Extend the pad past the shed on all sides so water runs off, not under.
Gravel pads: 4 to 6 in. Concrete slabs: 4 in is standard.
An estimate. Many areas require a permit for a shed above a certain size, and some require a specific foundation type or a setback from boundaries. Check with your local building department before you dig.
Calculated entirely in your browser. Nothing you enter is sent anywhere.
How to use it
Enter the shed size
The footprint, not the roof. Roof overhang does not sit on the foundation.
Add a pad overhang
Twelve inches each side is standard, so rain runs off the pad rather than under the shed.
Pick the type
Gravel drains and is far easier to build. Concrete is permanent and suits heavy equipment.
How the calculation works
For a gravel pad, volume is pad area times depth, with 25 percent added because crushed stone compacts. The result is the loose volume you order, not the compacted volume you end up with.
For a concrete slab, volume is area times thickness, converted to cubic yards and bag counts. A 60 lb bag yields about 0.45 cubic feet.
The bag count is shown so you can see when it stops being sensible. Above about one cubic yard you are mixing more than 60 bags by hand, and ready-mix is cheaper and substantially better concrete because it is batched properly rather than by eye.
What this calculator does not do
It does not know your local rules. Many areas require a permit above a certain size, and some specify the foundation type or a setback from boundaries.
It does not account for sloping ground, which means excavation at the high side or a retaining edge at the low side.
It assumes level, stable soil. Soft, peaty or recently filled ground needs a deeper base or concrete piers instead.
Questions
Gravel or concrete?
Gravel for most garden sheds: it drains, costs less and is far easier. Concrete if you are storing a vehicle or heavy machinery.
How thick should gravel be?
Four to six inches compacted. Six is safer, especially on softer ground.
Why extend past the shed?
So rain off the roof lands on the pad and drains away, rather than soaking the ground under the walls.
Can a shed go straight on soil?
You can, but the floor will rot and it will settle unevenly. The base is the cheapest part and the part that decides how long the shed lasts.
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