Concrete Block (CMU) Calculator
Calculate the number of concrete masonry units (CMU) needed for a wall, foundation, garage, or retaining wall. Get block count, mortar bags, and course calculations for 8x8x16 and other sizes.
🧱 Masonry🏗️ Blocks = Wall Area x 1.125 blocks/sq ft x (1 + Waste%)
Wall length (ft)
Wall height (ft)
Block size
Waste factor (%)
Please enter valid values.
Formula & Reference
| Variable | Formula | Units |
|---|---|---|
| Concrete Block (CMU) Calculator | Blocks = Wall Area x 1.125 blocks/sq ft x (1 + Waste%) | blocks + mortar |
Step-by-Step Examples
Example 1
8-ft Foundation Wall
30 ft x 8 ft, standard 8x8x16, 5% waste.
- Area = 240 sq ft | Courses = 12
- Blocks = 240 x 1.125 x 1.05 = 283.5 -> 284
- Mortar = 240/35 = 7 bags
✓ 284 blocks, 7 bags mortar
Example 2
4-ft Retaining Wall
15 ft x 4 ft.
- Area = 60 sq ft | 6 courses
- Blocks = 60 x 1.125 x 1.05 = 71
✓ 71 blocks
Example 3
Garage CMU Walls
40x25 ft garage, 10 ft walls, subtract 2 garage doors (9x8 ft each).
- Perimeter = 2x40+2x25 = 130 ft
- Area = 130x10 = 1,300 sq ft
- Minus 2 doors: 1,300 - (2 x 72) = 1,156 sq ft
- Blocks = 1,156 x 1.125 x 1.05 = 1,365
✓ ~1,365 blocks (after subtracting doors)
Real-World Applications
Foundation Work
CMU is standard for basement walls, crawl spaces, and stem walls.
Masonry Contractors
Generate accurate block counts for material orders and bids.
Retaining Walls
CMU retaining walls require engineering for walls over 4 ft in most areas.
Garage Construction
Many jurisdictions require CMU for detached garage fire separation.
Common Mistakes to Avoid
⚠️
Not accounting for partial courses
If wall height is not a multiple of 8 inches, the top course requires cut blocks. Include extra waste for this.
⚠️
Forgetting bond beams and lintels
CMU walls need horizontal reinforcement (bond beam blocks) every 4 feet and concrete lintels or U-blocks over openings.
⚠️
Ignoring rebar and grout
Structural CMU walls require vertical rebar in filled cells. This significantly adds to project cost beyond block and mortar.
Frequently Asked Questions
How many CMU per sq ft? ▾
Standard 8x8x16 with 3/8-inch mortar joints: 1.125 blocks per square foot of wall area.
How many mortar bags for CMU? ▾
One 80-lb bag of type S mortar covers approximately 35 sq ft of 8x16 CMU wall. About 3 bags per 100 sq ft.
Do I need a permit for a CMU wall? ▾
Yes for most structural CMU walls and retaining walls over 4 ft. Always check local codes; tall CMU walls usually require stamped engineering drawings.
CMU vs cinder blocks? ▾
Modern blocks are concrete masonry units (CMU) made with concrete aggregate. Traditional cinder blocks used coal ash and are rarely made today. The terms are interchangeable in practice.
How many courses in an 8-ft wall? ▾
An 8x8x16 block is 8 inches tall (including the 3/8-inch mortar joint = 7.625 + 0.375 = 8 inches). So 8 ft / 8 in = 12 courses exactly.