Size
Make the base the same size as the shed's floor, or very slightly larger. If the base sticks out too far, rain runs off the walls and pools on the concrete against the shed.
Layers
A typical base has three layers: the soil dug out and levelled, a compacted sub-base of crushed stone such as MOT Type 1, and the concrete slab on top. Many garden shed bases use around 75mm to 100mm of concrete. Larger outbuildings, workshops or soft ground may need more, so follow the shed maker's instructions and take advice if unsure.
Working out the concrete
Multiply length by width by depth in metres. A 3m by 2.5m base at 100mm deep needs 0.75 cubic metres. Add around 10% for uneven ground and waste, giving about 0.83 cubic metres. The concrete calculator turns this into bags.
Bags or ready-mix?
Bagged concrete is convenient for small bases but heavy work: at a typical 2,400kg per cubic metre, 0.83 cubic metres is around 80 bags of 25kg. For about a cubic metre or more, ready-mixed concrete delivered by lorry, or a mix-on-site service, is usually easier.
Formwork
Timber boards around the edge hold the concrete in shape. You need enough to go round the base once, so a 3m by 2.5m base needs 11 metres of board plus pegs.