Concrete Footing Calculator Australia — Post Holes & Bag Count
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How to calculate concrete for post hole footings in Australia
Post hole footings are cylindrical, so the volume formula is π × radius² × depth. Divide the diameter in millimetres by 2000 to get the radius in metres, then apply the formula to get m³ per hole. Multiply by the number of posts for total volume. One 20 kg premix bag covers 0.010 m³ and one 25 kg bag covers 0.0125 m³, so bag count is simply total m³ divided by the bag yield. Always add at least 10% for waste, over-excavation and uneven hole walls — it's cheaper than a second trip to the hardware store.
Post hole footing calculator — FAQs
How deep should post hole footings be in Australia?
Depth depends on post height, load and soil type. A general rule for residential work is that the footing depth should be at least one-third of the post height above ground, with a minimum of 450 mm. For deck and pergola posts in stable soils, 600 mm is typical. In reactive or loose soils (Class H, E or P under AS 2870) an engineer may specify 900 mm or deeper. Always check your local council and NCC requirements before digging.
What diameter post hole do I need for a fence post?
For a standard 75 × 75 mm timber fence post, a 200 mm diameter hole is the minimum — it gives roughly 60 mm of concrete around the post on all sides. For 100 × 100 mm posts a 250–300 mm hole is more appropriate. Structural posts for decks and pergolas typically need 300–450 mm diameter holes to provide adequate bearing area, especially in softer soils. Some engineers specify a belled-out (under-reamed) footing at the bottom for extra uplift resistance.
How many 20 kg bags do I need per post hole?
It depends on hole size. A 200 mm diameter × 600 mm deep hole is approximately 0.019 m³ — that's 2 × 20 kg bags. A 300 mm × 600 mm hole is about 0.042 m³ — 5 × 20 kg bags. A 450 mm × 900 mm hole is around 0.143 m³ — 15 × 20 kg bags. Use the calculator above with your exact dimensions rather than relying on rule-of-thumb figures.
Should I use premix bags or order ready-mix concrete for footings?
Premix bags are cost-effective and practical for small jobs — up to about 10–15 post holes. Beyond that, ready-mix from a batch plant (Boral, Holcim, Cockburn Cement) is usually more economical per m³ and less labour-intensive. A typical minimum order for ready-mix is 0.2–0.3 m³ depending on the supplier. For large decks or pergolas with many footings, get a quote from your local concrete supplier to compare.
Do I need a building permit for post footings in Australia?
If the footings are for a structure that requires a building permit (deck, pergola, shed over a certain size), then yes — the footings are part of that permitted work. A standalone fence under 2.1 m high generally does not need a permit in most states, but setback rules and council requirements still apply. Always check with your local council before starting any structural footing work. In most states the footing design must comply with AS 2870 for residential sites.