Concrete slab calculator

Volume. Length times width times thickness, with the thickness turned from centimetres into metres. A slab 4 m by 2.5 m and 10 cm thick is 4 × 2.5 × 0.1 = 1 m³.

Waste. Some concrete is always lost to uneven ground, spills and the last bit in the mixer. The calculator adds your waste percentage on top. 10% is the default, the same as the Construction Calc app. Use more for an uneven base or an awkward shape.

Bags. A 25 kg bag of premix makes about 0.0125 m³ of concrete once mixed. The volume with waste is divided by that yield and rounded up, because you cannot buy part of a bag. That same yield means about 2,000 kg of dry premix for every m³ of slab.

• The slab is a rectangle with an even thickness. Split an L shape into two rectangles and add them.
• The bags are 25 kg premix (cement, sand and gravel together) that yield about 0.0125 m³ each.
• It does not cover rebar, mesh, formwork or a sub-base of hardcore.

For a large slab, ready-mix delivered by truck is usually cheaper than bags. Ask the supplier for a quote in m³.