Concrete Slab Calculator
Calculate concrete yards and bag estimates for a rectangular slab, patio, pad, or walkway.
Planning estimate only. Verify product specifications, field measurements, safety requirements, and applicable codes before buying or building.
Reviewed September 6, 2026Runs in your browser
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How this calculator works
Cubic feet = length × width × (thickness ÷ 12). Cubic yards = cubic feet ÷ 27. The selected waste allowance is added last.
Worked example
A 20 × 12 ft slab at 4 in thick with 10% extra requires about 3.26 cubic yards.
Sources & assumptions
Volume is length × width × thickness with an editable waste allowance. Bag yields follow QUIKRETE Concrete Mix product data; structural thickness and reinforcement require project-specific review.
Frequently asked questions
Is four inches thick enough?
Four inches is common for many light-duty slabs, but soil, reinforcement, loads, frost, and local code can require a different design.
Should I round ready-mix up?
Concrete is normally ordered in practical increments. Confirm the supplier’s minimum order and rounding policy.