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Concrete Block (CMU) Calculator

Calculate how many standard 8×8×16 concrete blocks you need for a wall.

Result

Blocks Needed
142
Wall Area
120 sq ft (11 m²)

About the CMU Block Calculator

Concrete block (CMU) walls are estimated using a standard industry figure - 1.125 blocks per square foot - derived from the block's nominal face size including its mortar joint allowance.

How It Works

The calculator multiplies the wall's area by 1.125 blocks per square foot, then adds a waste allowance for cutting and breakage.

blocks = wall area × 1.125 × (1 + waste %)

Examples

A garden wall

A 20×6 ft wall (120 sq ft) needs about 142 blocks including a 5% waste allowance.

A taller retaining wall

Doubling the height to 12 ft on the same 20 ft length needs roughly double the blocks, since area scales directly with height.

Advantages

  • Uses the standard masonry-trade figure of 1.125 blocks per square foot
  • Simple area-based calculation for a fast estimate
  • Adjustable waste allowance for cutting and breakage

Common Mistakes

  • Not subtracting large openings from the wall area before calculating
  • Forgetting that mortar, rebar, and grout are separate materials
  • Applying the standard-block figure to half-height or specialty block sizes

Edge Cases to Watch For

  • This assumes standard full-size 8×8×16" CMU block - half-height, corner, and specialty blocks have different face dimensions and need their own coverage calculation.
  • Openings for doors, windows, and vents should be subtracted from the wall area before calculating for a more precise count, though the waste allowance partially absorbs this for smaller openings.
  • Rebar, mortar, and grout for filling block cores are separate materials not included in this block-count estimate.

Common Use Cases

  • Estimating concrete block needed for a garden or retaining wall
  • Budgeting a masonry project before ordering block
  • Comparing material needs for different wall dimensions
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why 1.125 blocks per square foot?

A standard concrete block (CMU) has a nominal face of 8×16 inches including its mortar joint allowance, which works out to 0.889 sq ft per block - dividing 1 sq ft by that gives 1.125 blocks per square foot, a standard figure used throughout the masonry trade.

Does this work for half-height or specialty blocks?

No - this assumes standard full-size 8×8×16" CMU block. Half-height, corner, or specialty blocks have different face dimensions and would need their own coverage calculation.

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Conclusion

1.125 blocks per square foot is the masonry trade's standard shorthand, derived directly from a standard block's face dimensions plus its mortar joint. It holds well for typical full-size CMU block; anything using specialty or half-height units needs its own calculation.