How this calculator works
The formula: (fence length ÷ post spacing) + 1 end post + 1 bag per corner extra. Each post hole needs a set amount of concrete that depends on the fence type and soil.
For a 24 m Colorbond fence with 2.4 m post spacing in clay soil, you need 11 posts (10 bays + 1 end post). At 1.5 bags per hole, that's 17 bags of rapid-set concrete. Add 1 extra bag per corner for added bracing strength.
Hole size and depth by soil type
Australian fencing standards recommend these hole specs for 1.8 m high Colorbond with 2.4 m post spacing:
- Rock: Ø200 mm × 400 mm deep. 1 bag per hole usually enough.
- Clay / normal soil: Ø200 mm × 600 mm deep. 1.5 bags per hole.
- Sand / loose fill: Ø200 mm × 900 mm deep. 2 bags per hole (deeper = more concrete).
Timber posts generally need slightly less concrete than Colorbond channel posts because the timber itself displaces hole volume. Pool fencing (1.2 m high, smaller posts) needs about 1 bag per post regardless of soil.
Using rapid-set concrete
Rapid-set concrete (Cement Australia Rapid Set, Boral Rapid Set, Bunnings generic) is the standard choice for fence posts. Key advantages:
- Sets in 15 minutes — no overnight bracing required
- No mixing — pour dry into hole, add water, done
- Cheap — around $7-9 per 20 kg bag at Bunnings
The process: dig hole → 50 mm gravel at the bottom for drainage → place post and brace it plumb → add about 2 L of water per bag to the hole → pour the concrete around the post → tamp → top up with more water if needed. Do NOT mix rapid-set in a wheelbarrow; it's designed to go straight in the hole.
Post spacing by fence type
- Colorbond: 2.4 m standard (matches panel width). Never wider.
- Timber paling: 2.4 m standard for 1.8 m fences. Reduce to 1.8-2.1 m for heavier or taller fences.
- Pool fencing: Varies by panel — typically 1.2-1.8 m.
- Chain wire / rural: 3-4 m between strainers.
Check your council rules before you dig
Boundary fences under 2 m high generally don't need council approval in most Australian councils, but front fences over 1.2 m often do. Corner blocks have extra visibility restrictions. Check with your local council (Brisbane, Gold Coast, Sunshine Coast all have online planning rules) before you buy materials.
If you're replacing a boundary fence between you and a neighbour, the Dividing Fences Act (varies by state) typically requires you to give written notice first and split the cost 50/50 for like-for-like replacement.