Concrete Block Calculator — How Many CMU Blocks Do I Need?
Estimate how many concrete blocks and bags of mortar you need for your wall, including openings and waste factor.
How to Use This Calculator
- Enter the wall length and height in feet.
- Select your block size (standard 8×8×16 is default).
- Enter how many identical walls you're building.
- Optionally add up to three openings (doors or windows) by width and height.
- Adjust the waste factor (10% is typical) and click "Calculate".
How the Calculation Works
wall_area_sqft = (length × height × num_walls) - sum(opening_areas)
blocks_per_sqft = 1.125 // standard 8×8×16
total_blocks = ceil(wall_area_sqft × blocks_per_sqft × (1 + waste_factor))
mortar_bags = ceil(total_blocks / 33) // 1 bag per ~33 blocks The calculator finds the total wall area, subtracts the area of doors and windows, and multiplies by an average number of blocks per square foot. A waste factor is added, and mortar bags are estimated based on typical coverage per 80lb bag.
Variables:
- length: Length of a single wall in feet
- height: Height of the wall in feet
- num_walls: Number of identical walls
- opening_areas: Sum of door and window areas in square feet
- waste_factor: Extra percentage of blocks for cuts, breakage, and waste
Pro Tips
- Always add at least 10% extra blocks for breakage and cutting around openings.
- Stagger vertical joints (running bond) for a stronger wall.
- Check your local building code for required reinforcement and footing size.
- Use mason's line and line blocks to keep each course straight and level.
Recommended Block Wall Tools
Mason's Line and Line Blocks
Keep your block courses straight and level.
View on Amazon →Block Chisel and Hammer
Cut and trim concrete blocks cleanly.
View on Amazon →Mortar Mix (80lb bags)
Pre-mixed mortar for laying block walls.
View on Amazon →Mason's Trowel
Essential hand tool for spreading mortar.
View on Amazon →48-inch Spirit Level
Check that every course is perfectly level.
View on Amazon →Jointing Tool
Tool for finishing neat mortar joints.
View on Amazon →Frequently Asked Questions
How many concrete blocks are in a square foot?
A standard 8×8×16 CMU block uses about 1.125 blocks per square foot of wall area, including mortar joints.
How much mortar do I need for concrete blocks?
On average, one 80lb bag of mortar will lay about 33 standard 8×8×16 blocks. This calculator uses that rule of thumb.
Should I add extra blocks for waste?
Yes. It's common to add at least 10% extra blocks to cover breakage, cuts around openings, and future repairs.
Do I need rebar in a block wall?
Most structural block walls require vertical and sometimes horizontal reinforcement. Always follow your local building code or an engineer's design.