Concrete Calculator
Concrete volume for slabs, footings, posts and steps, with bags of premix needed.
About this tool
Choose what you are pouring β slab, strip footing, round columns or post holes, or steps β enter the dimensions in metres or feet and inches, set a waste allowance, and see the volume in cubic metres, cubic yards and cubic feet, the amount to order and the number of premix bags at typical yields.
Working out how much concrete to buy
Concrete volume is length Γ width Γ depth for a slab or footing, Ο Γ radiusΒ² Γ depth Γ the number of holes for posts and columns, and for steps the sum of each step as a solid block. Measure the depth carefully: a 4-inch (100 mm) patio slab and a 6-inch (150 mm) driveway use 50% different amounts. Add 5β10% for formwork that bows, ground that is not level and spillage β running short mid-pour leaves a weak cold joint. For small jobs, premix bags are convenient; typical yields are about 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags and roughly 0.012 mΒ³ for a 25 kg bag, but check your brand. Beyond about a cubic metre (1.3 cubic yards), ready-mixed concrete delivered by truck is usually cheaper and far less work. General information, not structural advice: footings for walls, extensions and anything load-bearing should be specified by a builder or engineer. The volume calculator handles other shapes, the area calculator irregular plots, the tile calculator the finish, and the unit converter metric and imperial.
Frequently asked questions
How many bags of concrete do I need for a 10 Γ 10 ft slab?
At 4 inches thick that is 33.3 cubic feet (1.23 cubic yards). With 10% extra, about 62 bags of 80 lb at 0.6 ftΒ³ each β a job where ready-mix delivery is worth pricing.
How thick should a concrete slab be?
Typically 100 mm (4 in) for patios, paths and sheds and 150 mm (6 in) for driveways that take vehicles. Local building rules may specify more.
How much extra concrete should I order?
5β10% covers uneven ground, formwork flex and spillage. Running short mid-pour is far worse than a little left over.
Is anything saved?
No. The calculation runs in your browser and nothing is stored or sent.