How this calculator works
A rectangular slab of constant thickness has volume length × width × thickness. The bag count multiplies that volume by the waste allowance, divides by the printed yield of one bag and rounds up, because a partial bag cannot be bought. Order ready-mix by the cubic yard for slabs over about 1 yd³; bagged mix suits footings, pads and repairs.
Formula
V = L × W × (T ÷ 12) in ft³ with T in inches; yd³ = V ÷ 27; bags = ceil(V × (1 + waste%) ÷ yield).
Worked example
A 10 × 12 ft patio at 4 in thick with 10% waste: V = 40.0 ft³ (1.48 yd³), needing 74 eighty-pound bags, or 67 without waste.
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Frequently asked questions
Why round the bag count up?
Stores sell whole bags. Rounding 73.3 bags up to 74 also leaves a working surplus, which beats running short mid-pour.
How do 25 kg or 30 kg Canadian bags compare?
Yield scales roughly with mass: a 30 kg bag yields about 0.50 ft³ versus 0.60 ft³ for an 80 lb (36.3 kg) bag. The printed yield on the bag wins.
Does the waste allowance change the reported volume?
No. Waste is added only to the bag count. The volume and cubic yardage describe the measured slab before waste.