Concrete Block (CMU) Calculator

Enter your wall dimensions and instantly calculate how many concrete masonry units you need, how many bags of mortar to buy, and your total material cost.

Free to use No sign-up required Standard CMU block sizes included Mortar joints accounted for

✓ Block count with waste factor ✓ Mortar bags included ✓ Multiple block sizes supported ✓ Last verified May 2026

Reviewed by the AllConcreteCalculator.com editorial team — formulas cross-checked against NCMA TEK standards, May 2026.

Enter Your Wall Dimensions

Wall Length

Total horizontal run of the wall, end to end. Please enter a valid length greater than 0.

Wall Height

Measure from top of footing to top of wall. Please enter a valid height greater than 0.

Block Size (Nominal) Nominal size includes the 3/8 in mortar joint. Actual block is 3/8 in smaller in each dimension.

Waste / Overage Factor (%)

5% is standard for straight walls. Use 10% for walls with many cuts, corners, or openings.

Openings Area (doors, windows) (optional)

Sum the area of all doors and windows. Calculator will deduct blocks for these openings. Leave blank if none.

Price per Block (optional)

Leave blank to skip cost estimate. US average: $1.50–$4.00 per standard 8×8×16 CMU block.

Results appear instantly. No sign-up required.

Your CMU Block Estimate

Blocks Required (with waste)

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Blocks Needed

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Net Wall Area (sq ft)

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Blocks (no waste)

Mortar Required

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70 lb mortar bags

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60 lb mortar bags

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Cu ft of mortar

— Courses (rows)

— Blocks / course

— Block size

— Waste factor

Estimated Block Material Cost

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Block material cost only. Add mortar ($8–$14/bag), labor ($10–$17/sq ft), grout, rebar, and delivery for a complete project budget. Use our Full Project Estimator for a complete breakdown.

Note: Nominal block dimensions include the 3/8 in mortar joint
Step 1: Block face area (sq ft) = Nominal Width (ft) × Nominal Height (ft)
Step 2: Net wall area (sq ft) = (Wall Length × Wall Height) − Openings Area
Step 3: Blocks (no waste) = CEIL(Net Wall Area ÷ Block Face Area)
Step 4: Blocks with waste = CEIL(Blocks × (1 + waste% ÷ 100))
Step 5: Courses = CEIL(Wall Height (in) ÷ Nominal Block Height (in))
Step 6: Mortar ≈ 0.01 ft³ per block (industry standard for standard CMU)

Mortar yields: 70 lb bag ≈ 0.56 ft³ | 60 lb bag ≈ 0.48 ft³

How to Use This CMU Block Calculator

Measure your wall length and height. Use a tape measure to get the total horizontal run and vertical rise of your wall. Measure height from the top of the footing or slab to the top of the finished wall, not from grade. If your wall has multiple sections at different heights or lengths, run a separate calculation for each section and add the block totals together.
Select your block size. The standard CMU in the US is 8×8×16 in (nominal), which covers exactly 0.889 sq ft of wall face per block including the mortar joint. Select the correct nominal size from the dropdown — this matters because the calculator uses nominal dimensions (which already include the 3/8 in mortar joint) to count courses and blocks per course accurately.
Enter any openings and set your waste factor. Add the combined area of all doors and windows as a single square footage value — the calculator deducts these from your wall area before counting blocks. Use 5% waste for simple straight walls, 10% for walls with corners, curves, or many cuts. Do not go below 5%; broken blocks and mis-cuts are unavoidable on any CMU job.
Use the results to order materials. The block count is what you give your masonry supplier. The mortar estimate tells you how many bags of Type S or Type N mortar to buy. If you entered a price per block, the cost estimate gives your block-only material budget — add mortar, grout, rebar, and delivery separately when building your full project estimate.

⚠ Pro Tip: Always count your courses before ordering. Divide your wall height in inches by the nominal block height in inches (e.g., 96 in ÷ 8 in = 12 courses). If that number isn't a whole number, your wall height isn't divisible by your block size — you'll need to either adjust the height or cut blocks for the top course. Plan for this before the first block goes down, not after.

CMU Block Count Formula

The industry-standard method for estimating CMU block quantities uses nominal block dimensions (which include the mortar joint) and divides the net wall area by the face area of one block. Here's the step-by-step process:

Step	Formula	Example (20 ft × 8 ft, standard 8×8×16)
1. Block face area	Nom. W × Nom. H (in sq ft)	(16/12) × (8/12) = 0.889 sq ft
2. Wall area	Length × Height	20 × 8 = 160 sq ft
3. Blocks (no waste)	CEIL(Area ÷ Block face area)	CEIL(160 ÷ 0.889) = 180 blocks
4. Add 5% waste	Blocks × 1.05	CEIL(180 × 1.05) = 189 blocks
5. Courses	CEIL(Height in ÷ Block H in)	CEIL(96 ÷ 8) = 12 courses
6. Mortar volume	Blocks × 0.01 ft³	180 × 0.01 = 1.80 ft³ (≈ 4 bags of 70 lb)

CMU Block Count Reference Table

Standard 8×8×16 CMU blocks and mortar bags — 5% waste included. No openings deducted.
Wall Length	Wall Height	Wall Area	Blocks (5% waste)	70 lb Mortar Bags
10 ft	4 ft	40 sq ft	48	1
10 ft	8 ft	80 sq ft	95	2
20 ft	8 ft	160 sq ft	189	4
30 ft	8 ft	240 sq ft	284	6
40 ft	8 ft	320 sq ft	379	7
20 ft	10 ft	200 sq ft	237	5
50 ft	10 ft	500 sq ft	591	11
100 ft	8 ft	800 sq ft	946	17

Based on standard 8×8×16 CMU (nominal), 3/8 in mortar joints, 5% waste. Deduct blocks for openings separately.

Which CMU Block Size Should I Use?

Choosing the right CMU size affects structural capacity, insulation value, cost, and scheduling. The table below reflects standard US masonry practice aligned with ASTM C90 specifications.

Recommended CMU block size by application type. All dimensions nominal (include 3/8 in mortar joint).
Block Size (Nominal)	Actual Size	Best For	Notes
8 × 8 × 16 in	7⅝ × 7⅝ × 15⅝ in	Load-bearing walls, foundations, retaining walls	Most common CMU in US construction. 1.125 blocks per sq ft.
4 × 8 × 16 in	3⅝ × 7⅝ × 15⅝ in	Partition walls, non-load-bearing interior dividers	Half the weight and width of standard. Not for load-bearing use.
6 × 8 × 16 in	5⅝ × 7⅝ × 15⅝ in	Lightly loaded walls, above-grade partition	Good compromise between weight and strength.
10 × 8 × 16 in	9⅝ × 7⅝ × 15⅝ in	Moderate load-bearing, improved insulation	More core space for grout and rebar inserts.
12 × 8 × 16 in	11⅝ × 7⅝ × 15⅝ in	Heavy load-bearing, below-grade foundation walls	Maximum CMU width for standard applications.
8 × 4 × 16 in	7⅝ × 3⅝ × 15⅝ in	Cap block, bond beam, half-height coursing	Used for top of wall, window sills, and course adjustment.

Standard 8×8×16 blocks produce exactly 1.125 blocks per square foot of wall face (including mortar joint). Many experienced masons use this as a quick mental shortcut: multiply your wall square footage by 1.125, then add 5% waste. This calculator uses the exact formula — which produces the same result — but handles non-standard block sizes and opening deductions automatically.

Common Mistakes When Estimating CMU Blocks

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Using actual dimensions instead of nominal dimensions. The actual size of a standard CMU is 7⅝ × 7⅝ × 15⅝ in. The nominal size is 8 × 8 × 16 in — which includes the 3/8 in mortar joint. Always use nominal dimensions when counting blocks. Using actual dimensions will make your mortar-joint math impossible and underestimate the block count.
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Forgetting to check divisibility before laying the first course. If your wall height isn't evenly divisible by the nominal block height, you'll need to cut blocks for the top course — or adjust the design. Discovering this after you've poured the footing is an expensive problem. Always verify: wall height in inches ÷ nominal block height in inches = whole number.
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Not deducting for openings — but also not adding back half-blocks for jambs. Most estimators correctly deduct for window and door openings. Fewer remember to add back the blocks needed to form the jamb (vertical edges of the opening). On a single opening in an 8-ft wall, this can add 10–12 blocks. For walls with many openings, the net effect is smaller — but still worth accounting for on a tight budget.
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Ordering the wrong mortar type. Type S mortar (higher strength) is required for below-grade applications, retaining walls, and anywhere the wall is exposed to weather or lateral soil pressure. Type N mortar is acceptable for above-grade, non-load-bearing, or interior walls. Using Type N where Type S is required is a code violation in most jurisdictions and a structural risk in freeze-thaw climates.
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Underestimating grout for filled-cell walls. If your wall design calls for grouted cells (partially or fully filled cores), you need to calculate grout volume separately — this calculator estimates dry-stack or mortar-only walls. Grouted CMU walls are an entirely different quantity take-off. Use our CMU Core Fill Calculator to estimate grout volume.

Frequently Asked Questions

For the standard 8×8×16 in CMU (nominal), you need approximately 1.125 blocks per square foot of wall face area, including the 3/8 in mortar joint. This is calculated as 1 ÷ (nominal width in ft × nominal height in ft) = 1 ÷ (1.333 × 0.667) = 1 ÷ 0.889 = 1.125 blocks/sq ft. Always add at least 5% for waste, cuts, and breakage. For non-standard block sizes, the blocks-per-square-foot figure changes — this calculator handles any nominal CMU size automatically.
Nominal CMU dimensions include the 3/8 in mortar joint on two sides (length and height). A standard 8×8×16 in block has actual dimensions of 7⅝ × 7⅝ × 15⅝ in. When you lay blocks with 3/8 in mortar joints, each block plus its joint occupies exactly 8 × 8 × 16 in. This is why nominal dimensions are used for estimating — they make the math clean and allow courses to align with standard module heights (every 8 inches of wall height = exactly 1 course). Always use nominal dimensions in your calculations.
The industry standard for standard face-shell mortar bedding (not full mortar bedding) on 8×8×16 CMU is approximately 0.01 cubic feet of mortar per block. A 70 lb bag of mortar mix yields approximately 0.56 cubic feet when mixed. So for 100 blocks, you need about 1.0 ft³ of mortar, or roughly 2 bags of 70 lb mortar. For full mortar bedding (all webs filled), increase mortar volume by approximately 25%. Always add 10% to mortar estimates for waste and over-application.
Type S mortar is the standard specification for most CMU applications: below-grade foundation walls, retaining walls, exterior walls, and any wall subject to lateral loads or freeze-thaw exposure. It has a minimum compressive strength of 1,800 PSI and high durability. Type N mortar is acceptable only for above-grade, interior, non-load-bearing partitions in a protected environment. Never use Type M mortar (too rigid — it can cause block spalling) or Type O mortar (far too weak for structural masonry) on CMU walls.
Nationally, installed CMU walls cost $15–$30 per square foot for standard construction — covering blocks, mortar, labor, and basic wall ties. Simple garden walls or retaining walls at the low end; reinforced, grouted, commercial-grade construction at the high end. Block materials alone run $1.50–$4.00 per standard block depending on size and region, plus $8–$14 per bag of mortar. Rebar and grout for reinforced walls add $2–$5 per square foot. Labor is typically $10–$17 per square foot for experienced masonry crews. Highly ornamental block types, decorative finishes, or engineered structural walls will cost significantly more.
It depends on the structural requirements. Unreinforced CMU walls (hollow, mortar only) are acceptable for simple garden walls, low-height landscape walls, and some non-load-bearing partitions. Structural load-bearing walls, retaining walls, and any wall above a certain height in seismic or high-wind zones typically require grouted and reinforced cores — rebar placed in the cells and low-slump grout or concrete pumped in to fill them. Always check local building codes and your structural engineer's specifications. For a wall that could fail and injure someone, grouting is cheap insurance.
For external corners (L-shaped or U-shaped walls), measure each wall section from the outside face of the corner — this avoids double-counting the corner block. Run separate calculations for each wall leg and add the block totals. Corners themselves use the same standard blocks but require alternating the overlap pattern each course for a running bond. You will not need to cut blocks at standard 90-degree corners — the modular dimensions are designed to interlock cleanly. Increase your waste factor to 8–10% if the wall has multiple corners or changes in direction.
The terms are often used interchangeably in everyday speech, but they technically refer to different products. A concrete masonry unit (CMU) is made from Portland cement and aggregate — it is the modern, ASTM C90-standardized block used in all current construction. A "cinder block" refers to older blocks made with cinder (ash byproduct from coal combustion) as the aggregate — these are rarely manufactured today and are generally not permitted in structural applications. If you see "cinder block" at a home center, it is almost always a standard CMU, not an actual cinder block. For any structural application, specify ASTM C90 CMU.
Permit requirements vary by municipality. In most US jurisdictions, retaining walls over 3–4 feet in exposed height require a building permit and engineer-stamped drawings. Foundation walls, load-bearing structural walls, and any wall attached to a building almost always require a permit. Low decorative garden walls under 2 feet typically do not. Seismic zones, flood zones, and areas with expansive soils often have stricter requirements. Always check with your local building department before starting — working without a required permit can result in mandatory demolition at your expense.
For a curved wall, calculate the arc length (the actual curved distance along the wall face) rather than the chord (straight-line distance between endpoints). Use the formula: arc length = radius × angle in radians, or measure the curve directly with a flexible measuring tape. Enter this arc length as your wall length in this calculator. Curved walls require more cuts and adjustments than straight walls — use a 10–15% waste factor. For tight radii (under 10 feet), you may need special tapered or curved CMU units manufactured specifically for curved walls, which have a different blocks-per-square-foot ratio than standard units.