Shed Foundation Calculator
Planning where your shed sits? Our free shed foundation calculator estimates both popular options: pick a gravel pad to get the pad area, cubic feet, and cubic yards of gravel to order, or pick concrete piers to get the pier count, total concrete, and bag counts from your shed's length and width.
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Your results
| Gravel volume | — |
| Weight estimate | — |
| Rounded up to order | — |
Planning estimates only — round up when ordering and confirm with your local supplier.
How to use this calculator
Choose a foundation type: a gravel pad for a simple, cheap base on level ground, or concrete piers when the site is sloped, soggy, or the shed is large. Type in your shed's length and width in feet — everything updates instantly as you type.
For a gravel pad, set the gravel depth (4 inches is the default) and optionally extend the pad beyond the shed footprint with the pad extension — a 1-foot border on each side is common for drainage. For piers, adjust the pier diameter and depth to match your sonotubes (12 in × 36 in is the default).
How it works
Gravel pad: pad area = (length + 2 × extension) × (width + 2 × extension) square feet. Gravel volume = area × depth ÷ 12 cubic feet, then ÷ 27 for cubic yards. Weight assumes about 1.4 tons per cubic yard of crushed gravel. With the defaults, an 8 ft × 10 ft shed at 4 inches deep needs 8 × 10 × (4 ÷ 12) = 26.67 cu ft ≈ 0.99 cu yd.
Concrete piers: piers go at all four corners plus evenly spaced supports with at most 4 feet between them. Per side, spans = ceil(side ÷ 4) and piers on that side = spans + 1; subtract the 4 shared corners: total = 2 × (ceil(L ÷ 4) + 1) + 2 × (ceil(W ÷ 4) + 1) − 4 (minimum 4). An 8 × 10 shed needs 2 × (3 + 1) + 2 × (2 + 1) − 4 = 10 piers.
Concrete per pier: volume = π × (diameter ÷ 2)² × depth, all in feet. A 12-in × 36-in pier is π × (0.5 ft)² × 3 ft ≈ 2.36 cu ft. Bag counts assume one 80-lb bag yields 0.60 cu ft, always rounded up. All figures are planning estimates, not engineering specs.
Frequently asked questions
How much gravel do I need for a shed foundation?
Multiply shed length by width to get the pad area in square feet, then multiply by the gravel depth in feet (inches divided by 12): gravel = L × W × (depth in ÷ 12). For example, an 8 ft × 10 ft shed on a 4-inch pad needs 8 × 10 × (4 ÷ 12) = 26.67 cubic feet, or about 0.99 cubic yards. Round up when ordering — suppliers sell gravel by the cubic yard.
How many concrete piers does a shed need?
Place piers at all four corners plus evenly spaced supports with no more than 4 feet between them. Per side, spans = ceil(side ÷ 4) and piers = spans + 1, then add the four sides and subtract the 4 shared corners: total = 2 × (ceil(L ÷ 4) + 1) + 2 × (ceil(W ÷ 4) + 1) − 4. An 8 × 10 shed works out to 10 piers: 4 along each 10-ft side and 1 in the middle of each 8-ft side.
How deep should a gravel pad be for a shed?
4 inches is the common minimum for small sheds; 6 inches is better for larger sheds or soft soil. This calculator defaults to 4 inches — increase it in clay or poorly drained soil, and compact the gravel in 2-inch lifts so it stays level.
How much concrete goes in each pier?
For a 12-inch-diameter pier tube 36 inches deep: volume = π × (0.5 ft)² × 3 ft ≈ 2.36 cubic feet (about 0.09 cubic yards) per pier. That is roughly 4 eighty-pound bags of concrete mix per pier, and the calculator totals it all up for you.
Gravel pad or concrete piers — which is better for a shed?
A gravel pad is cheaper, faster, and works well for sheds up to about 120 square feet on level, well-drained ground. Concrete piers are better for larger sheds, sloped sites, or heavy loads because they keep the floor frame off the ground and resist settling. All results here are planning estimates — check local frost-line and permit rules before you build.