Free NZ concrete slab calculator. Cubic metres for a slab, path or pad, the number of 20kg bags and mesh sheets, with 10% overage on the concrete. No signup.
This calculator works out the volume of concrete in a slab, path or pad, how many 20kg bags that is, and how many sheets of mesh cover it. It counts material for a slab you have already decided the size of.
A 4 x 3 m slab at 100mm thick holds 1.2 cubic metres of concrete. The calculator shows 1.32 cubic metres to order, because that figure includes CostMate's allowance on top. If you are mixing on site, that is 165 bags of 20kg, and the mesh is 2 sheets.
For a ready-mix truck, give your supplier the cubic metres, not the bag count.
Volume is length times width times thickness, with CostMate's allowance of 10% added. The thickness options are 75mm for a path, 100mm for standard residential, and 150mm for a garage or driveway.
Bags are worked out at 0.008 cubic metres per 20kg bag: the maker's own figure for Dricon's 20kg bag (Cemix says 0.01 for its 20kg bags, so on a Cemix bag you will have a few over).
Mesh is counted in SE62 sheets (the seismic 665-equivalent) at 5.85m by 2.42m with a 250mm lap on ends and sides, as Fletcher Reinforcing's North Island data sheet gives them. (A sheet sold as 665 is smaller, 4.56m by 1.98m.) A lap is only used where two sheets actually meet, so the first sheet on each axis contributes its full dimension and every sheet after it contributes its dimension less the lap.
There is no separate wastage multiplier on the mesh count. The laps are the allowance. Adding a percentage on top would be counting the same thing twice.
Slab thickness, reinforcing and ground preparation are design decisions. Check them against your plans and NZ Building Code B1 (Structure), and confirm bag yields with your merchant, before you order or pour.