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.