Concrete Block Calculator – How Many CMU Blocks Do I Need?
Calculate concrete blocks (CMU) needed for walls, foundations, and structures
CMU Block Calculator
Most common size
Each door = 21 sq ft (3x7 ft)
Each window = 15 sq ft (3x5 ft)
⚡ Quick Answer
A standard 8×8×16 concrete block covers approximately 0.89 square feet (including mortar joints). For a wall with standard 3/8" mortar joints, you need about 1.125 blocks per square foot.
Building a concrete block wall, foundation, or retaining structure? This calculator helps you determine exactly how many CMU (Concrete Masonry Units) blocks you need, accounting for different block sizes, mortar joint thickness, and openings like doors and windows.
Dr. Snezana Lawrence
Mathematical Historian
15+ years experience
PhD from Yale University. Published mathematical historian ensuring precision in all calculations.
Education
PhD in Mathematical History - Yale University
Standard Concrete Block Sizes
| Block Size | Actual Dimensions | Coverage | Blocks/sq ft |
|---|---|---|---|
| 8×8×16 (Standard) | 7.625×7.625×15.625" | 0.89 sq ft | 1.125 |
| 10×8×16 | 9.625×7.625×15.625" | 1.11 sq ft | 0.9 |
| 12×8×16 | 11.625×7.625×15.625" | 1.33 sq ft | 0.75 |
| 6×8×16 | 5.625×7.625×15.625" | 0.67 sq ft | 1.5 |
| 4×8×16 (Half) | 3.625×7.625×15.625" | 0.44 sq ft | 2.25 |
How to Calculate Concrete Blocks
(Wall Area - Openings) × Blocks per Square Foot = Total Blocks
Example: 20 ft × 8 ft wall with 1 door
- • Wall area: 20 × 8 = 160 sq ft
- • Minus door: 160 - 21 = 139 sq ft
- • For 8×8×16 blocks: 139 × 1.125 = 156.4 blocks
- • Add 10% waste: 156.4 × 1.10 = 172 blocks
- • Round up to 172 blocks
Mortar Requirements
| Wall Area | 80lb Bags |
|---|---|
| 100 sq ft | 12 bags |
| 200 sq ft | 24 bags |
| 300 sq ft | 36 bags |
| 500 sq ft | 60 bags |
Rule of Thumb
- •3 bags of mortar per 100 blocks (8×8×16 size)
- •Each 80lb bag lays approximately 30-35 blocks
- •Add 10% extra for waste and repairs
Frequently Asked Questions
How many concrete blocks do I need for a 100 square foot wall?
For 8×8×16 blocks: 100 × 1.125 = 112.5 blocks. Add 10% waste = 124 blocks. For 12×8×16 blocks: 100 × 0.75 = 75 + 10% = 83 blocks.
What is a CMU block?
CMU stands for Concrete Masonry Unit. It's the technical term for concrete blocks, cinder blocks, or breeze blocks used in construction.
How much does a concrete block weigh?
Standard 8×8×16 hollow block weighs 38-40 lbs. Solid blocks weigh 50-55 lbs. 12×8×16 blocks weigh 50-55 lbs hollow, 65-75 lbs solid.
How many bags of mortar do I need per 100 blocks?
You need approximately 9-12 bags of 80lb mortar mix per 100 standard blocks, depending on joint thickness and block size.
Should I subtract door and window openings?
Yes, subtract openings. Standard door = 21 sq ft (3×7 ft), standard window = 15 sq ft. This prevents overordering.
What's better - 8 inch or 12 inch concrete blocks?
8-inch blocks for non-load bearing walls, partitions. 12-inch blocks for structural walls, foundations, retaining walls, or where extra strength is needed.
Dr. Snezana Lawrence
Mathematical Historian | PhD from Yale
Dr. Lawrence is a published mathematical historian with a PhD from Yale University. She ensures mathematical precision and accuracy in all our calculations, conversions, and academic score calculators. Her expertise spans computational mathematics and educational assessment.
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