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Calculate the amount of concrete needed for slabs, footings, and walls. Get instant results in cubic yards and cubic feet, plus the number of concrete bags required.
Where L = length (ft), W = width (ft), T = thickness (in). Divide by 324 to convert to cubic yards.
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Calculating concrete volume for rectangular slabs is straightforward:
Length and width should be in feet, thickness in inches. The formula divides by 324 to convert directly to cubic yards.
Slab: 20 ft × 15 ft × 4 in thick
= (20 × 15 × 4) ÷ 324
= 1200 ÷ 324
= 3.70 cubic yards
| Thickness | Cubic Yards | 80 lb Bags |
|---|---|---|
| 2" | 0.62 | 28 |
| 3" | 0.93 | 42 |
| 4" | 1.23 | 56 |
| 5" | 1.54 | 70 |
| 6" | 1.85 | 84 |
Ready-mix concrete typically costs $100-$150 per cubic yard, depending on location, mix design, and delivery distance.
An 80 lb bag yields approximately 0.60 cubic feet. You need 45 bags to make one cubic yard.
4 inches for sidewalks and patios, 4-6 inches for driveways, 6 inches for garage floors, and 8+ inches for heavy equipment.
For projects over 1 cubic yard, ready-mix is more economical. Bags are best for small repairs and projects under 1 cubic yard.
Concrete reaches walking strength in 24-48 hours but takes 28 days to reach full strength.
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