Shed foundation calculator

Two ways to hold a shed off the ground: a gravel pad on timber skids, or a grid of concrete piers. Enter the footprint once and get the skid count and the pier count side by side, with the material for each.

01 How the math works

Both methods start from the footprint. Skids run the length of the shed and are spaced across the width, so the runner count is the width divided by the spacing, rounded up, plus one for the closing runner at the far edge. Piers follow the same logic in two directions: beam lines across the width, piers along each line.

The spacing figures are not foundation constants. Skid and beam spacing are capped by the floor joist that spans between the runners; pier spacing is capped by the beam that spans between piers. The calculator uses the spacing you enter — check it against a joist span table before you trust the count.

02 Worked example

Take a common 8 ft × 12 ft shed (2.44 m × 3.66 m), 96 ft² of footprint (8.92 m²). That sits under the 200 ft² (18.58 m²) permit-exempt line and well under the 600 ft² (56 m²) light-frame frost-exemption line, so both methods are on the table.

Skid method. Skids run the 12 ft length, spaced across the 8 ft width at 48 in: ceil(96 ÷ 48) + 1 = 3 runners, 36 lin ft of 4×6. They sit on a gravel pad extended 12 in past every side — 14 ft × 10 ft × 4 in = 46.67 ft³ = 1.73 yd³ (1.32 m³), about 4,667 lb / 2.33 tons (2,117 kg) of crushed stone.

Pier method. Three beam lines across the width, three piers per line along the length at 6 ft: 3 × 3 = 9 piers. Each 8 in tube dug to 36 in holds 1.05 ft³, so 9.42 ft³ (0.35 yd³ / 0.27 m³) of concrete — 18 bags of 80 lb mix at 10% waste. Same footprint, two very different jobs: three timbers set on a bed of gravel, or nine holes dug to frost depth and filled.

03 When this calculator is wrong

The number that ends most shed-foundation arguments is the frost line, and it is the one this calculator can't know for you. A gravel-and-skid pad is a floating foundation: it is allowed to rise and settle as the ground freezes and thaws, and IRC R403.1.4.1 exempts a light-frame accessory structure of 600 ft² (56 m²) or less from frost-depth footings for exactly that reason. Concrete piers are the opposite — a pier that stops above the frost line will heave, and one corner lifting while the others stay put is what racks a shed door out of square. If you pier in frost country, the depth field has to reach the greater of the 12 in (305 mm) minimum and your local frost line, which runs from 0 in in the far south to more than 60 in (1,524 mm) in the coldest northern states.

• Spacing is span-limited, and the calculator trusts your input. A 2×6 floor joist and a 2×8 floor joist carry different spans, so the skid spacing that a 2×6 floor allows is not the one a 2×8 allows. Enter a spacing the floor frame can't actually span and the runner count is right for the wrong floor.
• Deck blocks sitting on grade are not piers to bearing. A precast pier block set on the surface counts as a floating support and only works if the whole foundation floats with it. Mixing surface blocks under one edge with deep piers under another gives you differential heave — the failure the single-method exemption is written to avoid.
• The pad volume assumes level ground. On a slope you excavate the high side and build up the low side; a 12×8 pad on a 6 in fall across the length can need 30–50% more stone than the flat calculation, most of it holding up the downhill edge.
• A straight tube ignores the footing pad. The pier volume is a plain cylinder. In soft or expansive soil the base needs a wider bearing pad or a bell, which the tube figure doesn't include — that is a soil-bearing question, not a geometry one.

04 What to do with the result

Check the footprint against the two thresholds first, because they decide the rest. At or under 200 ft² (18.58 m²) the shed is usually permit-exempt, and at or under 600 ft² (56 m²) a light-frame shed on gravel and skids is exempt from frost-depth footings — which is why the gravel method is the default for a standard backyard shed and the piers are the exception, not the rule.

Order the gravel by the ton, not the bag: the pad above is 2.33 tons, and pit-run and crushed stone are priced and delivered by weight. Round the skid stock up to the next standard length (6, 8, 10, 12, 14, 16 ft) so each runner is a single piece with no splice. For the waste factor on a small job, the flat 10% built into the bag count is on the low side — for a pad under 2 yd³ or a pour under 2 yd³, real waste on small batches runs closer to 15%, so add a bag or a few hundred pounds of stone rather than stopping the job to fetch more. The exception is a large, simple, flat pad, where 10% is generous.

05 Common questions

What is the cheapest foundation for a shed?

For a standard shed, a compacted gravel pad on pressure-treated skids. The 8×12 example above is 3 runners and about 2.33 tons of crushed stone, against 9 piers and 18 bags of concrete for the pier method — less digging, less material, and no concrete to mix.

Do I need a concrete foundation for a shed?

Usually not. A one-story detached storage shed of 200 ft² (18.58 m²) or less is typically exempt from a building permit (IRC R105.2), and a light-frame shed of 600 ft² (56 m²) or less is exempt from frost-depth footings (IRC R403.1.4.1). Both let a gravel-and-skid foundation carry the shed. Piers make sense on a slope, in loose fill, or where you want a fixed, level base — subject to local code.

How many piers do I need for a 10×12 shed?

With three beam lines across the 10 ft width and piers at 6 ft along the 12 ft length, that's 3 × (ceil(12 ÷ 6) + 1) = 3 × 3 = 9 piers. Widen the beam spacing to two lines and it drops to 6. The count follows the beam and joist spans, so set those first.

How deep should a shed foundation footing be?

A pier carried to bearing must reach the greater of the 12 in (305 mm) minimum footing depth (IRC R403.1.4) and the local frost line, which ranges from 0 in in the far south to more than 60 in (1,524 mm) up north. A floating gravel pad has no depth requirement — that is the whole point of the frost exemption for small accessory structures.

How much gravel do I need under a shed?

Take the footprint, add the pad overhang on every side, and multiply by the depth: (length + 2 × overhang) × (width + 2 × overhang) × depth. The 8×12 shed on a 14×10 ft pad 4 in deep needs 46.67 ft³ = 1.73 yd³ (1.32 m³), about 2.33 tons.