Concrete Block Calculator: Count CMU Blocks, Mortar and Cost
Block walls are counted two ways at once: the face area of the wall and the face area of one block plus its mortar joint. This concrete block calculator handles both. Enter the wall length and height, subtract doors and windows, pick your block size, and it returns the CMU count, mortar bags, and total cost. Useful for garden walls, garage walls, and foundation stem walls.
Masons use it to quote a job before the first course goes down. Homeowners use it before ordering a pallet. The block list covers six CMU sizes, from a 4-inch thin block to a 12-inch extra-wide. Mortar bags come from the same calculation, scaled by block face area instead of a flat per-block rule. Enter local prices and the total cost updates as you type.
How to Use the Concrete Block Calculator
Choose your currency from the top dropdown.
Enter the wall length and set the unit: ft, in, m, cm, mm, or yd.
Enter the wall height in whatever unit your tape measure reads.
Set the number of walls you’re building to the same height.
Pick a block type from the dropdown. 8″ × 8″ × 16″ standard CMU is the default.
Confirm the mortar joint thickness. Three-eighths of an inch is typical.
Enter total door and window area so the tool can subtract them.
Adjust the waste factor, then add your price per block and per mortar bag.
The Summary tab leads with the block count. Detailed Breakdown shows every intermediate figure, from block face area to mortar volume.
How the Concrete Block Calculator Formula Works
A block wall is a grid. Each block occupies its own cell plus half a mortar joint on each side. The calculator builds that grid mathematically and divides the wall into it.
Formula: Net wall area = (Length × Height × Wall count) − Door area − Window area
Formula: Block face area = ((Block length + Mortar joint) × (Block height + Mortar joint)) ÷ 144
Formula: Blocks needed = ⌈ Net wall area ÷ Block face area × (1 + Waste) ⌉
Formula: Mortar bags = ⌈ Blocks with waste ÷ (40 × (Block face area ÷ 0.4872)) ⌉
The 0.4872 in that last line is the face area of a standard 8″ × 8″ block with a 3/8″ joint, expressed in square feet. It’s the baseline the mortar coverage is calibrated against, so wider or taller blocks with larger faces consume more mortar per unit.
Every dimension gets converted to feet before multiplying. Inches convert at 1/12, meters at 3.28084, and yards at 3. Areas convert separately. Door and window openings can be entered in ft², m², or in², and the tool normalizes them before subtraction.
One thing the code does carefully: openings can never drive the net area below zero. If you accidentally type 600 ft² of windows on a 500 ft² wall, the tool clamps the result. It won’t return negative blocks.
Mortar bags come from an 80 lb reference bag covering 40 standard blocks at a 3/8″ joint. If your block face is larger, the coverage per bag drops, and the count rises. Smaller blocks go the other way.
Worked Example: A 28 ft × 9 ft Two-Wall Run
Say you’re building two exterior block walls, each 28 ft long and 9 ft tall, with one 21 ft² door opening and one 8 ft² window in the run. Standard 8″ × 8″ × 16″ CMU, 3/8″ joint, 10% waste. Blocks are $2.50 each, mortar is $8.50 per 80 lb bag.
Gross area: 28 × 9 × 2 = 504.00 ft²
Openings: 21 + 8 = 29.00 ft²
Net wall area: 504.00 − 29.00 = 475.00 ft²
Block face with joint: 16.375 × 8.375 = 137.14 in², or 0.9524 ft²
Blocks base: 475 ÷ 0.9524 = 498.7, rounded to 499
With 10% waste: 499 × 1.10 = 548.9, rounded to 549 blocks
Blocks per mortar bag: 40 × (0.9524 ÷ 0.4872) = 78.2
Mortar bags: 549 ÷ 78.2 = 7.02, rounded to 8 bags
Block cost: 549 × $2.50 = $1,372.50
Mortar cost: 8 × $8.50 = $68.00
Estimated total: $1,440.50
That total doesn’t count reinforcement, grout, or footing. Block walls need all three, and they belong on separate lines.
