Cinder 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 .

Cinder 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	Cinder 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

Cinder block and concrete block are sized the same way: nominally 16 inches long by 8 inches high, including the mortar joint. One block covers about 0.889 square feet, so a square foot of wall takes 1.125 blocks. You can enter any block length and height and the mortar joint, and the calculator works out the blocks per square foot: a 16 by 8 inch block with a 3/8 inch joint is 1.125 per square foot, a 12 inch block 1.5, and an 8 inch half block 2.25. We multiply your net wall area (after deducting any openings) by that figure, add waste, and round up to whole blocks.

Courses are wall height in inches divided by 8, and blocks per course is wall length in inches divided by 16. For mortar, an 80 lb bag of pre-mixed mortar mix lays about 12 blocks at 3/8 inch joints, so roughly 8 to 9 bags per 100 blocks. The old three-bags-per-100 figure 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 cover.

The difference between the two names is the aggregate, not the size. Traditional cinder blocks used coal cinders, making them lighter but weaker; most blocks sold today are true concrete masonry units and are stronger. Because the dimensions match, this calculator works for either. For load-bearing or tall walls, follow your engineer and local code, and plan grout and rebar for reinforced cells.

Pro tips and common mistakes

Know what you are buying. Real cinder blocks are lighter and weaker than modern concrete block. For anything load-bearing or below grade, use rated concrete masonry units, not salvage cinder block.
Build off a level footing. Lay the first course in a full mortar bed on a cured, level footing and check line and level before going up. The base sets up the whole wall.
Keep joints consistent. Hold a steady 3/8 inch joint. Starved or fat joints weaken the wall and slowly throw your course heights out over the full height.
Reinforce where needed. Tall or load-bearing walls need rebar in grouted cells. Size the steel with our rebar calculator and follow code for spacing and grout.
Have spares on hand. Cinder block chips and cracks easily on cuts. Keep a few extra on site so a broken block does not stop a course.

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

Is a cinder block the same as a concrete block?

They are the same standard size, so the count is identical. The difference is the aggregate: traditional cinder blocks used coal cinders and are lighter and weaker, while modern blocks are concrete masonry units and are stronger. Most blocks sold today are concrete, even when people call them cinder blocks.

How many cinder blocks do I need for a wall?

Multiply the wall area in square feet by 1.125 and add 5 to 10% for waste. A 10 by 8 foot wall (80 square feet) needs about 99 blocks with 10% waste.

How many cinder blocks are on a pallet?

A standard pallet of 8 inch block usually holds 90 to 108 blocks depending on the supplier. Check with your yard, since this calculator counts blocks, not pallets.

How tall can a cinder block wall be?

Unreinforced dry-stacked or lightly mortared walls should stay low. Taller and load-bearing walls need rebar in grouted cells and must follow local code and an engineer for anything structural.

References

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