Patio 3 m × 2.4 m × 10 cm
Input Length 3 m, width 2.4 m, thickness 10 cm, 10% waste, 25 kg bags.
Result 0.79 m³ of concrete over 7.2 m², so 64 bags and about 1,901 kg wet weight.
Concrete Slab Calculator
Home & RenovationWork out the concrete, bags and weight for a patio, shed base or driveway slab from length, width and thickness.
Your inputs
Result
Concrete volume
—
m³
- Slab area — m²
- Bags of mix —
- Wet concrete weight — kg
Change any input and the result updates instantly.
Formula and rules
- Volume = length × width × thickness × (1 + waste %)
- Area = length × width
- Bags = volume ÷ (bag weight in kg × 0.0005 m³ per kg)
- Wet weight ≈ volume × 2,400 kg per m³
- A 25 kg bag yields about 0.0125 m³, so 1 m³ needs about 80 bags
- Above roughly 1 m³, ready-mix delivery is cheaper than bagged mix
Worked examples
Shed base 10 ft × 8 ft × 4 in
Input Length 10 ft, width 8 ft, thickness 4 in, 10% waste, 60 lb bags.
Result 29.33 ft³ over 80 ft², so 62 bags and about 4,395 lb wet weight.
FAQ
How many 25 kg bags do I need for 1 m³?
About 80. A 25 kg bag of concrete mix yields roughly 0.0125 m³ once water is added, so 1 ÷ 0.0125 = 80 bags.
How thick should a concrete slab be?
10 cm (4 in) suits footpaths and patios, 10–15 cm (4–6 in) suits shed bases and light driveways, and 15 cm (6 in) or more is used where vehicles park.
Should I add reinforcement?
For a shed base or patio, a single layer of mesh set mid-depth limits cracking. Slabs carrying vehicles normally need mesh plus a compacted sub-base of 10 cm or more.
Disclaimer: Results follow the formula shown on this page and are for reference only. Real-world quantities vary with materials, technique and conditions, so allow a margin before ordering.