Concrete Calculator: Cubic Yards & Bags

Calculate concrete for slabs, footings and pads in cubic yards, plus 40/60/80 lb bag counts with waste.

✓ Formula shown✓ No signupUpdated 2026-10-22
Before you buy

Most people under-order. Add the waste allowance shown above, and buy one extra bag or box for touch-ups. Compare current prices for concrete:

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Worked examples
SlabConcrete (no waste)80 lb bags (+10%)
10×10 ft, 4"1.23 yd³62 × 80 lb
12×12 ft, 4"1.78 yd³89 × 80 lb
20×20 ft, 4"4.94 yd³245 × 80 lb
10×20 ft, 6"3.7 yd³184 × 80 lb
24×24 ft, 4"7.11 yd³353 × 80 lb
How the calculation works (sources & assumptions)

Volume (yd³) = L × W × (thickness ÷ 12) ÷ 27. Bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³ (typical manufacturer figures). Ready-mix trucks usually require a minimum load, so pours above about 1 yd³ are often cheaper by truck than by bag.

FAQ

How many bags of concrete for a 10x10 slab?

At 4" thick it is 1.23 yd³ (33.3 ft³), about 62 bags of 80 lb including 10% waste. That is usually a case for ready-mix delivery.

How thick should a concrete patio be?

4 inches is standard for patios and walkways; 5–6 inches for driveways carrying vehicles.

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