01 Concrete block wall calculator
Select your region and block size, enter wall length, height and any window or door openings, and get your exact block count, mortar bags and core-fill grout volume.
Need concrete for the trench underneath? Jump to our strip footing calculator to size the foundation pour.
02 The concrete block wall formula
US Imperial (16 × 8 in nominal)
Blocks = Net Wall ft² × 1.125 × (1 + waste)
UK · AU · NZ Metric
Blocks = Net Wall m² ÷ (Block L × Block H)
Masonry units are specified by nominal size (actual block dimension plus one mortar joint). An American 8 × 8 × 16 in CMU actually measures 7⅝ × 7⅝ × 15⅝ in; adding a ⅜ in mortar joint brings the repeating module right back to 8 × 16 in (0.889 ft²).
03 How to calculate blocks step by step
- Find gross wall area. Multiply total wall length by wall height (for stepped or gabled walls, split into rectangles and triangles).
- Subtract openings. Deduct the area of doors, windows and utility louvers.
- Multiply by blocks per unit area:
1.125 blocks/ft²for US 8×16 face CMU,10 blocks/m²for UK 440×215 mm blocks, or12.5 blocks/m²for AU/NZ 390×190 mm blocks. - Add 5–10% waste for half-cuts, corner closures and chipped units, then round up to a whole block.
- Estimate mortar & core grout. Divide blocks by 33 for 80-lb mortar bags, and compute cell-fill grout based on your reinforcement spacing.
04 Blocks needed for common wall sizes
| Wall size | Area | Order blocks (+7%) | 80-lb mortar | Grout (if 100% solid) | Common project |
|---|---|---|---|---|---|
| 16 × 4 ft | 64 ft² | 78 | 3 bags | 0.71 yd³ | Garden or planter wall |
| 20 × 6 ft | 120 ft² | 145 | 5 bags | 1.34 yd³ | Privacy / yard screen wall |
| 30 × 6 ft | 180 ft² | 217 | 7 bags | 2.01 yd³ | Property line fence wall |
| 24 × 8 ft | 192 ft² | 232 | 8 bags | 2.14 yd³ | Single-wall garage side |
| 40 × 8 ft | 320 ft² | 386 | 13 bags | 3.57 yd³ | Workshop or basement wall |
| 60 × 8 ft | 480 ft² | 578 | 19 bags | 5.35 yd³ | Full foundation perimeter run |
05 Mortar bags vs core-fill grout
DIYers frequently confuse masonry mortar with core-fill grout—they do very different jobs:
- Mortar (Type S or Type N) is a sticky, low-slump mix troweled onto face shells and head joints to bond courses together. Use Type S for structural, exterior and below-grade walls.
- Masonry grout is a high-slump, fluid concrete mix poured into the hollow vertical cells and horizontal bond beams to lock steel rebar to the block shell. Never pack stiff mortar into rebar cells—it leaves structural voids.
06 US CMU vs UK breeze blocks vs AU/NZ 20-series
United States (CMU)
- Nominal face: 16 × 8 in (3/8 in joint)
- Thicknesses: 4, 6, 8, 10 and 12 in
- Multiplier: 1.125 blocks per ft²
- Hollow 2-core units designed for rebar & grout
UK, Australia & New Zealand
- UK dense / aircrete (“breeze”): 440 × 215 × 100 mm → 10 blocks/m²
- AU & NZ 20-series hollow: 390 × 190 × 190 mm → 12.5 blocks/m²
- Standard metric mortar bed: 10 mm
07 What does a concrete block wall cost?
Material cost for standard 8×8×16 grey CMU blocks, mortar, rebar and core grout usually lands around $4.50–$7.50 per ft² of wall face in the US. Hiring a licensed masonry crew typically brings total installed cost (excluding deep excavation) to $15–$30+ per ft² depending on height, reinforcement, footing size and stucco or split-face finish.
08 Common block estimating mistakes
- Forgetting special shapes. Sash blocks, bullnose corners, bond-beam U-blocks and half-blocks cost slightly more and save hours of saw cutting.
- Skipping horizontal joint reinforcement. Ladder or truss wire every second course (16 in vertically) controls shrinkage cracks.
- Under-ordering grout. Open cores drink more grout than smooth pipe calculations suggest because block webs taper inward. Keep a 7–10% buffer.
09 Frequently asked questions
How many 8×8×16 concrete blocks do I need per square foot?
You need 1.125 standard 8×8×16 concrete blocks per square foot of wall area (or 112.5 blocks per 100 sq ft) before waste. That is because a nominal 16 in × 8 in block face including its 3/8 in mortar joint covers 0.889 sq ft.
How many concrete blocks are in a m² in the UK, Australia and New Zealand?
In the UK, standard 440 × 215 mm blocks with a 10 mm mortar joint require 10 blocks per m². In Australia and New Zealand, 390 × 190 mm (20-series) hollow blocks with a 10 mm joint require 12.5 blocks per m².
How many bags of mortar do I need per 100 concrete blocks?
Plan on roughly 3 bags of 80-lb pre-blended Type S mortar per 100 standard 8×8×16 blocks (each 80-lb bag lays about 32–35 blocks with 3/8 in joints). Using 60-lb bags, budget about 4 to 4.5 bags per 100 blocks.
How much core-fill grout does an 8-inch CMU wall take?
Solidly grouting every cell of an 8×8×16 hollow block wall takes about 0.25 cubic feet of grout per block, or roughly 1.04 cubic yards per 100 square feet. Grouting vertical cores at 48 inches on center uses roughly one-third of that volume.
What is the difference between cinder blocks and CMU blocks?
True cinder blocks used coal cinders as aggregate decades ago. Modern blocks sold at masonry yards are Concrete Masonry Units (CMUs) made with Portland cement, sand and crushed stone or lightweight expanded shale, though people still casually call them cinder blocks.
How much extra block waste should I add?
Add 5–7% extra blocks on straight walls and 10% on walls with corners, gable rakes, pilasters or multiple window and door openings where half-block cuts generate more breakage.
Does a concrete block wall need a footing and rebar?
Yes. Load-bearing, retaining and freestanding walls over 3–4 feet high normally sit on a continuous concrete strip footing below frost depth with vertical rebar dowels grouted into the block cores. Always follow local building codes.
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