Concrete calculator

This concrete calculator tells you how much concrete you need for a slab, base, footing, or path. Enter the length, width, and thickness in metric or imperial units and it returns the volume in cubic metres and cubic yards, the approximate weight, and an estimate of how many 20 kg or 25 kg bags of pre-mixed concrete to buy — with 5% added for waste and spillage.

How concrete volume is calculated

Volume is simply length × width × thickness, converted to cubic metres. The calculator handles the unit conversion for you, so you can enter the thickness in centimetres or inches even when the slab is measured in metres or feet. Concrete weighs about 2,400 kg per cubic metre, which is how the weight figure is worked out.

Bags vs ready-mix

For small jobs, bagged pre-mix is convenient — a 20 kg bag yields roughly 0.009 m³ of set concrete. For anything above about 0.5 m³, ordering ready-mix delivered is usually cheaper and easier. Always round up and order a little extra. For sub-base material under the concrete, see our MOT Type 1 calculator; for decorative finishes, the gravel calculator.

Frequently asked questions

How much concrete do I need for a 4 × 3 m slab at 10 cm?

That is 1.2 m³ before waste (about 1.26 m³ with 5% added). The calculator shows the exact bag count for your dimensions.

Disclaimer: Estimates only. Always confirm quantities with your supplier and allow extra for uneven ground.