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CalcBelt

Concrete block calculator

Enter the length and height of a wall to get how many concrete blocks (CMU or cinder blocks) to buy, the bags of mortar mix and the cost of the blocks.

Units
Wall

Add up every wall. Where to measure

From the top of the footing.

sq ft

Width × height of each opening, added up.

%

CMHA allows 5%.

Block size (joint included)

Nominal: a standard block is 15 5/8 in plus a 3/8 in joint.

Nominal: 7 5/8 in plus the joint. Width (4 to 12 in) doesn't change the count.

Not covered: grout, rebar and footings. Which cores get filled and how big the footing is are structural decisions; ask your building department or a structural engineer.

Concrete blocks

142 blocks

135 laid plus 5% for breakage and cuts

Wall area
120 sq ft
Courses
98 in each
Mortar mix, 80 lb
at least 11 bagsQuikrete's best case: up to 13 blocks a bag

Cost estimate

per block

Blocks: $334 for 142 blocks

Typical US price $1 to $5 per block (HomeGuide, Home Depot, October 2026). Mortar, sand and delivery are extra.

Built by a mason: roughly $1,800 to $3,600 at $15 to $30 per sq ft of wall, footing and reinforcement included (HomeGuide, one source).

How it's calculated
  1. Wall: 20 ft × 6 ft = 120 sq ft
  2. One block covers 16 in × 8 in = 0.89 sq ft, so 1.125 blocks per sq ft
  3. 120 sq ft × 1.125 = 135 blocks; add 5% = 141.75, rounded up to 142
  4. Mortar: 135 blocks ÷ 13 per bag (Quikrete's best case) = 10.4, so at least 11 bags

Estimates use manufacturer data sheets and industry guides (sources). Check local codes and your supplier's figures before you order.

How to use the concrete block calculator

  1. Add up the length of every wall and enter it with the wall's height above the footing.
  2. Enter the area of any doors or windows to take off.
  3. Keep the standard 16 × 8 in block face (joint included), or type the size of your block.
  4. Read the blocks to buy, with 5% for breakage, and the minimum bags of mortar mix.

How to calculate concrete blocks

Wall area = length × height − openings
Blocks = wall area × 1.125 (for 8 × 16 in blocks)
To buy = blocks × 1.05
Mortar = blocks ÷ 13, at least

Example: a garden wall 20 ft long and 6 ft high.

  • Wall area: 20 × 6 = 120 sq ft
  • Blocks laid: 120 × 1.125 = 135
  • With 5% for breakage: 141.75, so 142 blocks
  • Mortar: 135 ÷ 13 = 10.4, so at least 11 bags of 80 lb mortar mix
  • Courses: 6 ft is 72 in, or 9 courses of 8 in
Concrete block calculator diagram: a block wall seen from the front with its length and height measured from the top of the footing, and one standard block measuring 15 5/8 by 7 5/8 inches plus a 3/8 inch mortar joint, which makes 16 by 8 inches.
Where to measure: the length of all the walls and the height above the footing. Each standard block takes up 16 × 8 in of wall with its joint.

Where 1.125 comes from

A standard block measures 15 5/8 × 7 5/8 in on its face. With a 3/8 in mortar joint it takes up 16 × 8 in, which is 128 square inches, and a square foot is 144 square inches: 144 ÷ 128 = 1.125 blocks per square foot. CMHA, the concrete masonry industry’s association, adds 5% for waste and breakage, which gives its figure of 119 blocks per 100 sq ft.

The width of the block doesn’t change the count: CMHA notes the method works whatever the unit’s width, since thinner and thicker blocks share the same face. Corner, bond beam and other special blocks are counted the same way; CMHA suggests working them out separately and taking them off the total.

Mortar is a minimum

Quikrete rates an 80 lb bag of its Mason Mix at up to 13 standard blocks, so the calculator’s bag count is the least you’ll use. CMHA notes that how far mortar goes depends on the mason, non-modular layouts and how many returns and corners the wall has, so plan on more than the minimum.

What’s not covered

Grout, rebar and footings. Which cores are filled, where the steel goes and how big the footing is depend on the wall’s height, what it holds back and your local code. Ask your building department or a structural engineer. Once the footing is designed, the concrete calculator gives its concrete.

Questions and answers

How many concrete blocks do I need?

1.125 standard 8 × 8 × 16 in blocks per square foot of wall, plus 5% for breakage and cuts: CMHA puts it at 119 blocks per 100 sq ft. A wall 20 ft long and 6 ft high is 120 sq ft, so 135 blocks laid and 142 to buy. Take doors and windows off the wall area first.

How much does a concrete block weigh?

Roughly 21 to 35 lb for a standard 8 in hollow block, depending on how dense the concrete is. CMHA lists ungrouted 8 in hollow block walls at 24 to 39 lb per square foot, mortar included, and a square foot holds 1.125 blocks. Check the supplier's figure if the weight matters.

How much is a concrete block?

$1 to $5 per block, according to HomeGuide. Home Depot listed standard 8 × 8 × 16 in gray blocks at $1.85 to $2.85 in October 2026, depending on the region. Mortar, sand and delivery are extra.

How many concrete blocks are on a pallet?

Often 70 to 90 standard blocks: HomeGuide prices a pallet of 70 to 90 at $115 to $225. The count depends on the plant and the block size, so ask the supplier before you plan deliveries.

How many bags of mortar do I need for concrete blocks?

At least one 80 lb bag of Quikrete Mason Mix for every 13 standard blocks, since Quikrete says a bag lays up to 13. That's a best case: CMHA notes that the mason's skill, non-modular layouts, returns and corners all change how far mortar goes. For 135 blocks, plan on at least 11 bags.

What size is a standard concrete block?

It's sold as 8 × 8 × 16 in but measures 7 5/8 × 7 5/8 × 15 5/8 in. CMHA explains that blocks are made 3/8 in smaller than their nominal size so a 3/8 in mortar joint brings each one back to an 8 in module. Thinner and thicker blocks share the same 8 × 16 in face, so the count per square foot doesn't change.

How much does a block wall cost?

About $15 to $30 per square foot of wall installed, HomeGuide says, including the footing and reinforcement, or $5 to $10 per block for the labor alone. That puts a 20 × 6 ft wall at roughly $1,800 to $3,600. It's one cost guide's range, so get quotes.

Do I need rebar or grout in a block wall?

That isn't covered here. Which cores get grout and steel, and how big the footing must be, are structural decisions set by your local code and the wall's design. Ask your building department or a structural engineer before you build a retaining wall or a load-bearing wall.

Sources

Figures checked October 2026.