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 mid-pour, and ready-mix is rounded up to the nearest quarter yard, which is how it is sold.

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 no openings deducted.

A wall is more than blocks. It needs a level, cured footing below it, and most block walls carry vertical rebar in grouted cells plus a bond beam near the top, sized by an engineer or local code. If you set the grout option, the calculator estimates the core-fill grout for an 8 inch wall at roughly one cubic yard per 100 fully grouted blocks, so you can order it with the block. For openings, subtract their area and add a lintel. Tall or load-bearing walls and anything below grade need reinforcement and waterproofing, so confirm the design before you start laying.

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

References

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

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