Concrete Block Calculator
Enter the wall length and height and this concrete block calculator returns the number of blocks, the mortar bags, and the concrete needed to fill the cores. It works for standard concrete masonry units, cinder block and CMU walls, and it subtracts door and window openings.
Concrete Block Calculator
Block walls are counted by area, not volume. A standard block covers a set piece of the wall face, so the block count is the wall area divided by the face area of one block, with openings removed.
How to calculate concrete blocks
A standard 8 by 8 by 16 inch block, including its mortar joint, covers about 0.89 square feet of wall. That works out to about 112.5 blocks per 100 square feet of wall. Find your wall area, subtract openings, then multiply by 1.125 blocks per square foot.
Worked example, a block wall
Take a wall 20 feet long and 8 feet tall, with no openings. The area is 160 square feet. Multiply by 1.125 to get 180 blocks. Add about 5 percent for breakage and cuts, so buy around 189. Mortar runs about 3 bags of 80 pound mix per 100 blocks, so 180 blocks need about 6 bags, or 7 with waste.
Mortar and core fill
Two materials go with the block. Mortar bonds the joints, and grout or concrete fills the hollow cores where a wall needs extra strength.
| Item | Typical figure |
|---|---|
| Mortar | about 3 bags of 80 lb per 100 block |
| Full core fill, 8 in block | about 0.5 to 0.6 cubic feet per block |
| Partial fill (rebar cells only) | far less, only filled cells |
Many walls only fill the cells that hold vertical rebar, not every core, which cuts the concrete sharply. Fill and rebar spacing follow the structural drawings.
Related calculators
- Concrete wall calculator for a poured wall instead of block
- Footing calculator for the footing under a block wall
- Concrete mix and mortar calculator for mortar by type
- How to calculate concrete guide for formulas
Frequently asked questions
How many concrete blocks do I need?
Find the wall area, subtract openings, then multiply by about 1.125 blocks per square foot. A 20 by 8 foot wall needs about 180 blocks before waste.
How many blocks are in 100 square feet?
About 112.5 standard 8 by 8 by 16 blocks, since each one covers roughly 0.89 square feet of wall face with its mortar joint.
How much mortar do I need per 100 blocks?
About 3 bags of 80 pound mortar per 100 standard blocks, before waste. Add 5 to 10 percent for joint size and layout.
How much concrete fills a block core?
A fully filled 8 inch block takes about 0.5 to 0.6 cubic feet. Most walls fill only the cells with rebar, which uses much less.
What is the difference between a concrete block and a cinder block?
Cinder block is an older term for a lighter unit made with coal cinders. Modern units are concrete masonry units, or CMU. The count and layout math is the same.
References
- Inch Calculator, concrete block coverage references: https://www.inchcalculator.com/concrete-block-calculator/
- ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units: https://www.astm.org/c0090-22.html
- ASTM C270, Standard Specification for Mortar for Unit Masonry: https://www.astm.org/c0270-19ae01.html
- Portland Cement Association, masonry resources: https://cement.org
Methodology: block count uses wall area minus openings times about 1.125 blocks per square foot; mortar and fill use typical figures that vary with joint size and layout. Estimates are for planning. Confirm reinforcement and fill with a licensed engineer and local code. Recommended: attribute this page to a named author and add a review date.