Calculate how many cubic yards of concrete you need for any slab. Enter length, width, and thickness to get cubic yards, cubic feet, and equivalent bag counts.
Level your forms perfectly before the pour — rotating laser levels save time and prevent costly errors.
Shop Express Tools →This concrete slab calculator uses the standard formula: length × width × thickness (converted to feet) ÷ 27 = cubic yards. A 20×30 ft slab at 4 inches thick requires approximately 7.4 cubic yards — a typical residential order. Ready-mix concrete is ordered by the cubic yard; bags are sold in 40, 60, or 80 lb units (a 60 lb bag yields about 0.45 cubic feet, or roughly 60 bags per cubic yard). Use this calculator for driveways, patios, garage floors, pole barn slabs, and footings. Level your forms precisely with a rotating laser level before the pour to ensure a flat finished surface — correcting high or low spots after concrete sets is costly.
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Start free trialA 20×30 ft slab at 4 inches thick requires approximately 7.41 cubic yards of concrete (20 × 30 × 0.333 ft ÷ 27). Always add 10% for waste, giving you about 8.15 CY to order.
4 inches is standard for residential driveways and patios. 6 inches for heavy vehicle traffic or commercial use. Footings are typically 8–12 inches. Your engineer or local code will specify requirements.
For slabs larger than about 1 cubic yard (roughly 10×10 at 4"), order ready-mix concrete. Bags are practical for small pours like footings, fence posts, and repair work. Ready-mix is more cost-effective and consistent for large pours.
Rotating laser levels like the Spectra HL760 or Topcon RL-H5A are used to set form heights and screed rails at a consistent elevation. This ensures a flat, level slab across the entire pour area.
A cubic yard of concrete weighs approximately 4,000 lbs (2 tons). This is important for structural loading calculations and delivery truck capacity.