Quantity · Concrete block (CMU)
Concrete Block Calculator
Count standard CMU for a wall from length and height, subtract openings, add waste, and get a mortar-bag estimate — before you order from the yard.
Wall dimensions
Face coverage stays 0.888 sq ft for modular 8×16 faces; width changes wall thickness, not count.
Blocks to order
379
standard CMU · 8" × 8" × 16"
- Gross wall area
- 320 sq ft
- Net after openings
- 320 sq ft
- Raw block count
- 360.4
- With waste
- 378.4
Mortar mix
12 bags
≈ 3 bags of mortar mix per 100 blocks
Breakdown
- Net area 320 ÷ 0.888 sq ft/block = 360.4
- × (1 + 5% waste) = 378.4
- Round up to 379 whole blocks
Price data updated . National US ranges from published supplier and contractor averages. Not a bid. Local quotes vary.
How this concrete block calculator works
Modular concrete masonry units are sold by the piece, but you plan by wall face area. A nominal 8"×16" face with a standard mortar joint covers 0.888 square feet.
gross wall sq ft = length_ft × height_ft net wall sq ft = gross − openings blocks = net ÷ 0.888 × (1 + waste %) blocks to order = ceil(blocks) mortar bags = ceil(blocks_to_order ÷ 100 × 3)
Worked example. A 40 × 8 ft foundation wall is 320 sq ft. With no openings that is 320 ÷ 0.888 ≈ 360.4 blocks. Add 5% waste → 378.4, round up to 379 blocks. Mortar at 3 bags per 100 blocks → 12 bags. If you cut a 3 × 7 ft door (21 sq ft) out of that wall, net area drops to 299 sq ft and the order falls to about 354 blocks — still keep waste for the cut pieces around the opening.
Common CMU sizes and face coverage
Width is the thickness of the wall. Face module stays modular 8×16 for these stock units — that is why the block count does not change when you switch from 8" to 12" CMU of the same face.
| Nominal size | Face coverage | Typical use |
|---|---|---|
| 4" × 8" × 16" | 0.888 sq ft | Partition / veneer backup |
| 6" × 8" × 16" | 0.888 sq ft | Light exterior / interior walls |
| 8" × 8" × 16" | 0.888 sq ft | Standard foundation & wall CMU |
| 12" × 8" × 16" | 0.888 sq ft | Heavier foundation / retaining |
Concrete block calculator FAQ
- How many concrete blocks do I need per square foot?
- A standard modular 8×8×16 inch CMU covers about 0.888 square feet of wall face once the mortar joint is included. That is roughly 1.13 blocks per square foot before waste. Divide your net wall area by 0.888, then add 5% (or more for cut-heavy walls).
- Does this concrete block calculator include mortar?
- Yes. After rounding up the block count, the tool estimates mortar mix bags at about 3 bags per 100 blocks — a common rule of thumb for standard 3/8 inch joints. Specialty mortars, colored joints, and heavily tooled joints can use more.
- Should I subtract doors and windows?
- Yes. Enter the combined opening area in square feet. Leaving openings in the wall area over-orders blocks and understates how many half-blocks and cut pieces you will still need around openings — keep a little waste for those cuts.
- Do corner blocks and bond beams change the count?
- Corner, sash, and bond-beam units usually replace a standard stretcher one-for-one in the course count, so the face-area method still works for ordering total units. Order the specialty shapes as a subset of that total from your supplier’s list.
- Is an 8 inch block the same coverage as a 12 inch block?
- For modular 8×16 inch faces, yes — wall thickness (4, 6, 8, or 12 inch) changes structural width and weight, not how many blocks cover a given face area. Always confirm the face module with your supplier; some architectural units use different face sizes.
Need concrete for the footing or cores?
Block walls usually sit on a poured footing, and cells may be grouted. Now estimate what the pour will cost → · Bag calculator for small patches