Concrete Block Calculator
Enter the wall and this gives you the blocks, the courses it runs to and the mortar. A standard block plus its joint is exactly a 16 by 8 inch module, which is where the familiar figure of nine blocks per eight square feet comes from.
Blocks needed
189blocks
- Wall area
- 160.0 sq ft
- Blocks per sq ft
- 1.13
- Courses high
- 12
- Mortar
- 6 bags
- Includes a 5% waste allowance. The bare figure is 160.00 sq ft.
- A standard block measures 15 5/8 by 7 5/8 inches so that it lays out on a 16 by 8 inch module once the mortar joint is added — which is why the answer comes to almost exactly nine blocks for every eight square feet of wall.
- Mortar is worked out at about 33 blocks to an 80lb bag. Reinforced walls need grout in the cores on top of that, and rebar to your local code.
About the concrete block calculator
Concrete block is easier to count than brick because the sizing is deliberate. A standard block measures 15 5/8 by 7 5/8 inches, which is not an accident — add a three-eighths inch mortar joint and it lays out on exactly a 16 by 8 inch module. That works out at 1.125 blocks a square foot, so a hundred square feet of wall takes 113 blocks.
Courses are worth checking before you start rather than after. A course is eight inches with the joint, so wall heights that are a multiple of eight inches finish on a whole course. A wall designed to an awkward height means cutting every block in the top course, which is slow and looks it.
What this does not cover is the reinforcement. A retaining wall or anything carrying load needs rebar and grouted cores to a specification set by your local code, and that is a structural question rather than a counting one.
What it works out
- Block count for any block size and joint
- How many courses the wall runs to
- Mortar bags at the trade yield
- Doors and windows deducted
The formula
Blocks = (Wall area × (1 + Waste)) ÷ ((Block length + Joint) × (Block height + Joint))
A block occupies its own size plus one mortar joint in each direction, because the joint on the far side belongs to the next block along and the course above.
Standard block is sized so that this comes out round. The block measures 15 5/8 by 7 5/8 inches and the joint is three eighths, so the module is exactly 16 by 8 — 128 square inches, or 1.125 blocks in every square foot. Nine blocks cover eight square feet of wall.
Courses come from the same module. A course is the block height plus one joint, which is eight inches, so a wall height in whole feet finishes on a whole course only when the height is a multiple of eight inches. An eight-foot wall is twelve courses exactly.
Five per cent is the usual waste allowance. Block chips at the corners in handling but breaks far less often than brick, and a chipped block can still go into the middle of a wall.
- Wall area
- Length times height, less doors and windows.
- Block length
- The block as it measures. A "16 inch" block is 15 5/8 inches long.
- Block height
- The face height. An "8 inch" block is 7 5/8 inches high.
- Joint
- The mortar bed, three eighths of an inch as standard.
- Waste
- Extra for breakage and cuts.
A worked example
A twenty by eight foot garage wall in standard block at a three-eighths inch joint, with five per cent for breakage.
That works out to 189 blocks.
- Wall area
- 160.0 sq ft
- Blocks per sq ft
- 1.13
- Courses high
- 12
- Mortar
- 6 bags
Questions
How many concrete blocks do I need for 100 square feet?
113 blocks, at the standard 1.125 blocks a square foot. Add your waste allowance on top — at five per cent that comes to 119.
How many blocks are in a square foot?
1.125 for a standard block at a three-eighths joint, which is the same as saying nine blocks cover eight square feet. The figure is that round because the block is sized to lay out on a 16 by 8 inch module once the mortar is added.
Why is a 16 inch block only 15 5/8 inches?
So that it works out at sixteen inches once the mortar joint is on it. The name is the module the block lays out on, not the block. The same convention runs through masonry and it is why nominal and actual sizes differ.
How many courses high will my wall be?
The wall height divided by eight inches, since a block and its joint make an eight-inch course. An eight-foot wall is twelve courses; a ten-foot wall is fifteen. Heights that are not a multiple of eight inches mean cutting the whole top course.
How much mortar does block take?
About one 80lb bag for every 33 blocks. A hundred square feet of wall is 113 blocks and takes four bags. Buy an extra — running out mid-wall means raking out mortar that has started to set before you can carry on.
Do I need to fill the cores with concrete?
It depends what the wall is doing. A non-structural garden or partition wall usually does not. Anything retaining soil or carrying load needs grouted cores and rebar to a specification your local code sets, and that is a structural question rather than a quantity one.
How many blocks are on a pallet?
Typically 90 to 108 for standard 8x8x16 block, depending on the supplier and whether they are solid or hollow. A pallet weighs well over a ton, so check access and where it can be set down before it arrives.
What about half blocks at the ends?
A running bond needs a half block at alternate courses on every end and corner. Most suppliers sell them, and buying them is faster and neater than cutting full blocks. Count two per course of wall end, and order them separately from the figure here.