How Many Concrete Blocks Do I Need?

To find how many concrete blocks you need, multiply the wall's length by its height in feet to get the area, then multiply by 1.125, since a standard 8x8x16 CMU with a 3/8 inch mortar joint covers 0.889 square feet of wall face. Below is the formula, a block count table by wall size, how many mortar bags to buy, and a calculator that handles openings and waste for you.

Key takeaways

  • Blocks = wall area (sq ft) x 1.125 for standard 8x8x16 CMU, then add 5 to 10% for waste.
  • A 10 ft by 8 ft wall (80 sq ft) needs about 90 blocks, or 99 with a 10% waste allowance.
  • Buy about 8 to 9 bags of 80 lb pre-mixed mortar per 100 blocks, roughly 1 bag per 12 blocks.
  • Subtract door and window openings from the area before multiplying, and round up to a whole block.
  • Standard hollow CMU costs about 1.25 to 2.50 dollars per block delivered in 2026, or 250 to 330 dollars per pallet of 90.
A mason stacking gray concrete blocks in a running bond pattern on a new foundation wall
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The formula: wall area times 1.125

A standard concrete masonry unit (CMU) presents an 8 inch high by 16 inch long face to the wall, and with a 3/8 inch mortar joint it occupies a modular 0.889 square feet of wall area. Flip that fraction over and one square foot of wall needs 1.125 blocks.

So the whole calculation is two steps: find the wall area, then multiply by 1.125.

Wall area (ft2) = Length(ft) x Height(ft)Blocks = Wall area x 1.125 x (1 + Waste% ÷ 100)

Deduct any doors or windows from the area before multiplying, and always round the final count up. You cannot order half a block, and a wall short by even one course will stop the job.

Block count by wall size

These figures use standard 8x8x16 CMU with a 3/8 inch joint and a 10% waste allowance, which covers breakage and the cuts around corners.

Wall (L x H)Area (sq ft)Blocks (+10%)Mortar bags (80 lb)
10 x 8 ft80999
20 x 8 ft16019817
30 x 8 ft24029725
40 x 6 ft24029725
50 x 4 ft20024821

Every wall is different once openings and corners enter the picture, so treat this table as a starting budget. For your exact footprint, run the numbers through the concrete block calculator, which also returns courses and blocks per course.

How much mortar to buy

Mortar is the second line item, and it is easy to underestimate. An 80 lb bag of pre-mixed mortar mix lays roughly 12 standard blocks at a 3/8 inch joint, which works out to about 8 to 9 bags per 100 blocks.

That is different from the older rule of three bags per 100, which applies to bags of plain masonry cement mixed on site with separate sand, not to pre-mixed mortar mix bought bag for bag. Using the cement-only ratio on a pre-mixed product will leave you several bags short mid wall.

Buy one or two spare bags beyond the estimate. Running out mid course means a delay and a risk of mismatched color once the new batch arrives.

Choosing the right block width

The 1.125 blocks per square foot figure holds for any standard CMU width, because every size shares the same 8 by 16 inch face and only the depth changes. What changes is which block suits the job.

An 8 inch block is the default for loadbearing exterior walls and most foundation work. A 6 inch unit fits interior fire rated or partition walls, while 12 inch block is reserved for tall retaining walls or foundations carrying heavy loads. For the full chart of nominal versus actual dimensions across every width, see standard concrete block sizes.

If your project needs the block to also hold vertical rebar and grout, plan on ordering some open-cell or knock-out block for those courses instead of standard solid-face units.

A tape measure laid across a concrete block wall to count courses and calculate the block quantity

Worked example: an 8x8 foundation wall

Say you are building an 8 foot long, 8 foot high foundation wall section with standard 8x8x16 block. Area is 8 times 8, which is 64 square feet.

Multiply 64 by 1.125 for 72 blocks before waste, then add 10% to land at about 80 blocks. Mortar comes out to roughly 7 bags of 80 lb mix at one bag per 12 blocks.

Now say the wall has a 3 by 3 foot vent opening. Subtract that 9 square feet first: 55 square feet times 1.125 is about 62 blocks, plus waste. Always deduct the opening before multiplying, never after.

Reinforcement, grout, and code

Loadbearing and foundation block walls usually need vertical rebar dropped into the cells and those cells filled with grout, per the project engineering and local building code. That reinforcement does not change the block count, but it does add two more materials to your order.

Size the steel with a rebar calculator based on the spacing called out on your plans, and figure grout volume separately since it fills the hollow cores rather than the wall face. Bond beam block, with a U shaped channel on top, carries horizontal steel and is usually specified every few courses on structural walls.

Let the calculator handle openings and waste

Doing this by hand works for a single straight run, but it is easy to forget an opening or use the wrong waste factor for a wall with lots of corners. Enter your wall length, height, and any openings, and the tool returns the block count, courses, blocks per course, and mortar bags together.

For an L shaped or stepped wall, split it into straight sections, run each one through the calculator, and add the totals. That keeps the count accurate without forcing an irregular shape into one formula.

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 do you calculate the number of concrete blocks needed for a wall?
Multiply the wall's length by its height in feet to get the area, then multiply by 1.125, since a standard 8x8x16 block covers 0.889 square feet with a 3/8 inch mortar joint. Subtract any openings first, then add 5 to 10% for waste and round up.
How many concrete blocks do I need for a 10x8 wall?
A 10 by 8 foot wall is 80 square feet, which needs about 90 standard 8x8x16 blocks before waste, or roughly 99 once you add a 10% allowance for breakage and cuts.
How many bags of mortar per 100 concrete blocks?
Plan on about 8 to 9 bags of 80 lb pre-mixed mortar mix per 100 standard blocks at a 3/8 inch joint, which is roughly one bag per 12 blocks.
What is the difference between a concrete block and a cinder block?
A cinder block traditionally used coal cinder as its lightweight aggregate, while a modern concrete block, or CMU, uses gravel, sand, or expanded shale. Both share the same 8x8x16 nominal size and the same 1.125 blocks per square foot count, so the terms are used interchangeably today.
How much does a concrete block wall cost per square foot?
A standard hollow-core CMU wall runs about 10 to 20 dollars per square foot installed in 2026, with grouted and reinforced walls adding 3 to 6 dollars per square foot for the extra steel and grout.

References

Related

Concrete Block Calculator→Standard Concrete Block Sizes→Rebar Calculator→