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Construction & Home
Concrete Block Calculator — How Many Cinder Blocks for a Wall
How many cinder blocks and bags of mortar for a CMU wall
A concrete block calculator turns a wall's length and height into the two numbers you take to the masonry yard: how many concrete blocks to buy and how many bags of mortar to lay them. Order short and your wall stops a course shy of the top; order long and a pallet of $2–$3 blocks sits in the yard. This tool does the area math, adds a waste factor for breakage and cuts, and always rounds up to whole blocks — you can't lay half a block.
The whole estimate rides on one constant. A standard CMU (concrete masonry unit) — the gray 'cinder block' you see everywhere — has a nominal face of 16 in × 8 in. That face, including the standard 3/8-inch mortar joint, covers about 0.888 square feet, which means you need roughly 1.125 blocks for every square foot of wall. Flip the math and 9 nominal blocks lay up 8 square feet of wall — the rule of thumb masons quote on site.
The steps are short:
Wall area = length × height, both in feet. A 20 ft long wall built 8 ft high is 20 × 8 = 160 square feet.
Blocks (before waste) = area × 1.125. That 160 sq ft wall needs 160 × 1.125 = 180 blocks.
Add waste, then round up. With the default 5% for breakage and cut pieces, 180 × 1.05 = 189 → 189 blocks to buy.
Mortar is the second half of the order. As a working rule, one 70 lb bag of mortar mix lays about 33 standard blocks (some crews stretch a bag to ~37 with type-S). For the 180-block wall that's ceil(180 ÷ 33) = 6 bags. Bump the waste factor toward 10% if the wall has lots of corners, openings or end cuts, since every cut block wastes part of a unit.
This is specifically a concrete/cinder block estimator, and that matters: a clay brick wall runs about 7 bricks per square foot, nearly six times the count, and a landscape retaining-wall block is sized and stacked completely differently. Use the matching tool for those — see the related calculators below — so your count and your mortar bags actually match the units on the pallet.
📖 See also: How to Calculate a Tip (and Split the Bill)
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Wall area (sq ft) = length (ft) × height (ft) • Blocks (before waste) = area × 1.125 blocks per sq ft • Blocks to buy = ceil(area × 1.125 × (1 + waste% ÷ 100)) • Mortar bags = ceil(blocks ÷ blocks-per-bag) • Standard CMU face = 16 in × 8 in ≈ 0.888 sq ft (≈ 1.125 blocks/sq ft; 9 blocks per 8 sq ft) • Mortar: 1 bag (70 lb) ≈ 33 standard blocks • Default waste = 5% (bump to 10% for many corners, openings or cuts)
📰 Formula
• Wall area (sq ft) = length (ft) × height (ft) • Blocks (before waste) = area × 1.125 blocks per sq ft • Blocks to buy = ceil(area × 1.125 × (1 + waste% ÷ 100)) • Mortar bags = ceil(blocks ÷ blocks-per-bag) • Standard CMU face = 16 in × 8 in ≈ 0.888 sq ft (≈ 1.125 blocks/sq ft; 9 blocks per 8 sq ft) • Mortar: 1 bag (70 lb) ≈ 33 standard blocks • Default waste = 5% (bump to 10% for many corners, openings or cuts)
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Counting blocks by wall length only and forgetting to multiply by height for the full area.
- Using the brick rate of about 7 per square foot instead of 1.125 for concrete block.
- Forgetting to subtract large door or window openings, so the count runs high.
- Skipping the waste factor, so breakage and end cuts leave you a few blocks short.
- Rounding blocks or mortar bags down — you can't buy a partial block or bag.
💡 Tips
- Order a few extra blocks beyond the waste factor; matching a dye lot later is a hassle.
- Use 5% waste for a plain straight wall and 10% when there are corners, ends and openings to cut.
- Plan 1 bag of 70 lb mortar mix per ~33 blocks, plus a bag of sand-mix for the footing bed if needed.
- Lay out the first course dry to confirm the length lands on a whole or half block before mortaring.
- Half blocks and corner (return) blocks count as full units when you order — don't try to round them off.
Embed this calculator on your site
Copy the code below and paste it into the HTML of your site or blog.
<iframe src="https://www.calcnimbus.com/embed/concrete-block-calculator" width="100%" height="500" frameborder="0" style="border:1px solid #eee;border-radius:12px"></iframe>
❓ Frequently asked questions
How many concrete blocks do I need for a wall?
Multiply the wall length by its height in feet to get square footage, then multiply by 1.125 blocks per square foot and round up. A 20 ft by 8 ft wall is 160 sq ft × 1.125 = 180 blocks before adding waste.
How many concrete blocks are in one square foot?
About 1.125 standard blocks. A nominal 16 in × 8 in block face covers roughly 0.888 square feet including the mortar joint, so it takes about 9 blocks to lay up 8 square feet of wall.
How many blocks does an 80 lb or 70 lb bag of mortar lay?
A 70 lb bag of mortar mix lays roughly 33 standard 8x8x16 blocks. Crews using type-S sometimes stretch a bag to about 37. Divide your block count by 33 and round up to get bags.
What size is a standard concrete block?
A standard CMU is nominally 16 inches long by 8 inches high by 8 inches wide. Actual size is about 15-5/8 × 7-5/8 to leave room for a 3/8-inch mortar joint, which is why the math uses the nominal 16 × 8 face.
Is a cinder block the same as a concrete block?
For estimating, yes. 'Cinder block' is the common name and 'concrete masonry unit (CMU)' is the technical one; both use the same nominal 16 × 8 inch face, so this calculator covers either.
How do I figure blocks for a wall with a door or window?
Calculate the full wall area, then subtract the opening's area (its width × height in feet) before multiplying by 1.125. For small openings many people leave them in as part of the waste cushion.
How many courses of block is an 8-foot wall?
Each course adds 8 inches of height, so an 8-foot (96-inch) wall is 96 ÷ 8 = 12 courses. Block count still comes from total square footage, but courses help you check the height on site.
Why does the block calculator add a waste factor?
Some blocks crack in handling and end pieces have to be cut, wasting part of a unit. A 5% waste factor covers a plain wall; raise it to 10% when there are many corners, ends or openings to cut around.
How much does a concrete block wall cost?
Multiply the block count by the price per block and add the mortar bags times their price. Enter both prices in this calculator and it returns an estimated material cost for the wall.