Concrete Block Calculator
Calculate how many concrete blocks (CMU) you need for any wall. Get block count with 5% waste, courses, mortar bags, and cost estimate.
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Blocks Needed (5% waste)
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Courses —
Mortar Bags (70 lb) —
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Blocks Needed
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Courses —
Mortar Bags —
Wall Area —
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Block Quantities
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Total Blocks (all types) —
Mortar & Fill
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Mortar Volume —
Concrete Fill (yd³) —
Cost
Material Cost —
How to Use This Calculator
- Enter the wall length and wall height in feet.
- Select your block size (8×8×16 standard or 12×8×16 wide).
- Results show blocks needed with 5% waste, number of courses, and mortar bags.
Formula
Wall area = Length × Height
Block face area = 0.889 sq ft (both sizes)
Blocks = ceil(Wall area ÷ 0.889 × 1.05) | Courses = ceil(Height × 12 ÷ 8) | Mortar bags = ceil(Blocks ÷ 28)
Example
Example: 20 ft × 8 ft wall → 160 sq ft ÷ 0.889 × 1.05 = 189 blocks, 12 courses, 7 mortar bags.
Frequently Asked Questions
- Divide the wall area (length × height in sq ft) by the face area of one block. A standard 8×8×16 CMU has a face area of 8 in × 16 in = 128 sq in = 0.889 sq ft. Formula: Blocks = (Wall area ÷ 0.889) × 1.05 (including 5% waste). For example, a 20 ft × 8 ft wall = 160 sq ft ÷ 0.889 × 1.05 = 189 blocks. Blocks are also counted by courses: each course is 8 inches high, so an 8 ft wall = 12 courses. Multiply courses × blocks per row to verify your count. This calculator does both calculations automatically.
- One 70 lb bag of masonry mortar covers approximately 28 standard 8×8×16 CMU blocks. To find mortar bags needed: divide total block count by 28 and round up. For 189 blocks: 189 ÷ 28 = 6.75, so order 7 bags. Mortar coverage can vary based on joint thickness — standard CMU uses a 3/8-inch mortar joint. Thicker joints use more mortar. Pre-mixed mortar in bags is convenient; for large projects, mixing Type S mortar (one part Portland cement, one part lime, six parts masonry sand) from bulk materials is more economical.
- Each course of standard 8×8×16 CMU sits 8 inches tall after accounting for the mortar joint. Since 8 feet = 96 inches, you need 96 ÷ 8 = 12 courses to build an 8-foot wall. Each block is 7.625 inches tall (nominal 8 inches), and the 3/8-inch mortar joint makes up the difference to reach the nominal 8-inch module. The first course often sits on a footing without a mortar joint on the bottom, so plan your footing height carefully. To build to exactly 8 feet, set the footing top at the right elevation before laying the first course.
- The 8×8×16 CMU (8 inches wide, 8 inches tall, 16 inches long) is the standard residential block used for garden walls, basement walls, and above-grade construction. The 12×8×16 CMU (12 inches wide) is used for foundation walls, below-grade retaining structures, and load-bearing walls where greater structural depth is needed. The 12-inch block weighs about 55–60 lbs compared to 38–43 lbs for the 8-inch block, making it harder to handle manually. Both have the same face area (0.889 sq ft) and use the same mortar calculation. Structural applications should always be engineered.
- Yes — always add at least 5% extra to your block count to account for cracked or chipped blocks, corner cuts, and blocks that break during handling. For walls with many corners, openings, or irregular angles, use 8–10% waste. This calculator includes 5% waste by default in the results. When cutting blocks for corners or openings, a diamond blade wet saw or a masonry chisel is typically used. Having extra blocks from the same batch is also important for color and texture matching — blocks from different production runs can vary slightly.