Concrete calculator
Volume for slabs, footings and post holes — converted into bags or a ready-mix order, with a straight answer on which of the two you should actually be buying.
How this calculator works
Bag yield is mixed volume, not bag volume
A 20 kg bag of concrete mix does not make 20 kg worth of anything obvious — it makes about 9.3 litres of placed concrete once water is added and the mix is compacted. That yield figure is the only number that matters when converting volume into bags, and it is the one most calculators get wrong by using dry density instead. The yields used here:
| Bag | Yield (L) | Yield (m³) | Bags per m³ |
|---|
The bags-versus-ready-mix line sits at about half a cubic metre
Below roughly 0.5 m³, bags win: no minimum charge, no delivery window, and you can pour at your own pace. Above it, bags stop making sense fast. Half a cubic metre is already around 54 twenty-kilogram bags — over a tonne to carry, and more mixing than one person can do before the first batch starts going off. This calculator shows both costs side by side and tells you which way the job falls.
There is a second, harder constraint people miss: a slab has to be poured in one continuous operation. If you cannot mix and place fast enough to keep a wet edge, you get a cold joint — a plane of weakness right through the slab. Two people with one mixer can realistically place about 0.3 m³ an hour. Work out your pour time before you decide.
Ready-mix is ordered in half units
Plants batch in half cubic metres (or half cubic yards), and most charge a part-load fee below two or three units. The calculator rounds your volume up to the next half unit, which is what you will actually be invoiced for. Ordering exactly to the calculated volume is a mistake — running out with the truck gone is a far worse outcome than a wheelbarrow of surplus, so the allowance exists for a reason.
Excavated depths always run over. A trench dug in soft ground will take noticeably more concrete than its nominal dimensions, because the sides slump and the base is never flat. Measure the trench you actually dug rather than the one you set out, and keep the allowance at 10% for anything poured straight against earth.
Common questions
How thick should a slab be?
For a domestic path or a shed base on well-compacted hardcore, 100 mm with a reinforcing mesh is the usual answer. A driveway taking cars wants 100–150 mm; anything carrying a commercial vehicle needs designing rather than guessing. Thickness is only half the story — a 150 mm slab on soft, unprepared ground will crack where a 100 mm slab on a properly compacted sub-base will not.
Do I need rebar or mesh?
Mesh for slabs, bars for footings and anything that spans. A light mesh sheet in the middle third of a slab's depth controls cracking; it does not make the slab stronger in bending unless it is positioned correctly, and mesh sitting on the ground at the bottom of the pour does nothing at all. Use proper spacers.
Can I pour concrete in cold weather?
Below about 5 °C the hydration reaction slows dramatically, and if fresh concrete freezes before it sets the damage is permanent. If you must pour in the cold, keep the mix warm, cover the pour with insulating blankets and expect to wait much longer before striking formwork or loading it. Hot weather is the opposite problem: the surface dries faster than it cures, so you need to keep it damp.
How long before I can walk on it?
Light foot traffic after 24–48 hours in normal conditions, vehicles after about a week, and full design strength at 28 days. Curing matters more than most people expect — keeping the surface damp for the first several days produces a significantly stronger and less dusty slab than letting it dry out in the sun.
Why subtract the post from a post hole?
Because a post set into the hole occupies volume the concrete does not have to fill. Over eight holes with a 100 mm post it is a small correction, but it is free to include and it stops you buying an extra bag. If the post sits on top of a pad rather than in the concrete, set it to zero.
Related
- Waste factor reference table
- Tile calculator — for what goes on top of the slab
Method last reviewed 2026-09-18. Estimates for planning only. Structural concrete — foundations, retaining walls, anything carrying a building — should be designed by an engineer.