Masonry
Concrete Block (CMU) Calculator
A standard 8 in by 8 in by 16 in concrete block has a nominal face of 128 in², so it takes 1.125 blocks per square foot of wall. Enter the wall size and subtract doors and windows.
Formula
Net area = length × height - openings. Blocks = net area × 1.125, because one nominal 8 in × 16 in face covers 128 in² = 0.889 ft² and 1 ÷ 0.889 = 1.125. Multiply by (1 + waste%) and round up.
Worked example
A 40 ft by 8 ft wall with 40 ft² of openings: net area = 320 - 40 = 280 ft². Blocks = 280 × 1.125 = 315. With 5% waste: 330.75, so order 331 blocks.
Assumptions and limits
- Nominal 8x8x16 units with 3/8 in mortar joints.
- Corner, bond beam and half units are counted as standard units; adjust for special shapes.
- For estimation only. A licensed Professional Engineer (PE) must design and stamp any structural, foundation or life-safety element.
Frequently asked questions
How many concrete blocks are in a square foot?
1.125 standard 8x8x16 blocks, or 112.5 per 100 ft² of wall.
What are the actual dimensions of an 8x8x16 block?
The actual size is 7 5/8 in by 7 5/8 in by 15 5/8 in. The 3/8 in mortar joint brings it to the nominal size.
Do block walls need rebar and grout?
Foundation and retaining walls usually need vertical rebar in grouted cells. The IRC (Section R404) and TMS 402 set requirements; spacing depends on wall height and soil load.