BagsAndYards How much you need — and what it costs

Concrete Calculator (slab)

Enter your slab dimensions to get the volume, the exact number of bags (or cubic yards of ready-mix), an estimated cost, and which option makes sense.

How much concrete do I need?

ft
ft
in
%
You'll need about
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Bags (with overage)
Bags (no overage)
Ready-mix (delivered)
Estimated materials cost
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Estimate for standard concrete mix (e.g. Quikrete #1101 / Sakrete High-Strength). Bag counts round up and include your overage %. Cost is an estimate (as of June 2026, US national average) — confirm bag price and ready-mix delivery (often a 1 yd³ minimum + short-load fees) with your supplier. Not a substitute for a structural engineer on load-bearing work.

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How the concrete estimate is computed

The math is just volume, then a conversion to how concrete is sold:

  • Volume = length × width × thickness. We convert thickness from inches to feet, so a 10 × 10 ft slab at 4″ is 10 × 10 × (4 ÷ 12) = 33.3 ft³.
  • Cubic yards = ft³ ÷ 27 = 1.23 yd³ (a cubic yard is 27 cubic feet).
  • Bags = volume ÷ the bag's yield, rounded up. A standard 80 lb bag yields 0.60 ft³, so 33.3 ÷ 0.60 = 56 bags — then we add your waste % and round up.
  • Ready-mix is ordered by the cubic yard (in ¼-yard steps), with your overage added.

Bag yields (standard concrete mix)

BagYieldBags per cubic yard
40 lb0.30 ft³90
60 lb0.45 ft³60
80 lb0.60 ft³45

Yields are from the QUIKRETE #1101 and Sakrete High-Strength data sheets and are identical at the common 80 lb size — so the bag math is the same for any standard mix. Fast-setting and specialty mixes yield differently.

Worked example: 10 × 10 ft slab at 4″

  • Volume: 33.3 ft³ = 1.23 cubic yards
  • Bags (80 lb): 56 with no waste, 62 with 10% overage
  • Ready-mix: 1.5 cubic yards (rounded up to the ¼-yard increment)
  • At ~1.2 yd³, ordering ready-mix is usually the better call
Tip: running short means a cold joint or an expensive second trip — that's why a 10–15% overage is standard. Order a little extra, not a little less.

Popular slab sizes

Last updated: 2026-06-28.
Bag yields from the QUIKRETE #1101 and Sakrete High-Strength data sheets. Cost figures are US-national-average estimates and will vary by region and supplier.