Concrete Block Calculator
Enter your wall length and height and the block size. You'll get the block count with 5% for waste, plus an estimate of the mortar bags needed to lay them.
Results
How the concrete block calculator works
A standard concrete block (CMU) has a 16 × 8 inch face. With a 3/8" mortar joint that works out to almost exactly 1.125 blocks per square foot of wall — a number that holds whether the block is 4, 6, 8, 10 or 12 inches thick, because only the face size sets the count. Multiply by your wall area and add 5% for waste.
Formula: wall area (sq ft) × 1.125 × 1.05 = blocks
| Wall size | Area | Blocks |
|---|---|---|
| 10 × 8 ft | 80 sq ft | 95 |
| 20 × 8 ft | 160 sq ft | 189 |
| 40 × 8 ft | 320 sq ft | 378 |
Mortar, sand and reinforcement
Mortar depends on how you bed the block. With face-shell bedding (mortar only on the outer edges, the usual method) budget about 3 bags of premixed mortar per 100 blocks; full bedding roughly doubles that. If you mix your own, one bag of masonry cement plus sand lays a similar number.
- The first course sets the whole wall — lay it on a level, cured concrete footing and check it obsessively.
- Structural walls usually need vertical rebar grouted into the cores at set spacings, plus horizontal joint reinforcement — check your local code.
- Buy a few extra blocks: colours and batches vary, and a cracked block mid-course is easier to swap from your own stack than to match later.
Frequently asked questions
How many concrete blocks do I need per square foot?
For standard 16 × 8 inch blocks, 1.125 blocks per square foot of wall. This is the same for 4", 6", 8", 10" and 12" thick block because they all share the same face size.
How many blocks are in a 20x8 ft wall?
A 20 × 8 ft wall is 160 square feet, which needs about 189 standard blocks including 5% waste, plus roughly 6 bags of premixed mortar for face-shell bedding.
How much mortar do I need for concrete block?
About 3 bags of premixed mortar per 100 blocks with face-shell bedding — the common method. Full bedding (mortar across the whole block) uses closer to 6 bags per 100.
Are cinder blocks and concrete blocks the same for estimating?
For counting purposes, yes — both use the standard 16 × 8 inch face, so 1.125 per square foot applies. True cinder blocks (made with coal cinders) are largely a thing of the past; almost all "cinder blocks" sold today are concrete masonry units.