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.
Fence post concrete calculator — frequently asked questions
How deep should fence posts be set in the ground?
The standard rule in Australia is one third of the total post length in the ground. For a 1.8 m fence with a 2.4 m post, that's 600 mm deep. In Queensland's cyclone-prone coastal areas, go deeper — 750 mm to 900 mm — and check your local council requirements for wind ratings.
How many bags of concrete do I need per fence post?
For a standard 100 mm post in a 300 mm diameter hole at 600 mm depth, you need roughly 1.5 to 2 bags of 20 kg rapid-set concrete per post. The calculator gives you the exact figure based on your post size, hole diameter and depth.
Should I use rapid-set or standard concrete for fence posts?
Rapid-set (also called post-fix or no-mix concrete) is the easiest option for DIY fence posts — pour it dry into the hole, add water, done in 20–30 minutes. Standard concrete mix gives a stronger result for heavy gates or exposed coastal sites but requires mixing and longer cure time.
How far apart should fence posts be?
1.8 m to 2.4 m between posts is standard for timber paling and Colorbond fences in Australia. Closer spacing (1.5 m) suits exposed or windy sites. Gate posts should be set no more than the gate width apart plus 50 mm clearance each side.
Do I need council approval for a fence in Queensland?
In Queensland, most dividing fences up to 2 m high don't need approval. Fences over 2 m, front boundary fences over certain heights, or fences near watercourses may require a development application. Check with your local council before you start — rules vary significantly between LGAs.