Building with Cinder Blocks (CMU)
Concrete Masonry Units (CMUs), commonly known as cinder blocks or breeze blocks, are one of the most reliable and heavy-duty building materials in the world. They are universally used for constructing load-bearing basement foundations, massive retaining walls, industrial buildings, and structural firewalls.
Because block laying is highly labor-intensive and mortar sets quickly, knowing exactly how many blocks to order (and stage along your footing) is critical before you mix your first batch of mortar.
Standard CMU Dimensions
To calculate how many blocks you need, you must understand the true dimensions of a standard cinder block.
A standard block is referred to as an "8x8x16" block. However, its actual physical dimensions are 7-5/8" deep × 7-5/8" high × 15-5/8" long.
Why the difference? The missing 3/8 of an inch accounts for the standard thickness of the mortar joint. When the block is laid in the wall with mortar on the bottom and one side, it occupies exactly an 8" x 16" space. This makes calculating wall areas incredibly simple.
How to Calculate Block Quantities
Because a block (with mortar) is exactly 8 inches high and 16 inches long, we can determine its exact square footage on the face of the wall.
The Math
- Height: 8 inches (0.667 feet)
- Length: 16 inches (1.333 feet)
- Area of one block face =
0.667 ft × 1.333 ft = 0.889 square feet.
Wait, there is an even simpler industry rule! Since one block equals 0.889 square feet, dividing 1 by 0.889 gives you 1.125 blocks per square foot.
The Formula
To find the total number of blocks needed for a straight, solid wall:
- Find the total Square Footage of the wall (Length × Height).
- Multiply that area by 1.125.
- Add a 5% to 10% waste factor to account for broken blocks and half-block end cuts.
Total Blocks = (Wall Length × Wall Height) × 1.125
Example Calculation
You are building a detached garage and need to erect a block wall that is 20 feet long and 8 feet high.
- Wall Square Footage:
20 ft × 8 ft = 160 sq ft. - Multiply by 1.125:
160 × 1.125 = 180 blocks. - Add 5% waste:
180 × 1.05 = 189 blocks.
You should order a pallet containing at least 189 standard blocks.
Don't Forget the Specialty Blocks
Our calculator estimates the total square footage using standard "stretcher" blocks. However, a real-world wall requires specialty blocks:
- Corner Blocks: These have one flat end (no mortar groove) to create a smooth, finished 90-degree corner.
- Half Blocks: Used at the ends of walls to maintain the staggered "running bond" pattern without having to cut full blocks in half with a masonry saw.
- Bond Beam Blocks: U-shaped blocks placed at the very top of the wall. They are filled with horizontal rebar and solid concrete to tie the entire top of the wall together structurally.