Concrete Block Calculator: Blocks, Courses and Mortar
Enter the wall size and the openings you will leave, choose the block size, and get the block count, the courses and the mortar. After switching units, re-enter your measurements.
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Formula
net area = length x height - openingsblocks = round up (net area / 0.8889 ft2 x (1 + waste %))(nominal face 16 x 8 in)courses = round up (height / 8 in),blocks per course = round up (length / 16 in)mortar = block count (waste included) x 0.07 ft3for 8x8x16 units, scaled by thickness (4 in solid: 0.09 ft3)mortar bags = round up (mortar ft3 / 0.6)
Worked example
A wall 20 ft long and 8 ft high with no openings, 8x8x16 blocks and 5% waste:
- Area 20 x 8 = 160 ft2; 160 / 0.8889 = 180 blocks; x 1.05 = 189 blocks
- Courses: 96 in / 8 in = 12; per course: 240 in / 16 in = 15
- Mortar: 189 x 0.07 = 13.23 ft3; 13.23 / 0.6 = 22.05, so 23 bags of 80 lb (about 22 before rounding up)
Assumptions
- Nominal block face 16 x 8 in and 3/8 in joints.
- Practical NCMA-style mortar quantities: about 0.07 ft3 per 8x8x16 unit with face-shell bedding (scaled by thickness for other sizes), about 0.09 ft3 for solid 4 in units, each already including the normal 25% loss. The waste input is applied on top through the block count.
- An 80 lb bag yields about 0.6 ft3. Real mortar usage varies by mason, joint tooling and weather.
- Grout for filled cores, rebar, lintels and corner or half blocks are not counted.
Frequently asked questions
How many concrete blocks do I need per square foot?
A standard 8x8x16 block has a nominal face of 16 x 8 inches, which is 0.889 square feet including the mortar joint. So you need 1 / 0.889 = 1.125 blocks per square foot, or about 112.5 blocks per 100 square feet.
What does 8x8x16 mean?
The numbers are the nominal width (thickness of the wall), height and length in inches. The real block is 3/8 inch smaller in each direction (7 5/8 x 7 5/8 x 15 5/8) so that a 3/8 inch mortar joint brings it to the nominal size. Width changes the wall thickness and the mortar, not the number of blocks per area.
How much mortar does a block wall need?
This calculator uses a practical rule of about 0.07 cubic feet of mortar per 8x8x16 block with face-shell bedding, which already includes the usual 25 percent loss (solid 4 inch blocks use about 0.09, other widths scale with thickness). For 189 blocks that is 13.23 cubic feet, and with an 80 lb bag making about 0.6 cubic feet you need about 22 bags, so 23 when rounded up. Real usage varies by mason.
How do I count the courses?
Each course is 8 inches high including the joint. Divide the wall height by 8 inches and round up: an 8 foot wall is 96 / 8 = 12 courses. The last course may need cutting if the height is not a multiple of 8 inches.
Should I subtract doors and windows?
Yes, enter their total area as openings. A 20 square foot opening on a 160 square foot wall leaves 140 square feet, or 157.5 blocks, so 158 before waste. Keep a few percent for cuts and breakage.