Concrete calculator
A 10 × 10 ft shed base at 4 inches deep is 33.3 cubic feet, or 1.23 cubic yards. Add the usual 10% for spillage and an uneven sub-base and you order 1.36, which in practice means asking for 1.5 yards, because ready-mix is sold in quarter-yard steps and running short mid-pour is far worse than paying for a little extra.
Concrete volume is length × width × thickness, converted to cubic yards or cubic metres. A 10 × 10 ft slab at 4 inches thick needs 1.23 cubic yards. Add 5–10% for waste, spillage and an uneven sub-base, then round up to the quarter yard your supplier sells in.
How to work out concrete
The arithmetic is the easy part; the judgement is in the waste allowance. A slab poured onto compacted hardcore with clean formwork might only lose 3%. The same slab onto a dug-out garden with soft spots can swallow 15% before you notice, because concrete finds every dip. Footings are worse again; trench sides slump inward and the trench you measured is not the trench you fill. If the pour is one continuous piece and running short means a cold joint, buy the extra quarter yard.
Questions
About 45 bags of 80 lb (36 kg) pre-mix, or roughly 73 bags of 25 kg. Bag mixes yield less than their dry weight suggests because water adds no volume to the paste.
Four inches (100 mm) is standard for a shed base, patio or footpath. Six inches (150 mm) if a vehicle will park on it. Thickness drives volume linearly, so going from four to six inches adds half again to the order.
Yes. Ten per cent is the default here. Concrete is unforgiving: a short pour means a cold joint, whereas a surplus is a wheelbarrow of spoil.
No. Cement is the binder; concrete is cement plus sand, aggregate and water. This calculator sizes concrete. If you are buying cement to mix yourself, the cement calculator splits the order into its parts.
About 2.4 tonnes. That matters for access: a single cubic metre is heavier than most small vans can legally carry.