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 0.0093 m³ when mixed — roughly 108 bags per cubic metre (the same industry figure used across our concrete calculators). 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.