Concrete masonry units (CMU), often called cinder blocks, are made to a nominal 8 × 16 inch face. The actual block is 7⅝ × 15⅝ inches, and a ⅜-inch mortar joint brings each one to the nominal module. Each block therefore covers 8 × 16 = 128 square inches, or 8/9 of a square foot. That gives the industry figure of 1.125 blocks per square foot of wall, regardless of the block's width. This concrete block calculator multiplies your net wall area by that figure, adds waste, and estimates mortar and the grout needed to fill reinforced cores.
The Concrete Masonry & Hardscapes Association's TEK 4-2A, Estimating Concrete Masonry Materials, uses exactly this method and gives 119 conventional units per 100 ft² including about 5% waste. For brick veneer, use the brick calculator; for vertical bars in grouted cores, the rebar calculator can total the steel.
How to use the concrete block calculator
- Enter the wall length and height. For several walls, add their lengths together if they are the same height.
- Subtract the area of doors, windows and other openings.
- Choose the block size. Standard 4, 6, 8, 10 and 12-inch-wide blocks all have the same 8 × 16 face, so the count is the same; only the core volume differs.
- Choose whether cores will be grouted: none, every few cells (where vertical rebar goes), or all cores.
Formula
Core volumes used (2-core units): 6 in 0.17 ft³, 8 in 0.25 ft³, 12 in 0.39 ft³ per block (half-high 8 in assumed at half of 0.25). The mortar figure is a supplier rule of thumb from Titan America's block estimating guide.
Worked example
A garden wall 40 ft long and 4 ft high, 8×8×16 blocks, 5% waste, every fourth block grouted.
- Area: 40 × 4 = 160 ft².
- Blocks: 160 × 1.125 = 180, plus 5% = 189 blocks.
- Layout: 4 ft ÷ 8 in = 6 courses of 40 ÷ 1.333 = 30 blocks.
- Mortar: 189 ÷ 100 × 3 = 5.67, so about 6 bags.
- Grout: 189 ÷ 4 × 0.25 = 11.8 ft³ (0.44 yd³).
Blocks per square foot by unit size
| Unit (face, nominal) | Blocks per ft² | Per 100 ft² plus 5% waste |
|---|---|---|
| 8 × 16 (standard, any width) | 1.125 | 119 (CMHA figure) |
| 4 × 16 (half-high) | 2.25 | about 236 |
| 8 × 24 ("two-footer") | 0.75 | about 79 |
Standard widths share the 8 × 16 face, which is why the CMHA method "is independent of the unit width". Corner, bond-beam and pilaster units should be counted separately and subtracted from the field count.
Mortar, grout and reinforcement
Mortar bonds the blocks; grout is a fluid concrete poured into the hollow cores, usually around vertical rebar, to make the wall structural. Retaining walls and load-bearing walls are typically grouted at the bar locations shown on the plans, and sometimes solid. Bond beams (a course of U-shaped blocks filled with grout and horizontal bars) add more grout. CMHA cautions that rules of thumb "should be used with caution and checked against rational judgment", because non-modular layouts and many corners increase quantities. Footings are a separate pour; use the concrete slab calculator for those.
Caveats and assumptions
- Block walls above a few feet, retaining walls and load-bearing walls need engineering and usually a permit.
- Mortar and grout figures are rules of thumb; actual use depends on joint thickness, waste and core shape.
- Doesn’t count special units (corners, bond beams, caps) separately.
- Prices are illustrative; enter your local price.
Frequently asked questions
How many concrete blocks do I need per square foot?
1.125 standard 8×8×16 blocks per square foot of wall, because each block covers 8/9 ft² including the mortar joint.
How many blocks are in 100 square feet of wall?
112.5 before waste. CMHA’s TEK 4-2A gives 119 including about 5% for waste and breakage.
What is the actual size of an 8×8×16 block?
About 7⅝ × 7⅝ × 15⅝ inches. The ⅜-inch mortar joint brings it to the 8 × 16 nominal module.
How much mortar do I need for block?
A common supplier rule of thumb is about 3 bags of masonry cement mortar per 100 blocks; check the bag label for its yield.
How much grout fills an 8-inch block?
About 0.25 ft³ per standard two-core 8×8×16 block, so a cubic yard fills roughly 108 blocks.
Sources
- Concrete Masonry & Hardscapes Association — TEK 4-2A, Estimating Concrete Masonry Materials — 8/9 ft² per unit, 119 units per 100 ft² with 5% waste
- Titan America — Block Estimating Guide — units per ft², mortar bags per 100 block
- Chaney Enterprises — Masonry Shapes Catalog (estimating guides) — core fill volume per block
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.