How Deep Should a Concrete Pier Be?

Pier depth in Australia depends on soil class (per AS 2870), wind region, and load:

• Class A or S (sand/gravel): 450-600mm deep for fence posts, 600-750mm for deck/pergola
• Class M (moderately reactive): 600-750mm fence posts, 750-900mm deck/pergola
• Class H (highly reactive clay): 750-900mm fence, 900-1200mm deck/pergola, engineer often required
• Class P (problem): Always engineer-designed

Diameter rule of thumb: 3× the post width. So a 90mm post needs a 270-300mm pier.

Worked Example: Pergola Post Footings

You're installing 6 pergola posts on a Class M site in Brisbane. Each pier is 300mm diameter × 750mm deep.

• Volume per pier: π × (0.15)² × 0.75 = 0.053 m³
• Total: 0.053 × 6 = 0.318 m³
• Plus 5% wastage: 0.334 m³
• 20kg bags needed: 0.334 ÷ 0.011 = ~30 bags
• OR ready-mix: round up to 0.4 m³ (minimum 0.2 m³ truck delivery)

FAQs

How many bags of concrete in a 300mm × 600mm pier?

About 4-5 × 20kg pre-mix bags. The pier holds 0.042 m³ of concrete and each 20kg bag yields approximately 0.011 m³.

Is bagged concrete cheaper than ready-mix?

Bagged is cheaper for under 10 piers (under 0.5 m³ total). Ready-mix becomes cost-effective above that, with truck delivery typically $250-350 minimum charge.

Do I need an engineer for fence post piers?

Not for residential fences under 1.8m on Class A, S, or M soil. Engineer required for Class H/P sites, fences over 1.8m, or commercial work.

What MPa concrete should I use for piers?

20 MPa (N20) for residential fence and deck posts. 25 MPa for engineered structural piers. Higher MPa for commercial work per AS 3600.