How the slab calculator works
Volume is length × width × thickness, with thickness converted from cm or inches to metres. Waste is added, then divided by the yield of one bag.
Bag yield varies by mix and bag size. The figures here are typical for ready-mixed concrete; always check the bag label.
Before you order
- Measure the slab in several places; use the deepest average thickness.
- Add 5–10% for uneven ground, spillage and settlement.
- For large slabs, ready-mix concrete is usually cheaper than bags.
Last reviewed October 27, 2026
Frequently asked questions
How many bags of concrete for a 4 × 5 m slab 10 cm thick?
Wet volume is 2 m³; with 10% waste about 2.2 m³, or 176 bags of 25 kg at 0.0125 m³ per bag.
How much does a bag cover?
A 25 kg bag yields about 0.0125 m³; 40 lb about 0.011 m³, 60 lb about 0.017 m³ and 80 lb about 0.022 m³. Check the bag.
Should I add waste?
Yes. 5–10% covers uneven ground, spillage and thickness variation. Add more for rough subgrades.