Concrete Block Calculator — Blocks & Bricks for Walls
Wall Area (net)
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Blocks Needed (with waste)
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Blocks (without waste)
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Mortar Bags (80 lb) Estimated
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Estimated Block Cost
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Rows of Blocks
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Blocks per Row
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How Concrete Block Calculation Works
A concrete block calculator is a tool that estimates the number of concrete masonry units (CMUs) or bricks needed to build a wall, along with mortar, and cost. Concrete blocks are the standard building material for foundations, retaining walls, and structural walls in residential and commercial construction. According to the National Concrete Masonry Association (NCMA), over 4 billion concrete masonry units are produced annually in the United States, making CMUs one of the most widely used structural building materials.
The calculation divides the net wall area (total area minus doors and windows) by the face area of a single block including one mortar joint on each dimension. A standard 8x8x16 inch CMU with a 3/8-inch mortar joint covers approximately 0.89 square feet of wall face. The calculator then applies a waste factor for cutting, breakage, and fitting around openings. For related calculations, see our concrete calculator for filling block cores, or the brick calculator for standard brick projects.
The Block Estimation Formula
The standard formula for estimating block count, as used by masonry contractors, is:
Net Wall Area = (Wall Length x Wall Height) - Total Opening Area
Blocks Needed = Net Wall Area / Block Face Area (including mortar) x (1 + Waste %)
Worked example: A 40-foot-long, 8-foot-high garden wall with one 6x3 foot gate opening. Net area = (40 x 8) - (6 x 3) = 302 sq ft. Using standard 8x16 blocks with 3/8-inch mortar (face area = 0.89 sq ft): 302 / 0.89 = 339 blocks. With 5% waste: 356 blocks. At $1.75 per block, material cost is approximately $623 for blocks alone. Mortar: approximately 30 bags (80-lb) at $6 each = $180. Total materials: approximately $803.
Key Terms You Should Know
- CMU (Concrete Masonry Unit) -- the industry term for concrete blocks. Defined by ASTM C90 standard for load-bearing units and ASTM C129 for non-load-bearing units.
- Nominal vs. actual size -- a "standard" 8x8x16 block is actually 7-5/8 x 7-5/8 x 15-5/8 inches. The 3/8-inch difference accounts for the mortar joint, making the nominal (installed) dimensions exactly 8x8x16.
- Mortar joint -- the layer of mortar between blocks, typically 3/8 inch thick. Mortar types include Type N (general purpose), Type S (structural), and Type M (below-grade). Type S is most common for above-grade structural walls.
- Core fill -- concrete or grout poured into the hollow cores of CMUs for added strength. Required at corners, ends of walls, and around rebar in structural applications.
- Bond beam -- a horizontal row of special U-shaped blocks filled with concrete and rebar, placed every 4-8 feet of wall height for structural reinforcement.
CMU Block Sizes and Coverage
Concrete blocks come in several standard sizes for different applications. The NCMA and ASTM standards define these dimensions:
| Block Size (nominal) | Actual Size | Blocks per 100 sq ft | Typical Use |
|---|---|---|---|
| 8x8x16 | 7-5/8 x 7-5/8 x 15-5/8 | 112.5 | Standard walls, foundations |
| 12x8x16 | 11-5/8 x 7-5/8 x 15-5/8 | 112.5 | Foundation walls, retaining walls |
| 6x8x16 | 5-5/8 x 7-5/8 x 15-5/8 | 112.5 | Interior partitions, garden walls |
| 4x8x16 | 3-5/8 x 7-5/8 x 15-5/8 | 112.5 | Veneer, non-structural partitions |
| Standard brick | 3-5/8 x 2-1/4 x 7-5/8 | ~675 | Veneer, decorative, fireplaces |
Practical Examples
Garden retaining wall (20 ft long, 4 ft high): Wall area = 80 sq ft. Standard 8x16 blocks: 80 / 0.89 = 90 blocks. With 10% waste: 99 blocks. Mortar: approximately 8 bags. Material cost: about $230. For walls over 4 feet, you may need an engineer's design for reinforcement and drainage requirements.
Foundation stem wall (120 linear feet, 3 courses high): Using 12x8x16 blocks (wider for foundation use). Wall area = 120 x 2 ft = 240 sq ft. Blocks needed: 240 / 0.89 = 270 blocks + 5% waste = 284 blocks. With core fill and rebar at 32 inches OC, you also need concrete for filling -- approximately 0.5 cubic yards. Use our concrete slab calculator to estimate the footing concrete beneath this wall.
Brick mailbox column (16 x 16 inches, 4 ft tall): Perimeter = 64 inches = 5.33 ft. Wall area = 5.33 x 4 = 21.3 sq ft. Standard bricks: approximately 144 bricks. Mortar: approximately 2 bags. Material cost: about $130-$180 including bricks, mortar, and cap stones.
Tips for Working with Concrete Blocks
- Start with a level first course. The first course of blocks defines the accuracy of the entire wall. Set blocks on a mortar bed applied to a level concrete footing. Check every block for level and plumb before the mortar sets.
- Use the right mortar type. Type S mortar is standard for structural walls above and below grade. Type N is adequate for non-structural above-grade walls. Type M is specified for below-grade and high-lateral-load applications. Pre-mixed mortar (80-lb bags) is convenient for small projects; site-mixed mortar from bulk materials is more economical for large projects.
- Tool joints before mortar sets. Strike or tool the mortar joints when they are "thumbprint hard" (firm but not fully set, typically 30-60 minutes after placement). Tooled joints compress the mortar surface and improve water resistance.
- Plan for reinforcement. Building codes require vertical rebar at corners, intersections, each side of openings, and at maximum 4-foot intervals. Horizontal bond beams with rebar are required every 4 feet of wall height or 48 inches, per most building codes.
- Order 5-10% extra blocks. Breakage during handling and delivery, cutting waste around openings, and occasional defective blocks mean you will always need more than the calculated amount.
Frequently Asked Questions
How many concrete blocks do I need for a wall?
Calculate the net wall area (length x height minus doors and windows), then divide by the face area of one block including mortar joint. A standard 8x16 CMU with 3/8-inch mortar covers about 0.89 sq ft, so a 100 sq ft wall needs approximately 113 blocks. Add 5-10% waste for cutting and breakage. For a 20-foot-long, 8-foot-high wall with one standard door, you need roughly 150 blocks.
What is the standard size of a concrete block?
The most common CMU in the US is 8x8x16 inches nominal (7-5/8 x 7-5/8 x 15-5/8 actual) as defined by ASTM C90. The 3/8-inch difference between nominal and actual dimensions accommodates the mortar joint. Other standard sizes include 4x8x16 (half-width for veneer), 6x8x16 (partitions), and 12x8x16 (foundation walls). Standard modular bricks are 3-5/8 x 2-1/4 x 7-5/8 inches actual.
How much mortar do I need per block?
Approximately 3 bags (80-lb each) of pre-mixed mortar covers about 36 standard CMU blocks, which equates to roughly 8.5 bags per 100 blocks. For standard bricks, you need about 6.5-7 bags per 100 bricks. Mortar consumption increases with wider joints, core filling, and cold weather (mortar sets slower and more gets used in adjustments). Always purchase 10% extra mortar to avoid running short mid-project.
What type of mortar should I use for a block wall?
Type S mortar is the standard choice for structural block walls, both above and below grade. It has a compressive strength of 1,800 PSI and provides strong bond and good water resistance. Type N mortar (750 PSI) is adequate for non-load-bearing above-grade walls like garden walls and partitions. Type M mortar (2,500 PSI) is used for below-grade applications and walls subject to high lateral loads like retaining walls. Use the mortar type specified by your local building code or engineer.
How much does a concrete block wall cost?
Standard 8x8x16 CMU blocks cost $1.50-$2.50 each at home improvement stores as of 2025. Mortar adds approximately $0.50-$1.00 per block. Professional installation (including mortar, reinforcement, and labor) runs $10-$17 per square foot of finished wall. A 20-foot-long, 8-foot-high wall (160 sq ft) costs approximately $500-$700 for DIY materials or $1,600-$2,720 professionally installed, not including the footing.
Do concrete block walls need rebar?
Yes, most building codes require vertical rebar reinforcement in concrete block walls. Standard requirements include #4 or #5 rebar at corners, intersections, each side of openings, and at 48-inch maximum spacing for structural walls. Horizontal reinforcement (bond beams with rebar) is required every 48 inches of height. The rebar cells are filled with grout or concrete after the blocks are laid. Non-structural garden walls under 4 feet tall may not require rebar in some jurisdictions.