Concrete Calculator
Work out the cubic metres and number of 20kg bags of concrete you need, including a wastage allowance.
How it works
Volume is length times width times depth (converted to metres), increased by your wastage allowance. Bags needed is that volume divided by the yield of a standard 20kg premixed bag, rounded up to the next whole bag — you can’t buy a fraction of a bag.
Volume = Length × Width × Depth × (1 + Wastage %)
Worked example
A 4m × 3m slab, 10cm deep, 5% wastage:
- Volume
- 4 × 3 × 0.1 = 1.2 m³
- With 5% wastage
- 1.2 × 1.05 = 1.26 m³
- Bags needed
- 1.26 ÷ 0.01 = 126 bags
Frequently asked questions
About 100 bags — each 20kg premixed bag yields roughly 0.01 m³ once mixed with water. For anything much larger than a small path or footing, ready-mix delivery is usually far cheaper and less physically demanding than mixing dozens of bags by hand.