Estimate how much concrete you need for a slab, footing or pad based on its length, width and thickness.
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This calculator multiplies length × width × thickness to find the volume of concrete needed in cubic feet, then converts that to cubic yards (dividing by 27) since ready-mix concrete is typically ordered by the cubic yard for larger pours.
For bagged concrete, it uses the standard published yield for premixed bags: a 40 lb bag yields about 0.30 cubic feet, a 60 lb bag about 0.45 cubic feet, and an 80 lb bag about 0.60 cubic feet, then divides your total volume by the bag's yield and rounds up to a whole number of bags.
For small projects like fence posts or a small pad, bagged concrete mixed on-site is usually more practical. For larger pours (roughly a cubic yard or more), ready-mix concrete delivered by truck is typically more cost-effective and easier to place in one continuous pour.
For a 10 ft × 10 ft slab, 4 inches thick: volume = 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, or 33.33 ÷ 27 ≈ 1.235 cubic yards. Using 60 lb bags (yielding 0.45 cu ft each): 33.33 ÷ 0.45 ≈ 74.1, rounded up to 75 bags needed.
Entering thickness in feet instead of inches, this calculator's thickness field expects inches specifically, entering '4' meaning 4 feet instead of 4 inches would inflate the volume estimate twelvefold.
Ordering exactly the calculated amount with no buffer, real pours lose some material to spillage, uneven subgrade and over-excavation, most contractors add 5-10% on top of the theoretical volume.
Assuming a larger bag size always means fewer bags in a simple ratio, bag yield doesn't scale perfectly linearly with weight in casual estimation, always use the actual yield figure (cubic feet per bag) printed on the specific bag, not just its weight.
This calculator gives the exact theoretical volume. Many contractors add 5-10% extra to account for spillage, uneven subgrade, and over-excavation, consider rounding up your final order.
No, this calculator estimates concrete volume only. Reinforcement like rebar or wire mesh doesn't meaningfully change the concrete volume needed and should be planned separately.
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