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Concrete Calculator: Cubic Yards & Bags (60/80 lb)

Enter your slab or post-hole size in feet and inches to see cubic yards of concrete and how many 40, 60 or 80 lb bags to buy, with extra for waste.

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Good to know: Scope: how much concrete a flat slab (patio, shed pad, walkway) or a round hole or column (fence post, deck footing, tube form) needs, in cubic yards and cubic feet, and how many bags of pre-mixed concrete to buy. It uses US units (feet and inches), 1 cubic yard = 27 cubic feet, and the bag yields printed by QUIKRETE for its Concrete Mix (No. 1101): about 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb, 0.60 for 80 lb and 0.675 for 90 lb. You can type the yield from your own bag instead. The extra amount for spills and uneven ground is your choice. Not included: walls, steps, curbs and other shapes (split them into slabs), the gravel base, rebar or wire mesh, mixing water, ready-mix delivery minimums and prices, local building code rules for footing size and depth, and structural design. Everything stays in this browser tab; nothing is sent or saved. Independent ToolsKo estimate. Sources reviewed November 11, 2026.

How to use it (step by step)

  1. 1Pick the shape: a flat slab, or a round hole or column such as a fence post hole.
  2. 2Enter the size: for a slab, length and width in feet and thickness in inches (a 4-inch patio is common); for a hole, its width (diameter) and depth in inches. Add how many identical pours you have, for example 8 post holes.
  3. 3Pick your bag size and read the cubic yards and number of bags. Buy a bag or two more if your ground is uneven, and check the yield printed on the bag you buy.

Words to know

Cubic yard
A box 3 feet long, 3 feet wide and 3 feet high, which is 27 cubic feet. Ready-mix concrete trucks sell concrete by the cubic yard.
Bag yield
How much finished concrete one bag makes once you add water, in cubic feet. It is printed on the bag, for example 0.60 cubic feet for an 80 lb bag.
Slab thickness
How deep the concrete is, measured in inches. Divide by 12 to get feet: 4 inches is one third of a foot.
Waste allowance
A little extra concrete to cover spills, a low spot in the ground or forms that bulge, so you do not run short in the middle of a pour.

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Sources & methods

References checked November 11, 2026. Read each tool’s scope and assumptions before relying on its result.

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