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.
How to use this calculator
- Enter the number of posts (holes). For a deck with 8 stumps, enter 8. For a fence with 10 posts, enter 10.
- Enter the hole diameter in millimetres. Measure the inside diameter of the holes you plan to dig. Common sizes: 200 mm (fence posts), 300 mm (deck/pergola posts), 450 mm (heavy structural posts).
- Enter the hole depth in millimetres. This is the depth from ground level to the bottom of the hole. Typical residential: 450–600 mm for fences, 600–900 mm for decks.
- Select your bag size preference and check the wastage box if desired.
- Click Calculate. Results show m³ per hole, total m³, and bag counts.
Worked example: You are building a deck with 6 posts, using 300 mm diameter × 750 mm deep holes. Volume per hole: π × (0.15)² × 0.75 = 0.053 m³. Total for 6 holes: 0.318 m³. With 10% wastage: 0.350 m³. That is 35 × 20 kg bags or 28 × 25 kg bags. At this volume, it is often worth getting a mini-mix or ordering a small quantity of ready-mix rather than hand-mixing 35 bags individually.
Understanding your results
The per-footing volume tells you how much concrete each individual hole requires — useful for staging work or pouring one hole at a time. The total volume is the sum for all holes combined, which is what you need when ordering from a supplier.
Post hole footings are cylindrical, so the volume grows with the square of the diameter — doubling the hole diameter from 200 mm to 400 mm quadruples the concrete needed. This is why getting the hole diameter right before digging is important; over-sized holes can dramatically increase your material cost.
Common mistakes: Entering the diameter in centimetres instead of millimetres (a 30 cm hole is 300 mm); forgetting to include wastage for uneven or over-dug holes (cylindrical holes rarely come out perfectly round); and not allowing for the post itself — the calculator calculates the full cylinder volume, which includes the space the post will occupy. In practice, for post footings set in wet concrete, the post displacement is minor and is accounted for in the wastage factor.
Post hole footings in Australia — what you need to know
Post hole footings are one of the most common concrete tasks on Australian residential sites — they anchor fence posts, deck stumps, pergola posts, letterbox poles, and clothesline supports. Despite their apparent simplicity, getting footings right is essential for structural performance and compliance.
Footing depth: The standard rule of thumb for residential work is that the footing depth should be at least one-third of the above-ground post height, with a minimum of 450 mm for fence posts and 600 mm for deck and pergola posts. In loose, sandy, or clay soils (common across much of coastal and eastern Australia), deeper footings may be required. Your local council or a structural engineer can advise for your specific soil conditions. AS 2870 classifies reactive soil sites from Class S (stable) through to Class E (extreme) — reactive soils in Melbourne, Adelaide, and parts of Brisbane require more conservative footing designs.
Hole diameter: The hole should be at least 3× the post width to provide adequate concrete cover on all sides. A 100 × 100 mm timber post needs a minimum 300 mm diameter hole. Using a smaller hole is a common mistake that results in inadequate bearing area and increased risk of post movement.
Concrete placement: The standard Australian method is to pour rapid-set or general purpose premix concrete around the post in the hole and tamp to remove air pockets. The top of the footing should be finished with a slight fall away from the post to shed water and reduce moisture-related decay at the base. For timber posts in ground contact, use H4 or H5 treated timber rated for in-ground use — standard H3 treated pine is not rated for concrete-encased applications.
Permits: Footings for structures requiring a building permit (decks, pergolas, sheds above threshold size) are part of the permitted work and must be inspected before concrete is poured. Check with your local council and engage a registered building surveyor in most states.
Australian standards and references
- AS 2870:2011 — Residential Slabs and Footings: soil classification and minimum footing depth requirements for residential applications.
- NCC Volume 2 (Building Code of Australia) — requirements for Class 1 and Class 10 building footings, including acceptable construction manuals.
- AS 1684.2:2010 — Residential Timber-Framed Construction: references post footing requirements for timber framed structures.
- Boral/Cockburn product data sheets — bag yield figures: 20 kg = 0.010 m³, 25 kg = 0.0125 m³.
- Local council requirements — minimum footing depths and diameters may be specified for your soil zone; check with your council before digging.