Concrete Block Wall Calculator
Enter your wall’s length, height, and any door or window openings to get the exact number of concrete blocks and mortar bags you need, plus a total material cost.
Total area of doors, windows or other openings to subtract.
Covers cuts, breakage and layout waste. 5–10% is normal.
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Ordering tips
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Coverage & waste
Material profile
For a 30 ft long, 8 ft tall concrete block wall with no openings, using standard 8 x 8 x 16in CMU blocks (0.89 sq ft of coverage each) and a 10% waste allowance, you need 297 blocks and 9 bags of block mortar (33 blocks per bag). At $1.85 per block and $14 per mortar bag, that’s about $675.45 in materials.
Concrete blocks needed by wall size
These estimates use standard 8 x 8 x 16in CMU block (0.89 sq ft coverage), a 10% waste factor, and 33 blocks per 80lb mortar bag.
| Wall size | Openings | Units needed | Mortar bags |
|---|---|---|---|
| 10 ft × 4 ft | 0 sq ft | 50 blocks | 2 bags |
| 20 ft × 6 ft | 15 sq ft | 130 blocks | 4 bags |
| 30 ft × 8 ft | 0 sq ft | 297 blocks | 9 bags |
| 40 ft × 8 ft | 21 sq ft | 370 blocks | 12 bags |
| 50 ft × 6 ft | 0 sq ft | 371 blocks | 12 bags |
| 60 ft × 8 ft | 32 sq ft | 554 blocks | 17 bags |
Concrete block sizes and where to use them
| Type | Coverage / unit | Size | Best for |
|---|---|---|---|
| 8 x 8 x 16in standard CMU | 0.89 sq ft | 8 x 8 x 16in | Foundation & retaining walls |
| 10 x 8 x 16in CMU | 0.89 sq ft | 10 x 8 x 16in | Heavier load-bearing walls |
| 12 x 8 x 16in CMU | 0.89 sq ft | 12 x 8 x 16in | Structural / retaining walls |
How to calculate how many concrete blocks you need
- Measure the wall. Multiply length by height in feet to get total wall area.
- Subtract openings. Deduct the square footage of any doors, windows, or gaps from the total area to get net area.
- Add a waste allowance. Multiply net area by 1 plus your waste percentage (10% is standard for straight-run walls with few cuts).
- Divide by block coverage. Divide the waste-adjusted area by the coverage of one block — 0.89 sq ft for standard 8 x 8 x 16in CMU — and round up.
- Calculate mortar. Divide the block count by the number of blocks one 80lb bag of mortar will set (33 for standard block) and round up.
Worked example
Using the calculator’s defaults: a 30 ft long, 8 ft tall wall, no openings, 10% waste, standard CMU block, $1.85 per block, and $14 per mortar bag.
- Wall area (30 ft × 8 ft)240 sq ft
- Openings deducted0 sq ft
- Net area240 sq ft
- Area with 10% waste264 sq ft
- Blocks needed (264 ÷ 0.89, rounded up)297 blocks
- Mortar bags needed (297 ÷ 33, rounded up)9 bags
- Estimated total cost (297 × $1.85 + 9 × $14)$675.45
Block quantities for typical DIY wall sizes
| Wall size | Net area | Quantity needed |
|---|---|---|
| 8 ft × 4 ft | 32 sq ft | 40 blocks |
| 12 ft × 6 ft | 72 sq ft | 89 blocks |
| 20 ft × 4 ft | 80 sq ft | 99 blocks |
| 25 ft × 8 ft | 200 sq ft | 248 blocks |
| 40 ft × 6 ft | 240 sq ft | 297 blocks |
Ordering blocks and mortar
Order all your blocks in a single delivery when possible — CMU dimensions and color can shift slightly between production runs. Round up to full bags of mortar mix; a partial bag left over is cheaper than a mid-job run to the supplier. Add a few extra blocks beyond the waste-adjusted total for corners, cuts, and future repairs.
- Confirm block core type.Standard CMU is hollow-core; check whether your design calls for solid block at bond beams or anchor points.
- Match mortar type to application.Type S mortar is standard for load-bearing walls; Type N works for lighter garden walls.
- Add rebar and grout separately.This calculator covers block and mortar only — reinforced walls need rebar and grout fill quantities calculated on top.
Common mistakes
- Forgetting the waste factor.Cuts at corners, ends, and courses around openings create offcuts that can’t be reused — skipping waste allowance runs you short mid-wall.
- Measuring outside dimensions only.Wall length should follow the actual block coursing line, not just the finished footprint, especially at corners.
- Ignoring opening headers.Window and door openings need lintels or bond beams above them — factor that block and rebar in separately.
- Using the wrong coverage figure.0.89 sq ft per block applies to standard 8 x 16in face CMU; larger structural block or split-face units cover different areas.
- Underestimating mortar for wide joints.3/8in joints are standard, but thicker joints or block laid with more mortar per course will use bags faster than the coverage ratio suggests.
- Not accounting for the first course.The starter course often sits on a mortar or grout bed that uses more mortar than the coursing above it.
What this calculator assumes
Adjust your result if your project differs from these.
- Standard 3/8in mortar joints on all four sides of each block.
- Running bond coursing pattern (blocks overlap by half-length row to row).
- Coverage of 0.89 sq ft per standard 8 x 8 x 16in block, based on nominal face dimensions.
- 33 blocks per 80lb bag of Type S mortar mix, a typical yield for full-bed joints.
- Waste percentage covers cuts, breakage, and corner returns — not structural bond beams or rebar.
- Openings are deducted as simple rectangular area; irregular openings should be measured individually.
Common questions
How many blocks do I need for a 30 ft wall?
For a 30 ft long, 8 ft tall wall with no openings and standard 8 x 8 x 16in block, you need about 297 blocks including a 10% waste allowance.
How much mortar do I need per concrete block wall?
Roughly 1 bag of Type S mortar mix per 33 standard CMU blocks. A 297-block wall needs about 9 bags.
What size concrete block should I use?
Standard 8 x 8 x 16in block works for most foundation and garden walls. 10 or 12in block is used for taller or load-bearing structural walls.
How much waste should I add for a block wall?
10% is standard for straight walls with simple corners. Add more for walls with many openings, angles, or curves.
Do I need rebar for a concrete block wall?
Load-bearing and retaining walls typically need vertical rebar in the block cores and horizontal bond beams — check local code requirements.
How many blocks are in a course?
Divide your wall length in inches by 16in (the nominal block length) to get blocks per course, then multiply by the number of courses (wall height ÷ 8in).
What's the difference between 8, 10, and 12in block?
The number refers to the block’s width. Wider block carries more load and is used for taller or retaining walls; 8in is standard for most residential walls.
How do I calculate blocks around a window or door?
Measure the opening’s width and height in feet, multiply for square footage, and subtract that from the total wall area before dividing by block coverage.
Can I use this calculator for a foundation wall?
Yes — enter your foundation’s total linear length and height. Foundation walls commonly use solid-bottom or filled-core block; check your local code.
How much does a concrete block wall cost per square foot?
At $1.85 per block and 0.89 sq ft of coverage, material cost runs about $2.08 per sq ft before mortar — add roughly $0.53 per sq ft for mortar.
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Project
| Wall size | — |
| Openings deducted | — |
| Net area | — |
| Material | — |
| Waste allowance | — |
Shopping list
| Units needed | — |
| Mortar / mix | — |
| Estimated total cost | — |
Notes
Estimate generated by materialcalculate.com. Confirm quantities with your supplier before ordering.