Concrete volume calculator

How much concrete do I need?

Concrete is ordered by the cubic metre. For a slab or footing, multiply length by width by thickness, keeping everything in metres. For round post holes, the calculator works out the volume of each hole and multiplies by the number of holes. We add a 10% allowance for spillage and uneven ground.

Bagged concrete suits small pours like a few post holes. Once you get past roughly 0.4 cubic metres, ordering pre-mixed concrete delivered by truck is usually cheaper and far less work than mixing dozens of bags by hand. A 20kg bag yields only about 0.009 cubic metres, so volumes add up fast.

Job	Typical thickness
Garden path	75 to 100 mm
Driveway	100 mm or more
Shed slab	100 mm

For the full installed price including labour and finish, see the concrete and slab cost calculator.

Common questions

How many 20kg bags of concrete in a cubic metre?

Roughly 110 bags of 20kg pre-mix make up one cubic metre, since each bag yields about 0.009 cubic metres. That is why ordering delivered concrete makes sense for anything but small pours.

Should I order pre-mix or use bags?

Bags are fine for small jobs like post holes. For about half a cubic metre or more, pre-mixed concrete delivered is usually cheaper per metre and saves a lot of mixing.

How thick should my slab be?

A garden path is often 75 to 100mm, while driveways and shed slabs are usually 100mm or more. Check any engineering or council requirement for structural slabs.

These figures are estimates to help you plan and sense check quotes. They are not a quote. Site access, materials, finish and your location all move the final price. Always get two or three written quotes.