Concrete Block 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

Wall area (ft²) = Length(ft) × Height(ft)Blocks per ft² = 144 ÷ (Block length × Block height), adjusted for the mortar jointBlocks = (Area − openings) × blocks per ft² × (1 + Waste% ÷ 100)Courses = Height(in) ÷ block height Mortar ≈ 1 bag (80 lb mortar mix) per 12 blocks

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

Slab size	Concrete blocks	80 lb mortar bags
10 ft × 8 ft	99	9	Use →
20 ft × 8 ft	198	17	Use →
30 ft × 8 ft	297	25	Use →
40 ft × 8 ft	396	33	Use →
20 ft × 4 ft	99	9	Use →
50 ft × 6 ft	372	31	Use →

Method & assumptions

A standard concrete masonry unit (CMU) is nominally 16 inches long and 8 inches high, which includes the 3/8 inch mortar joint. That face covers about 0.889 square feet, so one square foot of wall takes 1.125 blocks. You can enter any block length and height and the mortar joint you are using, and the calculator works out the blocks per square foot: a standard 16 by 8 inch block with a 3/8 inch joint is 1.125 per square foot, a 12 inch block is 1.5, and an 8 inch half block 2.25. A wider joint slightly lowers the count. We multiply your net wall area (after deducting any door and window openings) by that figure, add the waste allowance, and round up, because you cannot buy part of a block.

Courses are the horizontal rows: wall height in inches divided by the 8 inch block height. Blocks per course is the wall length in inches divided by the 16 inch 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, which we round up. The old rule of three bags per 100 is for bags of masonry cement that you mix with sand on site, not pre-mixed mortar, so it under-counts the bags you actually buy. Half blocks at corners and openings are counted as whole units, which the waste factor helps absorb.

This estimate assumes a single-wythe running-bond wall of standard 8 inch CMU with no large openings deducted. For openings, partition layouts, or structural walls, confirm the count against your drawings and local code, and remember that reinforced cells will also need grout and rebar.

Pro tips and common mistakes

Start from a level footing. A block wall is only as straight as its base. Lay the first course in a full mortar bed on a level, cured footing, and check it for level and line before you go up.
Buy a few spare blocks. Breakage and corner cuts add up. The waste allowance covers most of it, but having a handful of extra blocks on site beats stopping a course to make a supply run.
Do not skimp on mortar. Keep joints a consistent 3/8 inch. Thin or starved joints weaken the wall and throw off your course heights over the height of the wall.
Plan for reinforcement. Tall or load-bearing walls need rebar in grouted cells. Size the vertical and horizontal steel with our rebar calculator and follow your local code.
Deduct big openings. For doors and large windows, subtract their area before estimating, then add a few blocks back for the cuts around the opening.

How we check this

Every formula is shown in full and verified with automated tests against manufacturer spec sheets and ASTM and ACI standards. We show our work so you can check it.

Frequently asked questions

How many concrete blocks do I need for a wall?

Multiply the wall area in square feet by 1.125, then add 5 to 10% for waste. A standard 16 by 8 inch block covers about 0.889 square feet, so a 10 by 8 foot wall (80 square feet) needs about 99 blocks with 10% waste.

How many blocks are in a square foot?

A standard 16 by 8 inch concrete block covers 0.889 square feet, so it takes 1.125 blocks to cover one square foot of wall. A 12 inch block takes 1.5 per square foot, and an 8 inch half block 2.25.

How many bags of mortar per 100 blocks?

An 80 lb bag of pre-mixed mortar lays about 12 standard blocks at 3/8 inch joints, so roughly 8 to 9 bags per 100 blocks. The old three-bags-per-100 rule is for bags of masonry cement that you mix with sand on site, not pre-mixed mortar mix.

How many blocks high is 8 feet?

Each course of standard block is 8 inches high, so an 8 foot (96 inch) wall is 12 courses high.

References

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