The formula
Volume (cu ft) = length × width × thickness (in) ÷ 12. Divide by 27 for cubic yards. For a round column, volume = π × radius² × depth.
Bag yields
A standard concrete mix bag yields about 0.30 cu ft (40 lb), 0.45 cu ft (60 lb) or 0.60 cu ft (80 lb) — as listed on the QUIKRETE® Concrete Mix data sheet. Always check the yield on the bag you buy.
How thick should a slab be?
- 4 in is common for patios and walkways.
- Driveways and slabs for vehicles are typically 5–6 in — follow your local code.
Frequently asked questions
How many bags of concrete for a 10×10 slab?
At 4 in thick, a 10×10 slab is about 33 cu ft (1.23 cu yd). With 10% extra that is about 62 bags of 80 lb mix.
Should I add extra?
Yes — 5–10% covers uneven subgrade and spills. Running short mid-pour is worse than leftover bags.
Sources
Last reviewed 2026-10-14. Estimates only — get written quotes from licensed local contractors before you decide.