Concrete Calculator
Work out the concrete volume and number of bags for a slab or footing.
Results update as you type.
About this calculator
A concrete calculator multiplies the length, width and thickness of a slab to give the volume of concrete needed — in cubic yards or cubic metres — and estimates how many bags of pre-mix that is. Use it for patios, paths, footings and slabs before you order or buy.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × thickness in consistent units. A 10×10 ft slab, 4 in thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, which is about 1.23 cubic yards.
How many bags of concrete per cubic yard?
An 80 lb bag yields roughly 0.6 cubic feet, so a cubic yard (27 cu ft) takes about 45 bags. Ordering ready-mix by the yard is usually cheaper once you pass a third of a cubic yard.
Should I add extra for waste?
Yes — a 5–10% allowance covers spillage, uneven sub-base and over-excavation so you do not run short mid-pour. Set it under advanced options and the volume, bags and cost all include it.
Results are estimates for general guidance only, not financial, medical or tax advice.