Concrete Bags for a Slab
A 50 lb bag yields 0.375 cubic feet, a 60 lb bag 0.45, an 80 lb bag 0.6. Slab volume divided by that yield, plus a waste allowance, is the number the yard will ask you for.
Form depth must match the thickness you pick, and the ground under it needs 3–4 in of compacted gravel, that volume is not concrete and is not in this math.
How the math works
- Volume = length × width × thickness ÷ 12, in cubic feet. A 10×10 slab at 4 in is 33.3 cu ft, 1.23 cubic yards.
- Bag yields (standard, verified against manufacturer labels): 50 lb bag = 0.375 cu ft, 60 lb = 0.45, 80 lb = 0.6. Roughly 81 lb of dry mix makes one cubic foot.
- Bag count = volume × (1 + waste%) ÷ yield, rounded up. At 4 in a 50 lb bag covers about 1.1 sq ft, an 80 lb bag about 1.8 sq ft.
- Ready-mix is priced per cubic foot delivered; a short mix under 1 cu ft carries a delivery minimum, so the bag route wins small and the truck wins above roughly 3 cu ft.
- Water: about 0.45 gal per 50 lb bag, roughly 3.5 gal per 80 lb bag. Too much water lowers strength, add it to the hole, not to the pile.
- Reinforcement is priced per square foot here, mesh for patios and walks, rebar for driveways and anything a vehicle parks on.
How many bags of concrete for a 10×10 slab 4 inches thick?
About 89 bags at 0.375 cu ft per 50 lb bag, roughly 56 at 80 lb. Order the waste allowance too, 10% more, so 98 bags of 50 lb or 62 of 80 lb.
Is 80 lb concrete cheaper than 50 lb bags?
Per cubic foot, usually yes, an 80 lb bag is 1.6× the material of a 50 lb bag and rarely costs 1.6× as much. The trade is handling, 80 lb bags are a two-person lift and mix in pairs, not one.
Is bagged concrete or a ready-mix truck cheaper?
Bags under about 3 cubic feet, truck above it. Bags run roughly $13–16 per cubic foot at 2026 shelf prices, delivered concrete is $12–18 per cubic foot with no hauling, no mixing, and no 60 aching backs.