How many concrete blocks do I need?
Multiply the wall area in square feet by 1.125. That figure holds for 4, 6, 8 and 12-inch block because they all share the same nominal 8 × 16 inch face — thickness changes the mortar and the grout, never the block count.
Mortar runs about 3 bags of pre-blended mix per 100 blocks for 8-inch units, less for thinner block and about 4 bags for 12-inch. One 80 lb bag lays roughly 33 standard blocks.
Worked example: a 40 ft × 8 ft wall is 320 square feet. At 1.125 blocks per square foot that is 360 blocks, plus 5% breakage gives 378. Mortar is 360 ÷ 100 × 3 = 11 bags. Filling every core of an 8-inch wall would take 320 × 0.635 ft × 0.45 ÷ 27 = 3.4 cubic yards of grout.
How the calculation works
A nominal 8 × 16 inch block face is 0.889 square feet, so 1 ÷ 0.889 = 1.125 blocks per square foot. The block itself measures 7⅝ × 15⅝ and the missing ⅜ inch on each dimension is the mortar joint — which is why the nominal figure works and measuring the actual block does not.
Courses = wall height in inches ÷ 8. A standard 8-foot wall is 12 courses exactly, which is not an accident; block dimensions are built around it.
Grout volume is wall area × wall thickness × the void fraction of the unit. A standard two-core 8-inch block is roughly 55% solid, so 45% of the wall volume is core. That gives about 1.05 cubic yards per 100 square feet with every core filled — halve it for cores at 32 inches, and take a third for cores at 48.
Deduct openings at their full area but expect to add some back. Every door and window needs a lintel or bond beam over it and full-height jamb cores either side, and those are usually grouted even in an otherwise hollow wall.
Typical concrete block cost, mid-2026
| Material | Unit | Typical range |
|---|---|---|
| 4 in CMU | each | $1.20 – $2.40 |
| 6 in CMU | each | $1.50 – $3.00 |
| 8 in CMU (standard) | each | $1.75 – $3.50 |
| 12 in CMU | each | $3.00 – $6.00 |
| Mortar mix, 80 lbLays about 33 block | per bag | $8.00 – $14.00 |
| Grout, ready-mix deliveredMinimum load charge applies | per yd³ | $140 – $220 |
| #4 rebar, 20 ft | each | $9.00 – $18.00 |
| Masonry labour | per block laid | $2.50 – $6.00 |
Pre-blended Type N or S mortar needs only water. If you are mixing from Portland and lime you also need masonry sand — roughly a cubic yard per 7 bags of cement.
What goes wrong on this job
- Order 5% over, and order it all at once. Block from a second pallet run weeks later can be visibly a different shade, which does not matter below grade and matters enormously on a garden wall.
- Dry-lay the first course before you mix anything. It is the only cheap chance to find out that the wall wants 24 blocks and a 4-inch sliver, and to move a corner rather than cut every course.
- Do not wet the block. Concrete masonry is not brick — soaking it makes it shrink as it dries and cracks the joints. Brick gets wetted; block gets kept dry.
- Grout and mortar are different products and are not interchangeable. Grout is a fluid, high-slump concrete that has to flow down a core; mortar is stiff by design so it holds a block up. Pouring mortar into cores leaves voids you cannot see and cannot fix.
- If any core is getting grouted, place the vertical bar before you start laying and thread the block over it. Trying to drop a 9-foot bar down a finished wall is a two-person job that usually ends in a broken face shell.
Frequently asked questions
How many blocks per square foot?
1.125 for standard 8 × 16 inch face block, whatever its thickness. Multiply your wall area by 1.125 and add 5% for breakage.
How many blocks are in a 100 square foot wall?
About 113 before waste, or 119 with a 5% allowance. That takes roughly 3.4 bags of mortar.
How many bags of mortar per 100 blocks?
About 3 bags of 80 lb pre-blended mix for 8-inch block. Thinner block needs a little less, 12-inch needs about 4.
How much grout do I need to fill block cores?
Roughly 1.05 cubic yards per 100 square feet of 8-inch wall with every core filled. Cores at 32 inches on centre take about half that; at 48 inches, about a third.
How many courses is an 8 ft block wall?
Twelve. A nominal course is 8 inches including the mortar joint, so 96 ÷ 8 = 12 exactly.