Shed Base Calculator

Work out the concrete, paving slabs or gravel and sub-base for a shed base, sized to fit your shed.

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How to Use This Calculator

What This Calculator Does

This shed base calculator works out the materials for the foundation your shed sits on — whether that's a poured concrete slab, a paving-slab base, or a plastic grid and gravel base. It sizes the base slightly larger than the shed, then estimates the sub-base, concrete or slabs and bedding you'll need. (For working out the shed building itself, see our shed material estimator.)

How to Use

  1. Choose metric or imperial units.
  2. Enter the footprint of your shed (length × width).
  3. Set how far the base should extend beyond the shed on each side — 50 mm is typical so rainwater drips clear of the walls.
  4. Pick the base type. For concrete, set the slab thickness (100 mm suits most garden sheds).

Worked Example

For a 3 m × 2 m shed on a 100 mm concrete slab with 50 mm overhang each side:

  1. Base size = 3.1 m × 2.1 m = 6.5 m².
  2. Concrete = 6.5 × 0.1 m = about 0.65 m³ (plus a little over-order).
  3. You'll also want around 1.3 tonnes of MOT Type 1 for a 100 mm compacted sub-base.

Tips

  • Always lay the base on a firm, compacted MOT Type 1 sub-base — see our MOT Type 1 calculator.
  • Fall/level: a concrete slab should be dead level; a gravel base should drain freely.
  • A concrete slab is the most durable for larger or heavier sheds and workshops; paving slabs or a plastic grid are quicker for small timber sheds.
  • For the concrete mix itself, our concrete calculator gives cement, sand and aggregate quantities.

Common Questions

What is the best base for a shed?

A level, well-drained base is essential. A concrete slab is the most durable and is best for larger sheds and workshops. Paving slabs or a plastic grid filled with gravel are quicker and cheaper, and are fine for small to medium timber sheds.

How thick should a concrete shed base be?

For most garden sheds a 100 mm (4 inch) slab over a 100 mm compacted MOT Type 1 sub-base is plenty. For heavy workshops or if vehicles will run over it, increase to 150 mm and add mesh reinforcement.

Should a shed base be bigger than the shed?

Yes — make it about 50 mm larger on each side so rainwater running off the roof drips clear of the walls rather than pooling against them. This calculator adds that overhang automatically.

How much concrete do I need for a shed base?

Multiply the base area by the slab thickness. For example a 3 × 2 m base at 100 mm needs about 0.6 m³ of concrete. The calculator adds a small over-order allowance and converts this into ready-mix bags.

Do I need a sub-base under a shed base?

Yes. A 100 mm compacted layer of MOT Type 1 hardcore gives a firm, free-draining foundation under concrete, slabs or a gravel grid, and stops the base sinking or cracking over time.

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