concrete · 4 min read
How to Estimate Concrete for a Slab
Cubic yards = area × thickness ÷ 27. Add 10% waste. Over ~1 cubic yard, switch from bags to ready-mix.
The math
Thickness in feet = thickness (inches) ÷ 12.
Cubic feet = length × width × thickness (ft).
Cubic yards = cubic feet ÷ 27.
Order 5–10% extra to avoid a short pour.
Bag vs ready-mix
80 lb bag ≈ 0.60 cu ft. 60 lb bag ≈ 0.45 cu ft. 40 lb bag ≈ 0.30 cu ft.
About 45 × 80 lb bags per cubic yard. Anything larger than ~1 yard is cheaper and faster as ready-mix.
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These results are estimates only. Confirm quantities, compaction, waste, and delivery requirements with your supplier or project professional before ordering materials.