Block Wall Calculator
Enter your wall dimensions to calculate the number of concrete masonry units (CMU blocks), bags of mortar, and cubic feet of grout fill needed.
Deduct for openings (optional)
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CMU blocks
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Bags of mortar (60 lb)
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Grout fill (ft³)
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Net wall area (ft²)
One 60 lb bag of mortar covers approximately 36 standard blocks. Grout fill estimate assumes every core is filled.
How to Calculate Concrete Blocks
The number of blocks equals the net wall area (length × height, minus openings) divided by the face area of one block. A standard 8×8×16 CMU has a face area of approximately 0.888 sq ft (the nominal 8" × 16" minus the mortar joint).
Block Sizes and Coverage
| Block (nominal) | Actual size | Face area | Blocks per ft² |
|---|---|---|---|
| 4×8×16 | 3⅝ × 7⅝ × 15⅝ | 0.444 ft² | 2.25 |
| 8×8×16 | 7⅝ × 7⅝ × 15⅝ | 0.888 ft² | 1.13 |
| 12×8×16 | 11⅝ × 7⅝ × 15⅝ | 0.888 ft² | 1.13 |
8-inch block (8×8×16 CMU) is the most common for load-bearing walls, retaining walls, and foundations.
CMU Wall Construction
Mortar
Type S mortar is used for below-grade walls, retaining walls, and exterior structural walls. Type N is used for above-grade non-load-bearing walls. A 60-pound bag of pre-mixed mortar covers approximately 36 standard blocks.
Reinforcement and Grouting
For structural walls, retaining walls, and seismic zones, hollow CMU cores are filled with grout (fine concrete) and vertical rebar is placed in selected cores. Common patterns:
- Every core — maximum strength; required for some retaining walls and all below-grade walls
- Every 4 ft — typical for load-bearing exterior walls
- Bond beam courses — horizontal reinforcement every 4 vertical feet with horizontal rebar
Foundation and Footing
CMU walls require a concrete footing below the frost line. A typical 8-inch block wall needs a footing at least 16 inches wide and 8 inches deep. Check local codes for frost depth and footing requirements.
Frequently Asked Questions
How many blocks do I need for a 10×10 room?
A 10 × 10 ft room has 40 linear feet of wall. At 8 ft height, that is 320 sq ft of gross wall area. Using standard 8×8×16 CMU (0.888 ft² each), you need approximately 360 blocks before accounting for openings and waste. Add 5–10% for cuts and breakage.
What is the difference between a CMU and a cinder block?
The terms are often used interchangeably, but technically they differ. CMU (concrete masonry unit) uses aggregate concrete. Traditional cinder blocks used coal ash (cinders) as aggregate and are no longer manufactured. Almost all modern "cinder blocks" are actually CMU.
Do I need a permit to build a block wall?
For retaining walls taller than 3–4 feet, most jurisdictions require a permit and engineered drawings. For garden or landscape walls under 3 feet, a permit is often not required. Always check with your local building department before starting.
