Concrete Bag Calculator

How many 40–90 lb bags of concrete for a slab or pad.

How it’s calculated

Cubic feet = length × width × thickness (in) ÷ 12 × (1 + waste%). Bags = cubic feet ÷ bag yield, rounded up.

Yields from the QUIKRETE Concrete Mix (No. 1101) data sheet: 40 lb ≈ 0.30 cu ft, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60, 90 lb ≈ 0.675 (Quikrete data sheet; 45 × 80 lb bags ≈ 1 cu yd per product listing). Other brands may differ — check the bag.

Worked examples

10×10 ft patio, 4 in thick, 80 lb bags: 100 × 4 ÷ 12 = 33.3 cu ft; ÷ 0.60 = 55.6 → 56 bags; with 10% waste 36.7 cu ft → 62 bags.

Check: 1 cu yd (27 cu ft) ÷ 0.60 = 45 bags, matching Quikrete’s figure.

Waste factor

Uneven subgrade, form bulging and spillage eat concrete. 5–10% extra is a common contractor cushion (rule of thumb, not a manufacturer figure). Above roughly 1 cubic yard, ready-mix delivery is often more practical than bags.

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