Concrete Block Wall Calculator
Estimate the number of CMU blocks and costs for your project.
How to Calculate Concrete Blocks for a Wall
Building a retaining wall, a foundation, or a garden feature requires precise planning. The Concrete Masonry Unit (CMU), commonly known as a cinder block, is the standard building block for these projects. To ensure you don't run out of materials mid-build, you need to calculate the total surface area of your wall and divide it by the surface area of the blocks you've chosen.
The CMU Calculation Formula
The standard nominal size of a concrete block is 8 inches by 16 inches. This nominal size includes the standard 3/8-inch mortar joint. To calculate the number of blocks needed:
- Calculate Wall Area: Length (ft) × Height (ft) = Total Square Footage.
- Determine Block Area: A standard 8×16 block covers 0.889 square feet.
- Divide: Wall Area ÷ 0.889 = Total Blocks needed.
- Add Waste: Multiply by 1.10 (for 10% waste) to account for cuts and breakage.
Standard CMU Dimensions Table
| Nominal Size (W x H x L) | Actual Size | Sq. Ft. Coverage |
|---|---|---|
| 8″ x 8″ x 16″ | 7 5/8″ x 7 5/8″ x 15 5/8″ | 0.889 sq. ft. |
| 4″ x 8″ x 16″ | 3 5/8″ x 7 5/8″ x 15 5/8″ | 0.889 sq. ft. |
| 12″ x 8″ x 16″ | 11 5/8″ x 7 5/8″ x 15 5/8″ | 0.889 sq. ft. |
Real-World Example
Imagine you are building a garden wall that is 30 feet long and 4 feet high using standard 8×16 blocks.
- Step 1: 30 ft × 4 ft = 120 sq. ft.
- Step 2: 120 sq. ft. ÷ 0.889 = 135 blocks.
- Step 3 (Waste): 135 × 1.10 = 149 blocks (rounded up).
Essential Tips for Your Project
The Footing: Never build a concrete block wall directly on dirt. You need a concrete footing that is twice as wide as the block and deep enough to reach below the frost line in your area.
Mortar Ratios: Usually, you will need about 3 bags of mortar for every 100 blocks. Ensure you factor this into your budget along with rebar and grout if the wall is structural.