Calculate the cubic metres of concrete needed for a slab from its length, width and thickness.
Concrete volume = length × width × thickness (with thickness in metres). A 5 m × 4 m slab at 100 mm needs 2.0 m³ of concrete.
Multiply the three dimensions in consistent units. The tool converts thickness from millimetres to metres for you. Order a little extra for spillage, uneven subgrade and over-dig — typically 5–10%.
5 m × 4 m × 0.10 m = 2.0 m³. Add 10% waste allowance and you'd order about 2.2 m³.
Assumes a flat slab of uniform thickness on a level, well-prepared base. Thickenings, edge beams and footings add volume — calculate those separately and add them.
Yes — add roughly 5–10% for spillage, an uneven base and slab edges. Running short mid-pour means a cold joint or an expensive second delivery, so a small over-order is cheap insurance.
Calculate each thickened section as its own rectangular volume (length × width × extra depth) and add it to the slab volume. This tool covers the flat slab portion.
A 20 kg bag of pre-mix yields about 0.009–0.010 m³. So 1 m³ needs roughly 100–110 bags — for anything above ~0.5 m³, ordering ready-mix is usually cheaper and easier.
Last updated 21 June 2026