An 8×8 ft wall of standard 8×8×16 CMU needs about 72 blocks — closer to 76 after a 5% waste allowance — not sixty-four “one block per square foot” guesses that leave you short halfway up the wall.
The short count shows up on the second course. You multiplied length × height, forgot the mortar joint in the module, skipped openings, and the yard run starts while the mixer still has mortar.
The block-count formula is:
blocks = ceil( (L × H − openings) × 1.125 × (1 + waste%) )
For US 8×8×16 CMU, the nominal module with a ~⅜ in mortar joint is about 1.125 blocks per square foot (112.5 per 100 sf). Subtract door and window areas first. Then ceil after waste. Estimates only — not engineering or building-code advice.
| Wall (L×H) | Area | Blocks | +5% waste |
|---|---|---|---|
| 8 × 4 ft | 32 sf | 36 | 38 |
| 8 × 8 ft | 64 sf | 72 | 76 |
| 10 × 8 ft | 80 sf | 90 | 95 |
| 12 × 8 ft | 96 sf | 108 | 114 |
| 20 × 8 ft | 160 sf | 180 | 189 |
How many 8×8×16 blocks for an 8×8 wall?
Net area is 64 sf. At 1.125 blocks/sf: 64 × 1.125 = 72 blocks. With 5% waste: 72 × 1.05 = 75.6 → ceil to 76. Mortar for that order is roughly 2–3 bags of Type S 80 lb on the static ~3 bags per 100 blocks rule of thumb — check the bag and your joint size.
Count the face module, not the bare block face. An 8×16 unit with mortar lands on a 8×16 nominal grid; dividing wall area by the raw 7⅝ × 15⅝ face undershoots the pallet.
The formula, on a tape
Work in feet and inches (inches default in the app). AU/UK/NZ crews can toggle millimetres — same math. This piece is CMU-focused; the app also handles brick counts when you need them.
- Gross wall area = length (ft) × height (ft)
- Net area = gross − door/window openings (and other voids)
- Blocks ≈ net area × 1.125 for standard US 8×8×16 CMU
- Order = ceil(blocks × (1 + waste%)) — never round down
- Mortar bags ≈ (ordered blocks ÷ 100) × 3 Type S 80 lb (estimate)
Example: 12×8 wall with one 3×7 ft door. Gross 96 sf − 21 sf = 75 sf → 75 × 1.125 = 84.375 → 85 before waste → about 90 at 5%.
In Masonry Takeoff Master you enter wall length and height, openings, and waste once — and get CMU (or brick) count plus a mortar estimate on your phone. Free for a simple wall; Pro unlocks extras at $9.99. Inches default; mm toggle for metric regions.
Three mistakes that short the pallet
Forgetting openings — or counting them twice. A door and a window are not “close enough to ignore.” Subtract net opening area before you multiply by 1.125. Do not subtract again after you already used a net area.
Counting without the mortar joint / module. Dividing by the actual block face (no joint) under-orders. The industry rule of thumb for 8×16 CMU is the nominal module: about 112.5 blocks per 100 sf (1.125/sf). Use the module, then ceil.
Skipping waste. Cuts, breakage, and bond pattern eat units. Five to ten percent is common on small walls; ceil after the percentage. Running out mid-wall means a yard run while the mortar sets up.
Cinder block vs CMU — same count math
Search often says “cinder block.” On a US jobsite that usually means the same 8×8×16 concrete masonry unit (CMU). Face shell and core fill differ by product; the module count for estimating block quantity is still length × height × 1.125 after openings. Structural fill, rebar, and code are separate — this guide is quantity only.
Worked example for a 20×8 garden wall
Gross 160 sf, no openings → 160 × 1.125 = 180 blocks. With 5% waste → 189. Mortar ≈ 189 ÷ 100 × 3 ≈ 6 bags Type S 80 lb on the static estimate. For the count plus openings and waste on your phone, open Masonry Takeoff Master. Related: how many bags of concrete, how much rebar for a slab.
Estimates only — not engineering or building-code advice. Measure net wall area. Use the module with mortar. Round the order up.
Open Masonry Takeoff MasterEstimate only — not engineering or building-code advice.