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Shed Foundation Calculator

Calculate the concrete needed for a simple slab foundation for a shed or small outbuilding.

Result

Concrete Needed
0.99 cu yd
80lb Bags (if mixing by hand)
45

For slabs larger than a few cubic yards, ready-mix delivery is usually more practical (and often cheaper per unit) than mixing dozens of bags by hand - each 80lb bag yields about 0.6 cubic feet of concrete.

About the Shed Foundation Calculator

A poured concrete slab is one of the more durable foundation options for a shed or small outbuilding, and getting the concrete quantity right matters whether you're ordering ready-mix or mixing bags by hand. This calculator converts a shed's footprint and slab thickness into both a cubic yard volume and an equivalent count of 80lb concrete bags.

How It Works

You enter the shed's length and width in feet and the slab's thickness in inches, with a help note flagging that 4 inches is standard for a light-duty shed slab. The calculator converts thickness to feet, multiplies by length and width to get volume in cubic feet, then converts that to cubic yards for ready-mix ordering and separately divides it by 0.6 cubic feet, the yield of one 80lb bag, to estimate a hand-mixing bag count.

volume (cu yd) = (length x width x thickness in feet) / 27; bags (80lb) = (length x width x thickness in feet) / 0.6

Formula & Methodology

Start by converting slab thickness from inches to feet by dividing by 12. Multiply that figure by the shed's length and width in feet to get the slab's volume in cubic feet. Divide by 27 to convert to cubic yards, the unit ready-mix concrete is quoted and delivered in, or divide by 0.6 to estimate how many 80lb bags of pre-mixed concrete the same volume would take if mixed by hand.

Examples

Small utility shed

Using the default 10 ft by 8 ft footprint at a 4 inch slab thickness, the volume works out to about 26.7 cubic feet, or roughly 0.99 cubic yards, which converts to 45 bags of 80lb concrete if mixed by hand.

Larger workshop shed

A 12 ft by 10 ft shed at the same 4 inch thickness produces a 40 cubic foot slab, equal to about 1.48 cubic yards, or 67 bags of 80lb concrete mix.

Advantages

  • Gives both a ready-mix cubic yard figure and a hand-mix bag count from the same inputs, useful whichever way you plan to source concrete.
  • Helps decide between ordering a ready-mix truck and buying bagged concrete based on how the total bag count compares to the project's scale.
  • Provides a quick way to compare material needs across different slab thicknesses before committing to a design.

Common Mistakes

  • Using a slab thickness heavier than needed for a light-duty shed, which increases concrete volume and cost without a structural reason.
  • Mixing dozens of 80lb bags by hand for a slab that would be faster and often cheaper per unit delivered as ready-mix.
  • Forgetting to add extra volume for a thickened perimeter footing or slab overhang if the specific shed design calls for one, since the base calculation only covers the flat rectangular footprint.

Edge Cases to Watch For

  • The bag count is always rounded up with a ceiling function, so a fractional remainder still requires a full additional bag.
  • The calculator's own note flags that for slabs beyond a few cubic yards, ready-mix delivery is usually more practical and often cheaper per unit than mixing dozens of bags individually.
  • The formula assumes a simple rectangular slab matching the shed's stated footprint exactly; it does not add extra volume for a thickened edge, footing, or any slab overhang beyond the building's wall lines that some designs call for.

Common Use Cases

  • Homeowners and DIYers planning a small outbuilding, shed, or workshop foundation and comparing bagged versus delivered concrete.
  • Contractors providing a quick material estimate for a straightforward slab foundation quote.
  • Buyers of prefab shed kits confirming the concrete pad meets the manufacturer's recommended footprint and thickness before pouring.
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Does a shed always need a full concrete slab foundation?

No - lighter sheds are sometimes built on a gravel pad, concrete deck blocks, or a treated wood skid foundation instead of a poured slab, which can be faster and cheaper, though a poured slab offers the most durable, level, and rot-resistant base, particularly worthwhile for larger sheds or ones used as a workshop.

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Conclusion

A simple slab volume calculation is the starting point for deciding how to source concrete for a shed foundation, whether that means a ready-mix delivery or a weekend of hand-mixing bags. This calculator gives both figures from the same footprint and thickness inputs so you can weigh the tradeoff before ordering.