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.
Concrete volume calculator — frequently asked questions
How do I calculate concrete volume for a post hole?
The formula is π × radius² × depth. For a 300 mm diameter hole (radius 0.15 m) at 600 mm deep: 3.14159 × 0.0225 × 0.6 = 0.042 m³. Then subtract the post volume if it's a large post. The calculator handles all of this automatically.
What mix ratio should I use for general concrete in Australia?
A 1:2:3 mix (1 part cement, 2 parts sand, 3 parts aggregate) gives a general purpose concrete suitable for footings, posts and non-structural slabs. For higher strength applications like driveways, use a 1:1.5:3 mix or specify a 25–32 MPa ready-mix.
How much does concrete shrink when it cures?
Concrete typically shrinks 1–3 mm per metre as it cures — small but enough to cause cracking in large pours without control joints. This is why the calculator adds a 10% buffer to the recommended order volume; shrinkage and spillage mean you always need slightly more than the raw calculation suggests.
Can I mix concrete by hand for post holes?
Yes — for individual post holes or small jobs under about 0.1 m³, hand mixing in a wheelbarrow or trough is practical. Anything larger benefits from a hired electric mixer. For jobs over 0.5 m³, pricing ready-mix is worthwhile — the cost difference is often less than you'd expect.
What is the minimum concrete strength for structural footings in Australia?
AS 3600 specifies a minimum of 20 MPa for residential footings in Australia. Most ready-mix suppliers offer 20, 25 and 32 MPa mixes — 25 MPa is a safe general choice for most residential footing and post applications.