How many cinder blocks do I need for a wall?
A standard 8x8x16 block lays at 16" x 8" of wall, so it takes 1.125 blocks a square foot — 90 for a 20' × 4' wall. Block counts for every common wall, with mortar.
A 20' × 4' wall takes 90 concrete blocks — 95 to order with 5% for breakage.
That is 6 courses of 15 blocks, and about 7 bags of premixed mortar at 13 blocks a bag. The same wall in modular brick is 540 bricks.
How that is worked out
- A block is made a mortar joint short of its nominal size, so laid with the joint it takes up 16" x 8" of wall: 128 sq in of face.
- 144 ÷ 128 = 1.125 blocks per square foot — the same 112.5 per 100 sq ft the masonry industry publishes.
- 20' × 4' = 80 sq ft, × 1.125 = 90 blocks, and 5% more is 95.
Blocks and bricks for a wall, 5% added for breakage
| Wall | Courses of block | Blocks | Blocks to order | Modular bricks |
|---|---|---|---|---|
| 8' × 2' | 3 | 18 | 19 | 108 |
| 10' × 4' | 6 | 45 | 48 | 270 |
| 20' × 4' | 6 | 90 | 95 | 540 |
| 20' × 6' | 9 | 135 | 142 | 810 |
| 40' × 4' | 6 | 180 | 189 | 1080 |
| 40' × 8' | 12 | 360 | 378 | 2160 |
Worth knowing
- "Cinder block" is what almost everybody calls it; what the yard sells now is concrete block, made with sand and gravel rather than cinders. The size and the count are the same.
- The count holds for 6", 8" and 12" thick block alike — thickness is not on the face. Corners and the ends of a wall want corner or end blocks with a finished face, so count those separately from the stretchers.
- Mortar yield is printed on the bag and varies with the product and the mason; 13 blocks a bag is only a typical figure for premixed mortar.
- Anything taller than a garden wall, or holding back soil, is a structural wall — the footing, the rebar and which cores are grouted come from the drawings or an engineer, not from a block count.
Work out your own numbers
- Concrete block and brick
Blocks or bricks for a wall with waste, the bags of mortar at your bag's own yield, and vertical rebar at your spacing.
- Concrete footings and slabs
Cubic yards for a run of post footings or a slab, and the bags of ready-mix that buys.
Related questions
- How many cinder blocks per square foot?
- 1.125, for a standard 8 × 8 × 16 block laid with its mortar joint — 112.5 per 100 sq ft of wall. Modular brick is 6.75 per square foot.
- How many blocks are in a 4 foot high wall?
- 6 courses, with 0.75 blocks per foot of length in each — so 4.5 blocks for every foot the wall runs. A 20' wall is 90.
- How many bags of mortar for 100 blocks?
- About 8 bags of premixed mortar at a typical 13 blocks a bag. The yield on your bag is the one to use — it depends on the product and how full the joints are buttered.