Concrete Block (CMU) Calculator
Wall length × height, minus openings, divided by the nominal 16 × 8 in block face — a figure that is pure geometry, because nominal block dimensions include the mortar joint by definition. Waste applies before the round-up, mortar bags use a cited manufacturer yield, and prices are yours alone.
Wall dimensions
Example: a 25 × 8 ft wall (200 sq ft) at 5% waste → 237 blocks and 18 mortar bags.
Block counts are shopping arithmetic — reinforcement, grouting, footings, retaining-wall design, and permits are for a licensed engineer or your building department.
Why 0.889 sq ft per block is exact, not estimated
A standard CMU is specified at 15⅝ × 7⅝ in, and its nominal size — 16 × 8 in — is the unit
plus one ⅜ in mortar joint, the convention the concrete masonry industry documents precisely so
walls tile in clean modules. That makes the per-block wall area definitional:
(16 × 8) ÷ 144 = 8⁄9 ≈ 0.889 sq ft. The worked wall: 200 sq ft ÷ 8⁄9 = 225 blocks — exactly, with no rounding — then 5% breakage waste lifts it to 236.25, purchased as
237 blocks with 0.75 to spare. Mortar:
225 laid blocks ÷ 13 per 80 lb bag = 18 bags (breakage
waste consumes no mortar, so bags are figured on the laid count).
The rest of the masonry aisle
For clay brick the same wall-area method runs on the brick calculator with BIA's per-size coverage; for flatwork underfoot, the paver calculator counts units from your paver's own dimensions. Filling block cores or pouring a footing? Size the concrete with the slab calculator or bag calculator.
Frequently asked questions
How many blocks are in 100 square feet of wall?
Exactly 112.5 before waste. A standard CMU is nominally 16 inches long and 8 inches high, and nominal dimensions are defined as the actual unit (15⅝ × 7⅝) plus one ⅜ inch mortar joint — so each block accounts for exactly (16 × 8) ÷ 144 = 0.889 square feet of wall, joint included, and 100 ÷ 0.889 = 112.5.
Is the mortar joint included in the count?
Yes, by definition. The nominal-dimension convention exists precisely so that block plus joint tiles the wall in clean 16 × 8 inch modules — that is why block walls lay out on 8 inch grids. You never add joints separately with nominal dimensions.
Where does the mortar bag figure come from?
The QUIKRETE Mortar Mix No. 1102 data sheet, which states an 80 lb bag "will lay up to 13 standard (8 in by 8 in by 16 in) blocks" — exactly the unit this tool counts. Note the "up to": joint thickness, technique, and dropped mortar all eat into it, so treat the bag count as a floor and round your purchase generously.
What about half blocks, corner blocks, and bond beams?
The tool counts stretcher-equivalent area. Real walls swap in specialty units — corners, halves at openings, lintels, bond beams — that replace standard blocks roughly one for one in area, so the total is close, but plan the specialty count from your coursing layout.
Does this tool tell me if my wall needs grout or rebar?
No — reinforcement, grouted cells, footing size, and whether the wall is structural at all are engineering and code questions. This page turns a wall area you already have into a shopping count, nothing more.
Everything computes locally in your browser. Sources for the mortar yield and the nominal-dimension convention are on the methodology page.