Show the math
- Convert to base units: Wall length 20 ft; Wall height 4 ft
- Wall area = 20 × 4 = 80 ft²
- Blocks per ft² = 1.125 (8 × 16 in nominal face including a 3/8 in joint)
- Blocks = 80 × 1.125 = 90; × (1 + 10%) = 99 → 99
How to measure
Measure the length of the wall and its height from the top of the footing or slab to the top of the highest course. For a wall with a stepped top, calculate each section separately. Measure each door or window opening and subtract its area.
A standard block is nominally 8 in high and 16 in long. Its real size is 3/8 in smaller each way, so that a 3/8 in mortar joint brings the module back to 8 × 16 in (NCMA TEK 2-1B). The count per square foot is therefore based on the nominal size.
The block count formula
One block face is 8 in × 16 in, which is 128 in², or 0.889 ft². Divide one square foot by that and you get 1.125 blocks per square foot of wall. The factor is the same for 4, 6, 8, 10 and 12 in wide units, because they share the 8 × 16 in face (NCMA TEK 2-1B). With a 5% allowance, CMHA TEK 4-2A states 119 units per 100 ft². Half-height 4 × 16 in blocks double the count to 2.25 per square foot.
Formula: Blocks = (wall area − openings) × 1.125 × (1 + waste%), rounded up.
Worked example: a 20 × 4 ft garden wall
- Convert to base units: Wall length 20 ft; Wall height 4 ft
- Wall area = 20 × 4 = 80 ft²
- Blocks per ft² = 1.125 (8 × 16 in nominal face including a 3/8 in joint)
- Blocks = 80 × 1.125 = 90; × (1 + 10%) = 99 → 99
That is 99 blocks with 10% waste (90 blocks net · includes 10% for breakage and cuts). Block walls are built in 8 in courses, so a 4 ft wall is six courses of 20 ft / 16 in = 15 blocks, for a total of 90 full blocks before cuts. Wall height should be a multiple of 8 in to avoid cutting every block.
Blocks for common walls
| Wall | Area | Net blocks | With 10% waste |
|---|---|---|---|
| 20 × 4 ft | 80 ft² | 90 | 100 |
| 30 × 4 ft | 120 ft² | 135 | 149 |
| 30 × 6 ft | 180 ft² | 203 | 223 |
| 40 × 8 ft | 320 ft² | 360 | 397 |
| 50 × 8 ft | 400 ft² | 450 | 496 |
| 100 × 6 ft | 600 ft² | 675 | 743 |
What the count does not cover
- Mortar: block walls use a lot of it. Estimate separately from the supplier’s coverage data.
- Grout and reinforcement: cores are grouted with reinforcement as the design specifies. Estimate grout volume from the plans.
- Footing: block walls sit on a concrete footing. Use the footing calculator.
- Special shapes: corners, bond beams, lintels and end blocks are different units.
- Cuts and breakage beyond the waste allowance on complicated walls.
Block wall or poured wall?
A poured wall uses forms and a concrete truck; a block wall needs skilled laying but no forms. For small garden walls and retaining walls under a few feet, block is common. For walls with strict structural requirements, check with an engineer. To price the poured alternative, use the wall calculator. Retaining walls and tall walls need an engineered design.
Mistakes to avoid
- Using volume instead of face area. Block counts are about area.
- Forgetting to subtract openings, or subtracting them without allowing for lintel blocks.
- Ignoring the footing and starting the wall at grade.
- Ordering exactly the net count. Cuts, corners and breakage use up 5 to 10% on a typical job.
- Mixing nominal and actual dimensions. Use nominal for counts.
Course heights and corners
Block walls grow in 8 in courses, so a wall height of 4 ft is six courses and 8 ft is twelve. The count is by area, but real walls have corners, ends and special units such as bond beam blocks, lintels and half-blocks. A 40 ft wall, 4 ft high, comes to 198 blocks with 10% waste. Order the corner and end units from the plan separately. Mortar is also needed: ask the supplier for its coverage figure for your block size and joint thickness.
Frequently asked questions
How many cinder blocks per square foot?
1.125 blocks per square foot for the standard 8 × 16 in face. Modern “cinder blocks” are concrete masonry units (CMU).
How many blocks for a 10 ft wall, 4 ft high?
40 ft² × 1.125 = 45 blocks, or about 50 with 10% waste.
What size is a standard concrete block?
Nominally 8 × 8 × 16 in. The actual unit is about 7⅝ × 7⅝ × 15⅝ in, and the 3/8 in joint makes up the difference.
Does the block thickness change the count?
No. The 4, 6, 8, 10 and 12 in units share the same 8 × 16 in face, so the count per square foot is the same.
How many blocks in a pallet?
It depends on the block size, usually about 90 to 108 for an 8 in unit. Ask your yard.
Do I need to fill block cores with concrete?
Only where the design requires reinforced and grouted cells. Check the plans or an engineer.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.