Concrete Needed for a 20 × 24 Foot Slab (6 Inches Thick)

A 20 ft by 24 ft concrete slab at 6 inches thick requires approximately 9.78 cubic yards of concrete (including a standard 10% waste factor). This equals about 8.89 cubic yards before waste. The surface area of the slab is 480.00 square feet. Use our interactive Concrete Slab Calculator to customize your waste factor and bag size for a more precise estimate.

For a 20×24×6″ driveway, you will need 440 bags of 80lb concrete mix. If you prefer other bag sizes, you would need 587 bags of 60lb mix, 704 bags of 50lb mix, or 880 bags of 40lb mix. An 80lb bag yields approximately 0.6 cubic feet of mixed concrete. For small projects, bagged concrete is cost-effective; for larger pours, consider ready-mix delivery. Use our calculator to compare bag sizes and costs.

The formula is: Volume (cu yd) = (Length × Width × Thickness in feet) ÷ 27. For a 20×24 slab at 6 inches: (20 ft × 24 ft × 0.50 ft) ÷ 27 = 8.89 cubic yards. Add 10% for waste and subgrade irregularities: 9.78 cubic yards total.

For a 20×24×6″ driveway, ready-mix concrete typically costs $125 to $175 per cubic yard delivered, totaling $1,222.50 to $1,711.50 for concrete alone. Using 80lb bags at approximately $7.50 each, bagged concrete would cost about $3,300.00 plus tax. For projects under 2 cubic yards, bagged concrete is often cheaper since you avoid the ready-mix minimum delivery fee of $100 to $150.