How to Calculate Concrete Volume
Work out the concrete volume for slabs, paths and footings in cubic metres — with waste allowances and ordering tips.
5 min read
The basic formula
Concrete volume for anything rectangular is length × width × depth, with all three measurements in metres. The answer is cubic metres (m³).
A slab 5 m × 3 m × 0.1 m (100 mm thick) is 5 × 3 × 0.1 = 1.5 m³.
Getting the depth right
Depth is where mistakes happen, because slab thicknesses are usually quoted in millimetres. Divide by 1,000 to convert: 100 mm = 0.1 m, 125 mm = 0.125 m, 150 mm = 0.15 m.
Common Australian thicknesses: 100 mm for garden sheds and pedestrian paths, 125–150 mm for driveways. Anything structural or vehicle-bearing should be specified by an engineer.
Allow for waste
Real pours always exceed the theoretical volume: the subgrade absorbs or displaces concrete, edges bulge slightly, and shapes are rarely perfect. A 10% allowance covers most jobs on reasonably level ground; use up to 20% on rough or sloping sites.
The 1.5 m³ slab with 10% waste needs about 1.65 m³ — order 1.7 m³ and you have comfortable margin.
Try the Concrete CalculatorReady-mix or bags?
Ready-mix trucks suit pours over roughly 0.5–1 m³ — they supply in 0.1 m³ increments and ask you to sign for the amount poured, so finalise your order before the truck arrives.
Bagged premix (20 kg bags yielding about 0.009 m³ each) suits small footings and repairs. The 1.65 m³ pour would mean about 184 bags mixed by hand — possible, but a truck is the saner choice.
Irregular shapes
Break complex slabs into rectangles, calculate each volume, and add them. For post holes and round footings, volume is π × radius² × depth — a 300 mm diameter hole 600 mm deep is about 0.042 m³ each.