Block Wall Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short.

Worked example

A 20 ft long by 8 ft high block wall with a 10% waste allowance. Inputs: Wall length 20 ft, Wall height 8 ft, Waste allowance 10 %. Result: 198 .

Blocks for common wall sizes

Calculated for standard 8 inch block with a 10% waste allowance. Tap a size to load it above.

Method & assumptions

Planning a block wall starts with the block count: a standard 16 by 8 inch CMU with a 3/8 inch joint covers about 0.889 square feet, so one square foot of wall takes 1.125 blocks. Enter your block length and height and the mortar joint and the calculator works out the blocks per square foot for you. We multiply your net wall area (after deducting door and window openings) by that figure, add the waste allowance, and round up. Courses are the wall height divided by the block height, and blocks per course is the wall length divided by the block length.

For mortar, an 80 lb bag of pre-mixed mortar mix lays about 12 standard blocks at 3/8 inch joints, so roughly 8 to 9 bags per 100 blocks, rounded up. The old three-bags-per-100 rule is for masonry cement mixed with sand on site, not pre-mixed mortar. Corner and opening cuts count as whole blocks, which the waste factor helps absorb. This estimate is for a single-wythe running-bond wall of standard 8 inch block, with door and window area removed by the openings field before the block count.

A wall is more than blocks. Use the footing, reinforcement, grout, bond beams, opening details, drainage and moisture-control provisions specified for the project. If you set the grout option, the calculator estimates core-fill volume for its stated 8-inch-wall assumption; verify that geometry against the selected units and grouting detail. The calculator does not size a footing, select reinforcement or design lintels.

Pro tips and common mistakes

• Pour the footing first. The wall is only as true as its footing. Pour a level, code-sized footing, let it cure, then snap your wall lines on top before the first course.
• Lay a dry course first. Set the bottom course dry, end to end, to space the blocks and plan your cuts before you spread any mortar. It saves rework once the mortar is down.
• Build the corners, then fill. Lead with the corners a few courses high, run a line between them, and fill the field to the line. That keeps the wall straight and plumb.
• Reinforce and grout. Most block walls need vertical rebar in grouted cells and a bond beam near the top. Size the steel with our rebar calculator and follow code.
• Keep joints and coursing tight. Hold a steady 3/8 inch joint and check level every course. Small joint errors compound into a leaning, out-of-level wall over the full height.

Frequently asked questions

Sources and references

These references and structured source records support the method and material data used on this page.

• Concrete masonry unit (Wikipedia)
• NCMA: concrete masonry construction
• ASTM C90 (load-bearing CMU spec)

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