Concrete Bags Calculator: How Many Bags Do You Need?

A 10×10 slab at 4 inches deep is 33.3 cubic feet. That’s 42 bags of 80 lb concrete — not 20, not 30. Concrete bag math is where home improvement projects go wrong most often because people underestimate volume and don’t account for bag yield.

Bag Yield by Weight

Bag Weight

Cubic Feet Yield

Bags per Cubic Yard

40 lb

~0.30 cu ft

~90 bags

60 lb

~0.45 cu ft

~60 bags

80 lb

~0.60 cu ft

~45 bags

80 lb bags are the most efficient for large pours — fewer bags to mix, carry, and dispose of. 40 lb bags are easier to handle but require 2–3× more trips. For most residential work (patios, footings, fence posts), 80 lb bags strike the right balance.

Slab vs. Footing vs. Post Holes

Slab: Multiply length × width × thickness (convert inches to feet). A 10×10 slab at 4“ = 10 × 10 × (4/12) = 33.3 cu ft.

Footing: Same math — length × width × depth. A footing that’s 18“ wide × 12“ deep and 20 feet long = 1.5 × 1.0 × 20 = 30 cu ft.

Post holes: Volume of a cylinder = π × r² × h. A 10-inch diameter hole × 24 inches deep = π × (0.417)² × 2 = 1.09 cu ft per hole. Four holes = 4.36 cu ft, about 8 bags of 80 lb.

Always Over-Order by 5–10%

Running out of concrete mid-pour is a serious problem — the already-poured section can begin to set before you return with more, creating a cold joint that weakens the finished slab. Order 5–10% extra and mix it anyway; a small overage is easy to dispose of, but a cold joint requires tearing out and re-pouring.

Structural Footings Are Different

This calculator is designed for general estimates — patios, posts, garden walls, shed slabs. Load-bearing footings for structures (decks, additions, retaining walls) require engineering review and may specify a minimum compressive strength (psi) that not all bag mixes meet. Consult a licensed contractor or structural engineer for anything load-bearing.

Frequently Asked Questions

How many 80 lb bags of concrete do I need for a 10x10 slab?

A 10×10 slab at 4-inch thickness is 33.3 cubic feet. At 0.60 cu ft per 80 lb bag, you need 56 bags. At 6-inch thickness (common for driveways), you'd need 84 bags. Always add 5–10% extra for waste and to avoid running short mid-pour.

What's the difference between 60 lb and 80 lb concrete bags?

Both use the same concrete mix — the difference is handling weight. An 80 lb bag yields 0.60 cu ft vs 0.45 cu ft for 60 lb. For large projects, 80 lb bags mean fewer bags to buy, carry, and mix. For smaller jobs or solo work without a helper, 60 lb bags are easier to manage.

How long does a bag of concrete take to set?

Bagged concrete products like Quikrete begin to set within 20–40 minutes of mixing and reach initial set (firm to the touch) in 1–2 hours. Full cure takes 28 days, though the concrete reaches most of its strength within 7 days. High-strength mixes and fast-setting varieties shorten these times.

Can I use bagged concrete for structural footings?

Pre-mixed concrete bags can be used for residential footings, but load-bearing applications (decks, additions, retaining walls) may require a specific compressive strength (typically 3,000–4,000 psi). Check local building codes and consult a structural engineer or licensed contractor before mixing your own concrete for any load-bearing foundation.