Free Concrete Block Calculator

A standard 8 x 8 x 16 in concrete or cinder block wall needs 1.125 blocks per square foot. Enter your wall size, block size, and any door or window openings to estimate how many blocks to buy, plus mortar bags, grout, waste, and material cost. Everything is calculated privately in your browser.

Block calculator

Wall size

Decimal feet. 3 ft 6 in is 3.5 ft.

Finished height including any cap course.

Block size

Actual size 7.625 x 7.625 x 15.625 in (W x H x L), about 46% hollow.

3/8 in (0.375) is standard for concrete block.

5% for simple walls, 10% or more with many cuts or corners.

Doors, windows, and openings

Rough opening sizes in decimal feet. Each opening is deducted by its quantity.

No openings. Add one for each door or window size in the wall.

    Mortar and grout

    Quikrete rates an 80 lb bag of Mortar Mix at about 12 standard blocks.

    0 for none, 100 for a fully grouted wall, or the share of reinforced cells.

    Prices (optional)

    Optional. Leave blank to skip cost.

    Optional, per 80 lb premixed bag.

    Optional, per 80 lb bag (about 0.6 cu ft).

    Try an example

    Your estimate

    Enter the wall length and height, then select Calculate blocks. Or load an example.

    Formulas and assumptions

    • Blocks: (wall area minus openings) / ((block length + joint) x (block height + joint)), then add waste and round up.
    • Nominal vs actual: presets use actual sizes 3/8 in smaller than nominal, so an 8 x 16 in face plus joints covers 0.889 sq ft.
    • Mortar: blocks to buy / blocks per 80 lb premixed bag. Other block sizes and joints are scaled by joint length, joint thickness, and block thickness compared with a standard block.
    • Grout: blocks x gross block volume x hollow share x grouted cells, plus waste. Bags assume about 0.6 cu ft per 80 lb bag.
    • Not included: footings, rebar, bond beam or corner units, caps, sand for site-mixed mortar, delivery, tax, and labor. Structural walls need engineering and local code review.

    How to calculate concrete blocks for a wall

    1. Measure the wall

      Measure the finished wall length and height in feet. Enter decimal feet, so 3 ft 6 in is 3.5 ft.

    2. Choose a block size

      Pick a standard CMU preset or enter custom actual block dimensions and the mortar joint thickness.

    3. Subtract openings

      Add each door, window, or vent opening with its width, height, and quantity so it is deducted from the wall area.

    4. Add waste, grout, and prices

      Set a waste allowance, the share of cells to grout, and optional prices, then review the itemized blocks, mortar, grout, and cost.

    Concrete block calculator FAQ

    How many concrete blocks do I need per square foot?

    A standard 8 x 8 x 16 in concrete block is actually 7 5/8 x 15 5/8 in on its face. With a 3/8 in mortar joint it covers 8 x 16 in, or 0.889 sq ft, so you need 1.125 blocks per square foot of wall before waste.

    How do I calculate cinder blocks for a wall with a door or window?

    Multiply wall length by height, subtract the area of each door and window, then divide the net area by the face area of one block plus its mortar joint. Add 5 to 10 percent for cuts and breakage, and round up to whole blocks.

    How much mortar do I need for concrete blocks?

    Quikrete rates an 80 lb bag of premixed Mortar Mix at about 12 standard 8 x 8 x 16 in blocks with 3/8 in joints. This calculator uses that rate by default, adjusts it for other block sizes, and lets you change the blocks per bag to match your product.

    How much grout fills a concrete block wall?

    A fully grouted 8 in block wall needs roughly 0.24 cu ft of grout per block. Many walls only grout reinforced cells and bond beams, so enter the share of cells you plan to fill. Your engineer or local code sets the actual grouting requirement.

    Is a cinder block the same as a concrete block?

    In everyday use, yes. Modern concrete masonry units (CMU) are made with portland cement and aggregate, while older cinder blocks used coal cinders. Both use the same modular sizes, so the block count math is the same.

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