Concrete Block Calculator
Wall square footage, block count, waste, and mortar estimate for standard 8×8×16 block.
Concrete Block (CMU) Calculator
Enter your wall dimensions and block size to get total block count, waste, and mortar estimate.
Drag to rotate · scroll to zoom · visual only, not used for the math
How We Calculate Exact Volume
Built on standard volume formulas, with a waste factor you control.
Inside-Face Dimensions
Always measure inside form board to inside form board. Check depth at multiple points to verify sub-base grade.
Standard Cubic Yard Formula
(L × W × (D ÷ 12)) ÷ 27 converts your dimensions into net cubic yards before any safety factor.
Waste & Spillage Factoring
We add a 5–20% contingency you control. Uncompacted base and form flex both eat into your pour.
How many concrete blocks the opening wall needs
The form opens on a 20 ft by 4 ft wall and a 16 by 8 inch block face. Wall area is 80.0 square feet. Block face area is 16 × 8 ÷ 144, about 0.89 square feet, so the count is exactly 90 blocks before waste. With the 10% waste already filled in, the card prints 99 blocks. Mortar is the field rule of one 80 lb bag of Type S per 33 blocks, rounded up: 3 bags. Cinder block, CMU, and concrete block use this same face count. This form does not calculate grout or concrete to fill the cores.
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Frequently Asked Concrete Questions
Direct reference answers compiled for job-site use.
A nominal 8×8×16 inch block covers 0.89 sq ft of wall face (16 in × 8 in, which already includes the 3/8 inch mortar joint). Divide your wall's square footage by the block's face area to get a base count, then add waste.
A rough field rule is about 3 bags of Type S mortar mix per 100 blocks, or roughly 33 blocks per 80lb bag. Always round up and keep extra on hand — running out mid-course is worse than a little leftover mix.
10% is standard for straight runs. Increase to 15% for walls with lots of cuts, corners, or openings for windows and doors.