Concrete Block Calculator
CMU blocks and cost for walls from area.
What it does
Concrete masonry units lay on nominal eight by sixteen inch faces, covering eight ninths of a square foot each once mortar joints count. Wall area divided by that face gives the count, rounded up with cuts for openings and corners.
The tool above takes wall area in either system with block price. It returns block count, echoed area, and cost.
Mortar joints explain the odd math. Real blocks measure three eighths short so joints complete the module, and coursing stays level across openings because of it.
Half blocks, bond beams, and lintels escape rectangle math. Corners and ends need halves while openings need steel spans, all ordered beside the field count.
How to use
- Switch the unit system first: imperial for ft2, metric for m2.
- Measure length times height minus big openings.
- Read the block count and add cuts allowance.
- Order halves, caps, and mortar alongside.
Formula
- 0.8889 ft2
- Nominal 8x16 in face with mortar joint share.
- 0.0741 m2
- 390x190 mm face with joint share.
- ceil
- Round up. Partial courses still need whole blocks.
Actual blocks run 7.625 by 15.625 inches; joints make up the module. Metric blocks vary by maker around the 390 by 190 standard.
Worked example
Example: 260 ft2 wall at $2.50 per block
Wall area 260 ft2 after openings.
- Divide: 260 / 0.8889 = 292.47.
- Round up: 293 blocks.
- Cost: 293 x $2.50 = $732.50 before mortar.
- With 5% cuts: about 308 blocks ordered.
Reference table
| Area | Blocks | At $2.50 |
|---|---|---|
| 100 ft2 | 113 | $282.50 |
| 260 ft2 | 293 | $732.50 |
| 500 ft2 | 563 | $1,407.50 |
| 1000 ft2 | 1125 | $2,812.50 |
Limits
- Field blocks only. Halves, caps, and lintels excluded.
- Mortar volume needs separate math.
- Openings estimated by area, not lintel schedule.
- Structural cells and grout excluded.
Common mistakes
- Using actual block size without joint share.
- Forgetting cuts around every opening.
- Skipping half blocks at corners and ends.
- Pricing blocks without mortar and delivery.
- Laying without level coursing lines.
Try it yourself
Quick checks. Answers sit tucked under each question.
128 ft2 wall. Blocks?
128 / 0.8889 = 144.0, so 144 blocks exactly.
Metric: 24.2 m2?
24.2 / 0.0741 = 326.6, so 327 blocks.
Questions
How many blocks per square foot?
About 1.125. One block covers 0.8889 ft2 with joints.
What is a CMU?
Concrete masonry unit: the formal name for concrete and cinder blocks.
How much mortar per block?
Roughly a third of a 60 lb bag per block varies widely. Masons estimate per wall.
Do I need rebar in block walls?
Structural and tall walls do: vertical bars with grouted cells per plan.
What about wall caps?
Cap blocks finish tops separately. Count them by linear feet.
Can beginners lay block?
Garden walls, yes with level lines. Structural walls want masons.