Shed Base Calculator
Estimate the materials for your shed base — concrete slab, gravel, paving slabs, or a timber frame. Get concrete bags, sub-base, gravel, sand, slab counts and timber lengths. Imperial or metric.
Base size
Tip: make the base 50–75 mm (2–3 in) larger than the shed footprint on each side so rain drips clear of the walls.
Base preview (to scale, top-down)
8 × 6 ftBase type
Typical: 4 in (100 mm) for sheds, 6 in (150 mm) for heavy workshops.
Compacted MOT Type 1 under the slab. Set to 0 to skip.
10% is a sensible default for spillage, off-cuts and broken slabs.
Materials estimate
About Shed Base Calculator
The Shed Base Calculator estimates the materials for the four most common shed-base builds: a poured concrete slab, a compacted gravel base, a paving-slab base, or a timber (bearer-and-joist) frame. Enter your base dimensions once and switch between base types to compare what each needs — concrete bags and sub-base, gravel volume and dumpy bags, slab counts and bedding sand, or joists, bearers and timber lengths.
A solid, level base is the most important part of any shed — it keeps the floor dry, the walls square, and the door swinging freely for years. The calculator handles imperial or metric units, adds a waste allowance, and shows approximate weights so you know whether you can carry the materials or need a delivery.
How to Use Shed Base Calculator
- 1 Pick your unit. Imperial (feet + inches) or metric (metres + centimetres).
- 2 Enter the base size. Use the shed footprint plus a small margin on each side so water drips clear of the walls. The base area updates as you type.
- 3 Choose a base type. Concrete slab, gravel, paving slabs, or timber frame. The inputs change to match — slab thickness, gravel depth, slab size, or joist spacing.
- 4 Set depths and the sub-base. A compacted sub-base (MOT Type 1 hardcore) under a slab or paving prevents sinking. Set it to 0 if your ground is already firm.
- 5 Add a waste allowance and read the materials panel. Volumes, bag counts, slab numbers, timber lengths and weights are all worked out for you.
Frequently Asked Questions
What is the best base for a shed? ▾
It depends on the shed and the ground:
- Concrete slab — the strongest, best for large or heavy workshops and permanent sheds.
- Paving slabs — solid and level, great for small to medium sheds, easier than concrete.
- Timber frame — quick, raises the shed off the ground, ideal on reasonably level soil.
- Gravel — cheap and free-draining, good for smaller sheds and bin/log stores.
How thick should a concrete shed base be? ▾
100 mm (4 in) of concrete over 75–100 mm of compacted sub-base is standard for most garden sheds. For a heavy workshop, machinery, or a vehicle, increase the slab to 150 mm (6 in) and consider adding steel reinforcing mesh.
How many bags of concrete do I need? ▾
This calculator assumes a 25 kg bag of pre-mixed concrete yields roughly 0.011 m³ (about 11 litres) once mixed — so around 90 bags per cubic metre. Bags are convenient for small slabs; above about ½ cubic metre, ready-mixed or mixing your own from cement, sand and aggregate is far cheaper. Always check the yield printed on the bag you buy.
Do I need a sub-base under the shed base? ▾
For concrete and paving, yes — a compacted layer of MOT Type 1 hardcore spreads the load and stops the base sinking or cracking on soft ground. On firm, well-drained soil you can reduce it, and for a simple gravel base the gravel itself acts as the drainage layer over a weed membrane.
How big should the base be compared to the shed? ▾
Match the base to the shed's actual floor size, or make it 50–75 mm (2–3 in) larger on each side so rain runs off the walls onto the base rather than pooling at the edge. Avoid making it much bigger — a wide concrete lip collects water and looks unfinished.
Are these estimates exact? ▾
They're close planning estimates using typical material densities and bag yields, plus your waste allowance. Real quantities vary with compaction, moisture, slab and bag sizes, and supplier products — round up and confirm against the figures on the bags or your delivery quote. Everything runs in your browser; nothing is saved or sent anywhere.