Concrete Block Calculator
Estimate how many concrete blocks (CMUs) you need for a wall project, including waste and estimated cost.
This concrete block calculator turns your wall's length and height, plus your chosen block size, into a preliminary area-based block estimate. Enter your dimensions, pick a standard 8x8x16, 6x8x16 or 4x8x16 block (or enter a custom size), set a waste percentage for cuts and breakage, and optionally add a price per block to get a block-only cost estimate before you order.
Switching units converts wall dimensions and preserves the selected physical block, waste and price. Metric presets are exact conversions of inch-based blocks. For a local 20 x 40 cm face, choose custom and enter 20 cm height and 40 cm length.
Your Estimate
Wall Area
160.0 sq ft
Block Face Area
128.0 sq in
Blocks Needed
180 blocks
Extra Blocks for Waste
18 blocks
Total Blocks to Buy
198 blocks
Solid rectangular wall area estimate. Base blocks round up first; extra waste blocks round up separately and are added once. Openings, bond layout, corners, cuts and special units require a separate takeoff.
Results Actions
Sizes are width (wall thickness) × height × length; height × length controls the count. Use nominal modular face dimensions that include the intended mortar-joint allowance; do not add that allowance twice. Width does not change face coverage. Confirm product dimensions and joint layout with the supplier. Block cost excludes mortar, grout, reinforcement, labor, tax and delivery.
Common Concrete Block Sizes
Dimensions above are width × height × length. The 4-inch-thick block has the same 8 × 16 inch face as the standard block; the half-high example has a 4 × 16 inch face. Nominal dimensions differ from physical unit dimensions because they include the modular joint allowance. See CMHA’s concrete masonry unit dimension reference and confirm your supplier’s product.
How to Use the Concrete Block Calculator
- 1Choose Imperial or Metric measurements.
- 2Enter your wall length and wall height.
- 3Select a nominal block size (width × height × length), or enter a custom nominal face height and length. Width is wall thickness, not face height.
- 4Set a project-specific waste percentage. The default 10% is an example.
- 5Optionally enter a price per block to see an estimated total cost.
- 6Read your results instantly: wall area, block face area, blocks needed, waste, and total blocks to buy.
Example Calculation
Say you're building a 20 ft long, 8 ft high wall using standard 8x8x16 blocks, with a 10% waste allowance and blocks priced at $2.50 each. Wall area = 20 x 8 = 160 sq ft. Block face area = 8 in x 16 in = 128 sq in. Converting the wall area to square inches (160 sq ft x 144) = 23,040 sq in. Blocks needed = 23,040 ÷ 128 = 180 blocks. Waste at 10% = 18 blocks, for a total of 198 blocks to buy. Estimated block cost = 198 x $2.50 = $495.00. Switching to Metric converts the same wall to 6.096 m × 2.4384 m and the face to 20.32 cm × 40.64 cm, preserving 198 blocks. A true half-high 4 × 16 inch face needs 360 base blocks plus 36 waste blocks, or 396 total for this same wall.
Accuracy & Assumptions
- Results are estimates only and are meant for planning purposes.
- The area method assumes a solid rectangular wall; it does not model bond patterns, openings, corners, lintels or special units. Prepare a separate layout-based takeoff for those details.
- Actual block coursing may not divide evenly and could require additional cut blocks.
- Mortar, rebar, grout and wall ties are not included in this estimate and should be calculated separately.
- Verify all wall measurements and consult a professional for structural or load-bearing walls.
Frequently Asked Questions
How is the number of blocks calculated?
We calculate the total wall area from your wall length and height, then divide it by the face area of a single block (height x length). A waste percentage is added on top to account for cuts and breakage, giving a preliminary total to buy. Base blocks round up first, then extra waste blocks round up separately and are added once.
Does this account for mortar joints?
The presets use nominal modular dimensions, which include a joint allowance. For custom entries, use the intended nominal face height and length; do not add the joint allowance twice. Confirm the module and joint layout for your product.
What waste percentage should I use?
Choose waste from the project layout, cuts and expected breakage. The 10% default is an example, not a universal rule. The purchase total already includes that allowance.
Does this calculator subtract doors and windows automatically?
No. These inputs describe a solid rectangular wall. Prepare a separate net-area and coursing takeoff for openings, corners, lintels and special units; the calculator does not model those details.
What about mortar, rebar and grout?
This calculator only estimates the number of concrete blocks. Mortar, vertical rebar, horizontal reinforcement and grout for filled cores should be calculated separately based on your wall design.
This calculator provides an estimate for planning purposes only. Block sizes, mortar joint widths and wall designs vary by supplier and project — always confirm exact quantities with your mason or supplier before ordering.