Enter the wall length and height and any openings. The calculator uses the method the National Concrete Masonry Association gives for estimating units: divide the net wall area by the face area of one block, then add an allowance for waste and breakage.
Cinder / CMU blocks calculator
How it works
A standard block is 7-5/8 × 15-5/8 in, and with a 3/8 in mortar joint it covers a nominal 8 × 16 in of wall, or 8/9 of a square foot.
- Net area = length × height − openings
- Blocks = net area ÷ face area per block × (1 + waste %), rounded up
So a standard block wall takes 1.125 blocks per square foot before waste. With NCMA's 5% allowance that is 119 blocks per 100 sq ft (NCMA TEK 4-2A, Table 1). The method works for any block width because only the face size matters.
Worked example
A 40 ft garden wall, 4 ft high, in standard 8 × 16 in block:
- Wall length: 40 ft
- Wall height: 4 ft
- Area of openings to subtract: 0 sq ft
- Block face size (nominal): 8 x 16 in (standard block)
- Waste and breakage: 5%
The calculator gives:
- Blocks to buy: 189
- Net wall area: 160.0 sq ft
- Blocks before waste: 180.0 (1.125 per sq ft)
- Layout check: 6 courses of about 30 blocks
160 sq ft ÷ 0.889 sq ft per block = 180 blocks, plus 5% = 189. That is 6 courses of 30 blocks, with the extra covering cuts and breakage.
Material and waste notes
- Corner, bond-beam, pilaster and half blocks are counted separately in real take-offs; subtract them from the total and order them by type.
- Walls with many corners, returns or odd lengths use more units than the area method suggests (NCMA TEK 4-2A).
- "Cinder block" is the everyday name; most blocks sold today are concrete masonry units (CMU) made with ordinary aggregate.
- Reinforcement, grout and footing size for a retaining or structural wall come from the design or local code. This tool only counts blocks.
Frequently asked questions
How many cinder blocks are in 100 square feet?
About 113 standard 8 × 16 in blocks, or 119 with NCMA's 5% allowance for waste and breakage.
How many blocks per square foot?
1.125 standard blocks per square foot of wall (one block covers 8/9 sq ft including its mortar joint).
How many blocks in one course?
Each standard block adds 16 in of length, so divide the wall length in inches by 16. A 40 ft wall is 480 ÷ 16 = 30 blocks per course.
Does wall thickness change the count?
No. A 6, 8 or 12 in wide block has the same 8 × 16 in face, so the count is the same. Width changes the weight, the mortar bedding and the grout volume.
Sources
Last reviewed: 25 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.