A standard 8 × 8 × 16 in concrete block covers 0.889 sq ft of wall once the mortar joint is counted, so a wall takes 1.125 blocks per square foot, or 112.5 per 100 sq ft. Add 5% for breakage and you buy 119 blocks for every 100 sq ft. Cinder block, cement block, concrete block and CMU (concrete masonry unit) all count the same way.
How the block count is calculated
Following NCMA TEK 04-02A, the net wall area is divided by the face of one block. The face is the same for 4 to 12 in block, so the count does not change with the width.
blocks = (length × height − openings) ÷ 0.889 sq ft × (1 + waste)
Height is rounded up to whole 8 in courses. The count is then split into what you order: half blocks where every other course ends at a free end or jamb, bond beam blocks for the top course, and standard blocks for the rest.
Worked example: a 20 ft block wall, 4 ft high
The calculator opens on this wall: 20 ft long, 4 ft high, 8 in block, both ends free, a bond beam on top and rebar cells grouted every 48 in.
- 4 ft ÷ 8 in = 6 courses, and 20 ft × 4 ft = 80 sq ft of wall.
- 80 sq ft ÷ 0.889 = 90 blocks fill the wall.
- Half blocks: 2 free ends × 3 alternate courses = 6, plus 5% = 7 half blocks.
- Bond beam: 20 ft ÷ 16 in = 15, plus 5% = 16 bond beam blocks.
- Standard blocks: (90 − 3 − 15) × 1.05 = 75.6, so 76 blocks.
- Total to buy: 99 blocks.
- Mortar: 90 × 1.15 ÷ 13 blocks per bag = 8 bags of 80 lb Quikrete Mason Mix.
- Core fill: 80 sq ft × 6.1 cu ft per 100 sq ft = 4.88 cu ft in the rebar cells, plus 20 ft × 0.33 cu ft/ft = 6.6 cu ft in the bond beam. That is 11.48 cu ft, or 18 bags of 80 lb grout at 0.66 cu ft each.
- Vertical rebar: 240 in ÷ 48 in + 1 = 6 bars, each 4 ft long.
Block sizes: nominal and actual
Block is sold by its nominal size, which includes one 3/8 in mortar joint. The block itself is 3/8 in smaller in every direction (NCMA CMU-TEC-001), and the IRC sets bed and head joints at 3/8 in (R606.3.1).
| Nominal size | Actual size |
|---|---|
| 4 × 8 × 16 in | 3⅝ × 7⅝ × 15⅝ in |
| 6 × 8 × 16 in | 5⅝ × 7⅝ × 15⅝ in |
| 8 × 8 × 16 in | 7⅝ × 7⅝ × 15⅝ in |
| 10 × 8 × 16 in | 9⅝ × 7⅝ × 15⅝ in |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in |
Half blocks (8 × 8 in face) finish the ends of running bond. A corner block has one finished end and replaces a stretcher at every course of a corner, so it is counted inside the standard blocks.
Cinder blocks for a foundation
Switch to Foundation and enter the outside length and width. The walls overlap at the four corners, so the calculator lays block along the centerline: 2 × (length + width) − 4 × the block width. A 30 × 20 ft foundation in 8 in block runs 97 ft 4 in; at 8 ft high that is 12 courses, 876 blocks in the wall and 921 to buy with the bond beam and 5% waste, 78 bags of mortar, 25 vertical bars and 79.6 cu ft of grout. At that volume, order grout or concrete by the yard; the concrete footing calculator sizes the footing the wall sits on.
Cinder blocks for steps
In Steps mode each step is one 8 in course, and every course is as many block rows deep as the treads above it. Three steps, 4 ft wide with 16 in treads, take 3 blocks per row and 6, 4 and 2 rows: 36 blocks, 38 with waste, and 4 bags of mortar. Most block steps get a concrete cap or tread; the concrete steps calculator works out the concrete for a cast set instead.
Block wall by square feet
Square feet mode skips the layout: 100 sq ft is 112.5 blocks, 119 with waste, and 10 bags of mortar, without half or bond beam blocks.
Core fill, bond beams and rebar
NCMA TEK 04-02A Table 3 gives grout per 100 sq ft of wall by width and by how far apart the grouted cells are: 6.1 cu ft for 8 in block at 48 in, 36.1 cu ft when every core is filled. Filling every core of the example wall takes 28.9 cu ft, or 35.5 cu ft with the bond beam: about three times the partial fill. A bond beam (a U-shaped course with horizontal rebar) takes about 2.0 linear ft per 80 lb bag in 8 in block (Table 5). NCMA does not recommend grouting 4 in block, so the calculator turns core fill off for it.
The rebar spacing default of 48 in is the maximum the IRC allows in Seismic Design Category D0 and D1 (R606.12.3.2); your plans, engineer or building department set the real spacing and bar size. Count the bars and their length here, then work out the steel for the footing with the rebar calculator.
Mortar bags per block
Quikrete rates an 80 lb bag of Mason Mix or Mortar Mix at up to 13 blocks, Sakrete 12 for its Type S, and NCMA's field average is 16. The calculator adds 15% mortar waste, the low end of BIA's 15 to 25%. For Type N or S, or mixing your own, open the mortar calculator.
What to work out next
- A brick veneer or a brick cap on the wall: the brick calculator.
- Bags of concrete to fill cores or pour a cap: the concrete bag calculator.
- What the footing concrete will cost: the concrete cost calculator.
- Every block, brick and concrete tool in one place: concrete and masonry calculators.