Free NZ concrete calculator. Volume, bag count, and waste allowance for footings, slabs, pads, and post holes.
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.
Volume is length times width times thickness, with 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 0.010 cubic metres per 20kg bag. That yield is the same one CostMate's estimating engine uses for fence and deck footings, calibrated against real on-site bag counts.
It is worth knowing why that number is what it is. This page used 0.012 until 2 September 2026 — but 0.012 is the yield of a 25kg bag, and it was printed under a heading that said 20kg bags. Every bag count on this page came out roughly 17% short. If you used this calculator before that date, that is why you ran out.
Mesh is counted from sheets 6.0m by 2.4m with a 150mm lap. 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.
The old formula charged a lap at both ends of every sheet including the first, then multiplied the whole-sheet count by 1.1 and rounded up again — a third allowance on the same axis. A 6 by 4m slab came out at 5 sheets when 2 cover it, because the sheet is itself 6m long and one row spans the length on its own.
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.