Work out the concrete volume for a slab or footing and the bags of pre-mix.
Bag yield varies by product (e.g. 40kg ≈ 0.018 m³).
| Net volume | 2.00 m³ |
| Waste | 0.10 m³ |
| Total | 2.10 m³ |
| Bags | 117 |
Ordering concrete is unforgiving — run short mid-pour and the job is ruined. This calculator works out the volume you need for slabs, foundations and other rectangular pours, plus an estimate of bags.
Volume = length × width × thickness, all in metres. A slab 4 m × 3 m at 0.10 m thick needs 1.2 m³. Divided by the yield of a standard bag (often around 0.011–0.015 m³ per 25 kg bag), that is roughly 80–110 bags.
For anything large, ready-mix delivered by the cubic metre is usually cheaper and easier than mixing bags by hand — use the volume figure when you order.
It depends on the bag. A 25 kg bag of pre-mixed concrete yields roughly 0.011–0.015 m³, so a cubic metre takes about 65–90 bags. Check the yield printed on your bag.
Yes, add about 5–10%. Ground that is uneven or slightly over-dug absorbs more than the ideal calculation, and running out during a pour causes a weak cold joint.
Yes, a small allowance covers waste and uneven surfaces.