Calculate the concrete volume and number of bags needed for strip, pad and pier footings.
Pre-mixed concrete is sold in 25 kg bags (about 12.5 litres each) in the UK and 60/80 lb bags in the US. Large pours are cheaper from a ready-mix truck.
Roughly 80 bags of 25 kg pre-mixed concrete make up one cubic metre, as each bag yields about 12.5 litres of mixed concrete.
Mixing by hand is fine up to about 0.3–0.5 m³. Above that, ordering a ready-mix truck is usually cheaper and far less work.
Trenches are rarely perfectly even and some concrete is always lost to spillage and over-dig, so 10% is sensible.
Footings should reach undisturbed, firm ground below the frost line — often 450–900 mm in the UK, but always check local building regulations.
A strip footing is typically two to three times the width of the wall it supports; heavily loaded walls need wider footings designed by an engineer.
Calculate how much concrete you need for slabs, footings, columns and walls in cubic yards or metres.
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