Concrete Block (CMU) Calculator
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142 blocks in 9 courses
How to calculate concrete blocks
Block walls are laid out with nominal sizes, which include one mortar joint. A standard US block is 7⅝ × 7⅝ × 15⅝ inches, but with ⅜-inch joints it occupies exactly 8 × 16 inches of wall, or 0.889 square feet. That is why you need 1.125 blocks per square foot.
Worked example: a 40 ft garden wall
A wall 40 feet long and 4 feet high in 8-inch block is 160 ft². At 1.125 blocks per square foot that is 180 blocks, or 189 with 5% extra for cuts and breakage, laid in 6 courses. At 13 blocks per bag it needs 15 bags of mortar mix, and grouting the reinforced cores at 32 inches on center takes about 11.81 ft³ of grout.
Blocks per square foot and per square meter
| Block | Nominal face (with joint) | Blocks per unit area |
|---|---|---|
| US 8 × 8 × 16 in (and 4, 6, 10, 12 in widths) | 8 × 16 in | 1.125 per ft² (12.1 per m²) |
| UK 440 × 215 mm | 450 × 225 mm | 9.88 per m² |
| Australian 390 × 190 mm (15.01, 20.01 series) | 400 × 200 mm | 12.5 per m² |
Filling block cores with grout
Hollow block walls are often reinforced with vertical rebar in some cores, which are then filled with grout or a pourable core-fill concrete. Retaining walls and walls in high-wind or seismic areas may be fully grouted. The calculator lets you choose every core or a spacing that matches your rebar layout, such as every 32 or 48 inches.
Core volume depends on the block design. The defaults (about 0.25 ft³ per standard 8-inch block, roughly 7 liters for a 20.01 series block) are approximations; use the figure from your block supplier if available. UK dense concrete blocks are usually solid, so grout is not offered for them.
Tips for block walls
- Start with a footing. A block wall needs a level concrete footing below it. Size it with the concrete slab calculator.
- Plan lengths in block units. Designing walls in multiples of 8 inches (200 mm in Australia) saves cutting.
- Order extra. 5% covers normal breakage and cuts; walls with many corners or openings need more.
- Check the rules. Retaining walls above a modest height, and any wall that supports a structure, generally need engineering and a permit.
Frequently asked questions
How many concrete blocks do I need per square foot?
Standard 8 × 8 × 16 inch blocks lay at 1.125 blocks per square foot of wall, because each block plus its mortar joint covers 16 × 8 inches (0.889 sq ft). A 100 sq ft wall needs about 113 blocks before waste.
How many blocks are in a square meter?
UK standard 440 × 215 mm blocks lay at about 10 per square meter with 10 mm joints. Australian 390 × 190 mm blocks lay at 12.5 per square meter.
How much grout does it take to fill block cores?
A fully grouted 8-inch block wall takes roughly 0.25 cubic feet of grout per block, or about one cubic yard per 108 blocks. Walls are often grouted only in the cores that hold vertical rebar, which the calculator handles with the grout spacing option. Core volume varies by block design, so the figure is editable.
How many blocks does one bag of mortar lay?
It depends on the joint thickness and the product, so check the bag. Premixed 80 lb mortar bags are commonly labeled for about a dozen standard 8-inch blocks. The calculator uses 13 by default and lets you change it.
Are cinder blocks and concrete blocks the same thing?
In everyday use, yes. True cinder blocks made with coal cinders are rare today; what stores sell as cinder block is almost always a concrete masonry unit (CMU), so the same calculation applies.