Shed Base Concrete Calculator

About Shed Base Concrete Calculator

Shed Base Concrete Calculator sizes the slab for a garden shed, summer house or garden building base. A 3 m x 2.4 m base at 100 mm thick needs 0.72 m3, or 0.79 m3 with a 10 percent waste allowance, which is about 66 bags of 25 kg mix or 40 bags of 40 kg mix. A 10 ft x 8 ft base at 4 inches comes to roughly 1.09 cubic yards with waste, around 49 bags of 80 lb mix. Shed bases sit right at the volume where bag mixing is still sensible, typically under 1 m3, so they are a realistic weekend job for one or two people. The usual advice is to make the base the same size as the shed footprint or up to 50 mm larger on each side, not smaller, so the shed walls are fully supported and rainwater drips clear of the timber. Thickness is normally 100 mm over a compacted hardcore sub base, which is enough for a timber or metal shed; a heavy garden room or workshop with machinery should be treated as a loaded slab at 150 mm.

How to use Shed Base Concrete Calculator

  1. Choose metric or imperial input
  2. Enter length, width, and depth of your slab
  3. See volume and bag count in both metric and imperial instantly

Frequently Asked Questions

How much concrete do I need for a shed base?
Multiply base length by width by thickness. A 3 m x 2.4 m base at 100 mm thick needs 0.72 m3, or about 0.79 m3 with a 10 percent waste allowance, roughly 66 bags of 25 kg mix. A 10 ft x 8 ft base at 4 inches needs about 1.09 cubic yards with waste.
How thick should a shed base be?
One hundred millimetres, or 4 inches, over 75 to 100 mm of compacted hardcore is standard for a timber or metal shed. Go to 150 mm for a heavy garden room, a workshop with machinery, or anything storing a vehicle or ride on mower.
Should the base be bigger than the shed?
Match the shed footprint or go up to 50 mm larger on each side. A base smaller than the shed leaves wall plates unsupported, and a base much larger collects standing water against the timber unless you slope it away.
Can I mix a shed base by hand?
Yes. Most shed bases are under 1 m3, which is achievable with a mixer over a day. Pour continuously in one session rather than across two days so the slab sets as a single unit without a cold joint through the middle.
Do I need a sub base under a shed base?
Yes on soil or turf. Excavate and compact 75 to 100 mm of hardcore first so the slab is supported evenly. The sub base is separate from the concrete and does not count toward the concrete volume calculated here.

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