Engineering Content Strategist at ConcreteCalculatorz.com
Engineer Nazim is an engineer dedicated to breaking down complex structural and concrete problems into practical, actionable data for DIYers, contractors, and construction experts. From concrete calculations to field guide logistics, he focuses on real-world solutions that work on-site.
Latest posts by Engineer Nazim (see all)
- How much will a 20kg bag of concrete cover? - July 30, 2026
- how do I calculate how much concrete I need? How to calculate Concrete volume - July 30, 2026
- Concrete volume calculator vs manual measurement: which is more accurate? - July 28, 2026
A 20 kg bag of premix concrete yields approximately 0.01 m³ (10 litres) of finished concrete once mixed with water.
Coverage by Thickness
| Thickness | Area Covered (Metric) | Area Covered (Imperial) |
|---|---|---|
| 25 mm (1 in) | ~0.4 m² | ~4.3 ft² |
| 50 mm (2 in) | ~0.2 m² | ~2.1 ft² |
| 75 mm (3 in) | ~0.13 m² | ~1.4 ft² |
| 100 mm (4 in) | ~0.1 m² | ~1.1 ft² |
Quick Formula
To find how many 20 kg bags you need:
Bags = (Length × Width × Depth in metres) ÷ 0.01
Then add 10% extra for waste, spillage, and uneven subgrade.
Example: A 2 m × 1.5 m slab at 100 mm thick:
- Volume = 2 × 1.5 × 0.10 = 0.3 m³
- Bags = 0.3 ÷ 0.01 = 30 bags
- With 10% waste = 33 bags
When to Switch to Ready-Mix
Bagged concrete is practical for projects under 0.5 m³ (about 50 bags). For anything larger, ordering ready-mix is more economical and ensures consistent strength.
For bigger jobs, use the free concrete calculator at ConcreteCalculatorz.com to convert your dimensions into cubic metres, cubic yards, and exact bag counts — including the recommended waste buffer.
