Concrete Calculator
Enter a slab, path or footing, and any post holes, to see how many cubic metres of concrete you need and roughly how many premix bags that is.
Slab, path or footing
Post holes (fence or deck posts)
- Concrete needed
- 0.600 m³
- With waste allowance
- 0.660 m³
- 20 kg premix bags
- about 73
Need someone to do the job? Find builders, landscaping & gardening or fencing & decking near you and contact them directly, with no lead fees.
How it works
- Slab volume = length × width × depth. Post hole volume = π × radius² × depth, times the number of holes.
- The waste allowance covers uneven ground and spillage. 10% is a common starting point.
- Bags assume about 2,200 kg of dry premix per cubic metre, so a 20 kg bag makes roughly 0.009 m³. Yields vary by brand: check the yield printed on the bag.
These are estimates for planning and comparing quotes, not a substitute for a professional's measurements.
Common questions
How thick should a concrete slab be?
Paths and patios are commonly 75 to 100 mm thick, and driveways are usually at least 100 mm with reinforcing. For anything structural, such as a house floor or a retaining wall footing, follow the engineer's or council-approved design.
Should I use premix bags or ready-mix?
Bags suit small jobs like setting fence and deck posts. For bigger pours, ask a concrete supplier about ready-mix delivery and their minimum load, which is often cheaper and gives a more even pour.
How deep should fence post holes be?
A common rule is about a third of the post's height above ground, and at least 600 mm for fence posts. Use deeper holes in soft ground or for gates and tall fences.