How to calculate concrete volume for Australian projects
Concrete volume is simply length × width × thickness, all in metres. A standard 100 mm slab under a garden shed (3 m × 4 m) works out to 1.2 m³ before waste margin. For post holes, the calculator uses the cylinder formula: π × radius² × depth × number of holes.
How many 20 kg bags per cubic metre?
A 20 kg bag of pre-mixed concrete (Boral, Cement Australia, or Rapid Set from Bunnings) yields approximately 0.0095 m³ when mixed — roughly 105 bags per cubic metre. For small pours like post holes and letterbox footings, bags are practical and easy. Above about 0.5 m³, hiring a mixer or ordering a short-load from a ready-mix supplier is usually cheaper and faster.
Slab thickness — what's right for my project?
- 75–100 mm — garden path, light foot traffic only
- 100 mm — standard residential slab, shed base, alfresco area
- 125–150 mm — carport, light vehicle loads
- 150–200 mm — driveway with regular car or truck traffic
Always check with your local council and a structural engineer for anything load-bearing or larger than 10 m².
Post hole sizing guide
The general rule: hole diameter 3× the post width, depth at least one-third of post height above ground (minimum 400 mm). For a 1.8 m Colorbond fence: 300 mm diameter × 600 mm deep. For a 2.4 m gate post: 400 mm diameter × 800 mm deep.
When to order ready-mix instead
Ready-mix is typically cheaper per m³ above 0.5–1 m³. Most suppliers deliver a minimum of 0.2–0.3 m³ with a short-load surcharge below 1 m³. Get three quotes — prices vary significantly by region across Australia.