How It Is Calculated
The calculator first finds the net wall area by subtracting the area of openings from the total wall area. Then it divides that net area by the face area of one standard block (8/9 ft²) to get the number of blocks needed before waste. The waste factor is applied to both the block count and the concrete volume. Concrete volume is based on the block width and the core fill percentage, using standard fill volumes per block.
W = wall length (ft), H = wall height (ft), O = openings area (ft²)
Net area = W × H − O
Blocks before waste = Net area × 9/8
Blocks with waste = Blocks before waste × (1 + F/100), rounded up
Concrete per block (ft³): 6 in → 0.15, 8 in → 0.20, 12 in → 0.30
Concrete volume (ft³) = Blocks before waste × (C/100) × Concrete per block × (1 + F/100)
Concrete volume (yd³) = Concrete volume (ft³) / 27
Where F = waste factor (%) and C = core fill percentage (%).
Assumptions
- Blocks have nominal dimensions of 8 inches high by 16 inches long, giving a face area of 8/9 ft² per block.
- Concrete core fill volumes for full grout are based on industry averages: 0.15 ft³ for 6-inch blocks, 0.20 ft³ for 8-inch blocks, and 0.30 ft³ for 12-inch blocks.
- The waste factor applies to both block count and concrete volume.
- Openings are subtracted from the gross wall area before calculating block count.
- The core fill percentage is applied uniformly to all blocks.
Worked Example
Suppose you have a wall that is 10 ft long and 8 ft high, with no openings. You are using 8-inch wide blocks, filling 100% of the cores, and allowing 10% waste.
Net area = 10 ft × 8 ft − 0 ft² = 80 ft²
Blocks before waste = 80 × 9/8 = 90 blocks
Blocks with waste = 90 × (1 + 10/100) = 99, rounded up to 100 blocks
Concrete per block (8 in) = 0.20 ft³
Concrete volume (ft³) = 90 × (100/100) × 0.20 × (1 + 10/100) = 90 × 0.20 × 1.1 = 19.8 ft³
Concrete volume (yd³) = 19.8 / 27 = 0.73 yd³
Results: Number of blocks = 100 blocks, Concrete volume = 19.8 ft³, Concrete volume = 0.73 yd³.
Practical Tips
- Always measure wall length and height accurately, and subtract the area of all doors, windows, and other openings.
- Consider ordering a few extra blocks beyond the waste factor to account for cuts and future repairs.
- If you are also building a foundation, use the Concrete foundation calculator to estimate the concrete needed for footings.
- For fence posts, the Concrete calculator for fence posts can help you determine the concrete required for post holes.
- If you are working on a concrete slab, the Concrete slab calculator provides estimates for slab volume.
- Double-check your core fill percentage: 100% for full grout, 50% for every other cell, etc.
- Use the waste factor to account for breakage, spillage, and irregular cuts.
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