Concrete blocks, often called cinder blocks or concrete masonry units (CMUs), are sold by the piece, so the calculator turns wall area into a block count. A standard 8x16 block covers 0.889 sq ft of wall face, including its joint. Mortar comes as pre-mixed bags, and each bag lays a set number of blocks.
How to use this calculator
- Measure the wall's length along its face, in feet, and its height from the bottom course to the top. For a stepped or sloped wall, run each section as its own wall and add the blocks.
- Measure each door and window in feet, multiply width by height, and add the results. Type that total under More options, in the openings field, which asks for square feet.
- Leave the waste at 5 percent, the masonry association's allowance for cuts, chips and breakage. Raise it for a wall with many corners or with openings that do not land on full blocks.
- Under More options, check the extra mortar for waste. The default, 15 percent, is the low end of the 15 to 25 percent range in masonry guides. Raise it for a first-time job, or in hot or windy weather.
- Compare the Blocks result with Blocks in the wall. The gap is the waste you chose. Order the blocks and the mortar together, so the job does not stall while you wait.
Worked examples
Backyard garden wall
Wall: 30 × 3 ft
- Blocks
- 107
- 80 lb bags of mortar mix
- 10
- Wall area
- 90 sq ft
- Blocks in the wall
- 102 blocks
Shed wall with a door
Wall: 12 × 8 ft · Openings to subtract: 21 sq ft
- Blocks
- 89
- 80 lb bags of mortar mix
- 9
- Wall area
- 75 sq ft
- Blocks in the wall
- 85 blocks
Wall with windows
Wall: 24 × 8 ft · Openings to subtract: 40 sq ft · Extra for waste: 10%
- Blocks
- 189
- 80 lb bags of mortar mix
- 17
- Wall area
- 152 sq ft
- Blocks in the wall
- 171 blocks
Patio wall with extra mortar
Wall: 20 × 4 ft · Extra mortar for waste: 25%
- Blocks
- 95
- 80 lb bags of mortar mix
- 10
- Wall area
- 80 sq ft
- Blocks in the wall
- 90 blocks
Blocks to buy for common wall sizes at a standard and a higher waste allowance
| Wall | Extra for waste: 5% | Extra for waste: 10% |
|---|---|---|
| 12 × 8 ft | 114 | 119 |
| 20 × 4 ft | 95 | 99 |
| 30 × 6 ft | 213 | 223 |
| 40 × 4 ft | 189 | 198 |
Each cell: Blocks. Inputs not in the table use the calculator's starting values.
How the block count is built
The calculator multiplies the wall's length by its height, subtracts any openings, and divides what is left by the face one block covers. That gives the blocks in the wall. It then adds the waste and rounds up, since a supplier will not sell part of a block.
The masonry association's 5 percent is a starting point. It covers cuts at corners and ends, plus blocks broken in transit or on site. A wall with many corners or openings is likely to need more.
Thickness does not change the count, because the face of a block is the same whatever its depth. Mortar works differently, as the next section explains.
Mortar and the bags you need
The bag count divides the block count, plus mortar waste, by the number of blocks one bag lays. Mortar makers' data sheets list 12 blocks laid by one bag, within a range of 12 to 13 blocks. The calculator uses the lower number, which gives more bags.
Mortar lost to stiffening, spills and leftover mix is what the mortar waste covers. For a different block size, check the product label. For a brick wall, use the mortar calculator, which handles both units.
Reinforcing, footings and retaining walls
Reinforcing steel, grout-filled cores, bond beams and footings are structural. The building code or an engineer decides whether each one is needed, and this calculator sizes none of them. Retaining walls above a modest height also need an engineer.
A block planter that holds back soil works like a retaining wall. Ask the building department about its height and whether a permit applies, then work out the fill with the raised garden bed soil calculator.
If the wall will get a brick face, count that layer with the brick calculator. Keep those totals apart from the block totals, because the units differ.
Mistakes to avoid
- Ordering only the blocks in the wall. That figure has no waste, and the cut blocks at the ends and around openings come out of the extra.
- Keeping the default waste on a busy layout. Corners and short ends use far more blocks than a straight run.
- Using the block count to judge the design. It tells you what to buy, not whether a wall is safe to build.
Common questions
How do I turn wall area into a block count?
Divide the wall's area in square feet, with openings subtracted, by 0.889 sq ft. Then add the waste and round up. Doing that sum by hand is a useful check on the calculator.
How many bags of mortar do I need for concrete blocks?
The calculator divides the block count, plus the default mortar waste of 15 percent, by the blocks one bag lays, which is 12 blocks. Bag yields vary by product, so read the label before you order.
Do concrete block walls need rebar?
That depends on the wall and the building code. The code or an engineer decides on reinforcing steel, grout-filled cores and bond beams. This calculator counts only blocks and mortar, so ask the building department before you build.
How many concrete blocks do I need for a hundred square feet of wall?
As a rule of thumb, plan on 113 blocks for every hundred square feet of wall. That comes from each standard block covering 0.889 sq ft of wall face. Add the waste allowance of 5 percent on top, or let the calculator do it.
How much mortar do I need for a hundred square feet of block wall?
A rough figure is 8.5 cubic feet of mortar for every hundred square feet of block wall, and that already includes waste. The calculator's bag count is the easier number to order from.