This concrete block calculator estimates how many standard CMU blocks a wall needs — plus the mortar to lay them — from the wall’s length and height.
Based on standard 8″ × 8″ × 16″ CMU with ⅜″ mortar joints (1.125 blocks per sq ft). Mortar estimate assumes premix bags at roughly 25 blocks per bag — confirm against your product’s yield, and add for grout-filled cells separately if the wall is reinforced.
How CMU Estimating Works
A standard concrete masonry unit (8×8×16 inches nominal) plus its ⅜″ mortar joint covers 16⅜″ × 8⅜″ of wall — working out to exactly 1.125 blocks per square foot. Wall area times 1.125, plus waste, is the block count every estimator starts from. If your wall needs a poured footing or slab beneath it, the Concrete Calculator handles the yardage; for brick-faced projects, the Brick Calculator covers the veneer. All in our Construction Calculators collection.
Worked Example (Verified)
A 20 ft × 8 ft wall (160 sq ft):
- Blocks: 160 × 1.125 = 180 blocks
- With 5% waste: 189 blocks — order 190
- Mortar: ≈ 8 bags of 80 lb premix (at ~25 blocks per bag)
CMU Facts That Matter on Site
- Nominal vs. actual: an “8×8×16” block actually measures 7⅝ × 7⅝ × 15⅝ — the missing ⅜ everywhere is the mortar joint. The 1.125 rate already accounts for it.
- Course planning: each course is 8 inches tall; walls laid out in 8″ multiples (of height and, mostly, 8/16″ of length) minimize cutting — design openings to block modules.
- Half blocks and bond beams are separate SKUs: corners, jambs and lintel courses need them. A typical wall order adds a handful of halves and bond-beam units per run.
- Grout and rebar for reinforced cells are estimated by cell count and spacing, not by wall area — structural walls need an engineer’s schedule, not just this count.
Related Tools
- Brick Calculator — brick walls and veneer
- Concrete Calculator — footings and slabs
- Construction Calculators — the full collection
Frequently Asked Questions
How many concrete blocks are in a square foot?
1.125 standard 8×16 blocks per square foot of wall, including standard ⅜″ mortar joints.
How many blocks do I need for a 20×8 wall?
160 sq ft × 1.125 = 180 blocks; with 5% waste, order about 190.
How much mortar per 100 blocks?
Roughly 4 bags of 80 lb premix (at ~25 blocks per bag) — actual yield varies with joint width and waste; confirm on the bag’s coverage chart.
What size is a standard concrete block?
Nominal 8″ high × 16″ long (thicknesses 4, 6, 8, 10, 12″), actual dimensions ⅜″ smaller in each direction to leave room for mortar.
Should I subtract doors and windows?
For big openings, yes — subtract their area first (the calculator uses gross wall area). Small windows are usually absorbed by the waste allowance.
Is this concrete block calculator free?
Yes — free, instant, no sign-up. Structural walls need engineering beyond any count — consult local code for reinforcement requirements.