Concrete Block Calculator
CMU blocks and mortar bags for a block wall of any size.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Block walls estimate cleanly because a standard CMU with its mortar joint covers an exact 16 × 8 in of wall face — 1.125 blocks per square foot, every time. This calculator turns wall dimensions into blocks and mortar bags.
How it works
A standard block's face is 16 × 8 in (0.89 sq ft), so a wall takes 1.125 blocks per square foot. One 80 lb bag of mortar lays roughly 33 blocks.
The nominal 8 × 8 × 16 in block is actually 7⅝ × 7⅝ × 15⅝ — the missing ⅜ in is the mortar joint, which is why the nominal size lands exactly on layout lines. Wall area × 1.125 gives the block count with no separate joint math.
Mortar estimating uses the mason's rule of one 80 lb bag per 30–35 blocks laid. The calculator figures a bag per 33; add a bag or two for the learning curve if it is your first wall, since dropped and re-tempered mortar is real waste.
Blocks = wall length × height × 1.125, rounded up. Mortar = blocks ÷ 33 bags (80 lb), rounded up.
Worked examples
A 20 ft long, 8 ft tall garage wall. → 180 blocks and 6 bags of mortar.
160 sq ft × 1.125 = 180 blocks; 180 ÷ 33 = 5.5, rounded up to 6 bags.
A 30 ft long, 4 ft garden wall. → 135 blocks and 5 bags.
120 sq ft × 1.125 = 135; 135 ÷ 33 = 4.1 → 5 bags.
Tips & gotchas
- Order about 5% extra block for cuts and breakage — half blocks at corners and ends come out of full units unless you buy purpose-made halves.
- Use Type S mortar for anything structural or below grade; Type N is for above-grade, non-load-bearing work.
- Structural walls also need grout and rebar in cores per the plan — that is a separate estimate from the laying mortar.
FAQ
Do I fill the cores?
Structural walls get grout and rebar in cores per the plan; a garden wall usually only fills the cores holding rebar.
Cinder block vs concrete block?
Same thing in modern stores — 'cinder block' is the colloquial name for a concrete masonry unit (CMU). True cinder aggregate blocks are largely historical.
How many blocks can I lay in a day?
A mason lays 150–200; a careful first-timer might do 50–75. Mix mortar in small batches so it doesn't stiffen before you use it.
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