Concrete Calculator
Calculate ready-mix volume or the number of pre-mixed bags for a slab, a footing, or a set of post holes.
How we calculate
For a slab we multiply length × width × thickness. For post holes we use π × radius² × depth, times the number of holes. We then add your waste allowance and convert to cubic yards, cubic metres and whole bags, rounding up.
Tip: Running short mid-pour leaves a cold joint you can't undo — always round up rather than down.
Assumptions & things to watch
- Volumes include your chosen waste allowance — concrete is far cheaper to over-order than to run short mid-pour.
- Ready-mix suppliers usually have a minimum load; below about 1 m³ (1.3 yd³) bagged mix is often cheaper.
- Structural slabs and footings should follow local building codes — check depth, reinforcement and frost line requirements.
Estimates only — always double-check quantities with your supplier before ordering.
Quick tips: Concrete
How thick should a concrete slab be?
4 in (10 cm) suits patios, sheds and footpaths. Go to 6 in (15 cm) with reinforcement for driveways or anything carrying vehicles.
How many bags of concrete are in a cubic yard?
About 45 × 80 lb bags, or 60 × 60 lb bags, per cubic yard. In metric it's roughly 108 × 25 kg bags per cubic metre.
Bagged mix or ready-mix delivery?
Under about 1 cubic yard (0.75 m³), bags are usually cheaper and easier. Above that, ready-mix saves a lot of mixing and gives a more consistent pour.
How deep should post holes be?
Roughly one third of the post's above-ground height, and always below the local frost line. For a 6 ft fence post, 24–36 in (60–90 cm) is typical.
How long before concrete can be used?
You can usually walk on it after 24–48 hours, but it reaches most of its strength at 7 days and full design strength around 28 days. Keep vehicles off for a week.