area before dividing, and to count corner, pilaster and bond beam units separately. Enter the net area in the first field.
Sources and last check
Information, not professional advice.