Concrete Calculator
Work out how much concrete you need in cubic metres for a slab, footing or post holes – plus the number of 20 kg premix concrete bags and an estimated cost in AUD.
Formulas shown on this page · Last reviewed October 2026
How the concrete calculator works
volume (m³) = length (m) × width (m) × thickness (mm) ÷ 1,000
bags = volume × (1 + waste) ÷ bag yield
A 20 kg bag of dry concrete mix makes roughly 0.0093 m³ of concrete, so you need about 108 bags per cubic metre. Check the yield printed on the bag – it varies a little between brands.
Typical slab thickness
| Project | Thickness |
|---|---|
| Garden path, shed base | 75–100 mm |
| Patio | 100 mm |
| Driveway | 100–125 mm with reinforcing mesh (SL72 or SL82 is common) |
| Garage floor | 100–150 mm |
Bags vs ready-mix
Bags from Bunnings and local landscape yards are convenient for post holes and small pads under about 0.5 m³. Above that, a ready-mix truck is usually cheaper and gives a more consistent pour. Many ready-mix suppliers charge a short-load fee for orders under about 3 m³.
At the default prices, 1 m³ costs about $972 in 20 kg bags versus about $330 delivered as ready-mix. Prices vary by region, so enter your local quotes above.
Frequently asked questions
How many 20 kg bags of concrete are in a cubic metre?
About 108 bags. A 20 kg bag yields roughly 0.0093 m³ once mixed.
How much concrete do I need for a 3 × 3 m slab?
At 100 mm thick, a 3 × 3 m slab needs 0.9 m³ – about 0.99 m³ with 10% extra, or 107 bags of 20 kg.
How much extra concrete should I order?
Add 5–10% for spillage, uneven ground and formwork bowing. Running short mid-pour is far more costly than a little left over.
How do I convert mm to metres for the formula?
Divide millimetres by 1,000: 100 mm = 0.1 m, 125 mm = 0.125 m, 150 mm = 0.15 m.