How many concrete blocks do I need for a wall?
Wall area in square feet = length x height, less any openings. A block counts at its nominal face, so blocks = wall area x 144 / (nominal height x nominal length in inches), rounded up. The 144 is the square inches in a square foot (12 x 12). [1]
CMHA says the specified size of a block is typically 3/8 in less than its nominal size, so that a 3/8 in mortar joint keeps the module. [2] That is why the count uses the nominal 8 x 16 in face: the joint is already inside it.
We found no source for an extra on blocks, so Extra to order starts at 0. Ask your supplier how much to add for cuts and breakage.
Worked example
A wall 20 ft long and 4 ft high is 20 x 4 = 80 square feet. A block with a nominal face of 8 in x 16 in covers 8 x 16 = 128 square inches, so a square foot of wall takes 144 / 128 = 1.125 blocks. 80 x 1.125 = 90 blocks, with no extra. As an example, an extra of 5% gives 94.5, which rounds up to 95 blocks.
What size block do I need?
Angelus Block says a nominal dimension is the specified dimension plus the thickness of the mortar joint, and that sizes are stated in the order width, height and length. [3] Its table gives the specified size of each nominal size.
| Nominal size (W x H x L) | Specified size (in) |
|---|---|
| 4x8x16 | 3.625 x 7.625 x 15.625 |
| 8x8x16 | 7.625 x 7.625 x 15.625 |
| 12x8x16 | 11.625 x 7.625 x 15.625 |
The 8x8x16 block
An 8x8x16 block is 8 in wide, 8 in high and 16 in long, nominal. Its specified size is 7.625 x 7.625 x 15.625 in, which is 3/8 in under each nominal dimension. The preset in this calculator is that block: a nominal face of 8 x 16 in.
How much does a concrete block weigh?
We found no source we can cite for the weight of one block, because it depends on the size, the cores and the concrete. Ask your supplier, then open Weight, core fill and price (optional) above and enter it under Weight of one block to see the weight of the wall.
What CMHA does give is the density of the concrete. It sorts concrete masonry into three classes: lightweight is less than 105 pounds per cubic foot, medium weight is 105 pounds per cubic foot or more and less than 125 pounds per cubic foot, and normal weight is 125 pounds per cubic foot or more. [2]
Should you fill concrete blocks with concrete?
We found no source we can cite for when the cores of a block wall are filled. Ask your local building authority or the designer of the wall.
To count the concrete if you do fill them, open Weight, core fill and price (optional) above and enter the Concrete in the cores of one block from your supplier. The calculator multiplies it by the blocks as measured, and shows cubic feet and cubic yards.
How many blocks for a 10x10 wall?
Blocks for four example walls, with no extra. A 10 x 10 ft wall is 100 square feet, which is 112.5 blocks, so 113. The sizes are examples to show the sum.
| Wall | Area (ft²) | Blocks before rounding | Blocks |
|---|---|---|---|
| 10 x 10 ft | 100 | 112.50 | 113 |
| 20 x 4 ft | 80 | 90.00 | 90 |
| 30 x 6 ft | 180 | 202.50 | 203 |
| 40 x 8 ft | 320 | 360.00 | 360 |
The mortar joint
The nominal size already includes the joint, so the count needs no extra for mortar.
How much does a concrete block cost?
We state no price. Ask local suppliers what one block costs, then enter it under Weight, core fill and price (optional) above to see the total for the blocks to order. The site does not store the prices you type.
Common mistakes and limits
- Counting with the specified size. The specified size is typically 3/8 in less than the nominal size, and the mortar joint fills the difference. [2]
- Reading a size in the wrong order. Angelus Block says sizes are stated as width, height and length. [3]
- Forgetting the openings. Enter doors and windows under Openings to take away.
- What this tool does not cover: mortar, reinforcement, the footing and caps and corners.
Frequently asked questions
Are concrete blocks and cinder blocks the same?
We found no source we can cite for the difference. The units this page counts are concrete masonry units, whose sizes and properties the Concrete Masonry & Hardscapes Association describes. Ask your supplier what you are buying. [2]
Do concrete block walls need rebar?
We found no source we can cite for whether a block wall needs reinforcing bar. Ask your local building authority or the designer of the wall. Our rebar calculator counts bar for a slab only.
Can concrete blocks be used for a foundation?
We found no source we can cite for this. Ask your local building authority, because local code decides.
How thick should the mortar joint be?
CMHA says specified block sizes are typically 3/8 in less than nominal, so that a 3/8 in mortar joint keeps the module. That is why this calculator counts a block at its nominal size. [2]
How do I read a block size such as 8x8x16?
Angelus Block says dimensions must be stated in the order width, height and length, so an 8x8x16 is 8 inches wide, 8 high and 16 long. Nominal sizes include the mortar joint, and the specified size is 3/8 in smaller. [3]
Sources and references
- National Institute of Standards and Technology (NIST). NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (SP 811, accessed November 2026). Used for: the inch and foot relationship (12 inches in a foot).
- Concrete Masonry & Hardscapes Association (CMHA). CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (2023, accessed November 2026). Used for: the nominal and specified block sizes, the 3/8 inch mortar joint, and the density classes.
- Angelus Block Co.. Dimensions and Sizes for CMU (accessed November 2026). Used for: how block sizes are named and the specified size of each nominal size.
How we use sources: methodology.
Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.