Construction
Concrete slab calculator
Enter slab dimensions (length × width × thickness) and a waste factor. The calculator returns volume in m³, weight in kg, and the number of 50 kg premix bags you'll need to buy.
Formula
Concrete volume is straightforward (L × W × T), but you almost always need more than the bare minimum. Waste comes from spillage, uneven excavation, and the inevitable 'one more shovelful'. A 10% waste factor is standard for small slabs; 5% for big ready-mix pours.
Fresh concrete weighs about 2400 kg/m³ (varies 2200-2500 depending on aggregate and water content). For load calculations on a foundation or truck capacity, this is the number you want.
DIY-friendly 50 kg premix bags yield about 0.022 m³ each when mixed with water. For volumes above ~1 m³, a ready-mix delivery is usually cheaper and faster than dozens of bags.
Examples
- 014×3 m, 15 cm thick, 10% waste→ Volume 1.8 m³ × 1.1 = 1.98 m³ · Weight 4752 kg · 90 bags (or 2 m³ ready-mix)
- 023×3 m patio, 10 cm thick, 10% waste→ 0.9 × 1.1 = 0.99 m³ · 45 bags · ~$140 in premix
- 031×1 m footing, 30 cm thick→ 0.3 × 1.1 = 0.33 m³ · 15 bags
FAQ
- Under ~1 m³: bags. 1-3 m³: depends on access and time available — bags need lots of mixing labor. 3+ m³: ready-mix truck is faster, cheaper per m³, and gives a uniform pour. Truck minimums are usually 3-4 m³.