Fence Post Calculator — Posts, Hole Depth & Concrete
Calculate how many fence posts you need, how deep to set them, and how much concrete per post hole. Enter fence length and post spacing to get post count and total concrete bags for the whole run.
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How the post count is calculated
- 1Post count: fence length ÷ post spacing + 1 end post, rounded up
- 2Hole depth: 1/3 of the post's above-ground height, minimum 600 mm (2 ft)
- 3Hole diameter: 3× the post width (a 100 mm post gets a 300 mm hole)
- 4Concrete per hole: π × (diameter/2)² × depth, minus the post volume
- 5Total concrete: per-hole volume × post count
- 6Gates need a larger post and a deeper footing on both sides
Worked example
Fence: 30 m (≈ 100 ft) long, posts at 2.4 m (8 ft) spacing, 200 mm diameter holes 600 mm deep.
14 posts (13 spans + 1). Concrete: 14 × 0.019 m³ = 0.27 m³ ≈ 23 bags of 25 kg premix.
Frequently asked questions
How many bags of concrete per fence post?
A 200 mm diameter hole at 600 mm deep takes about 0.019 m³ — roughly 1.6 bags of 25 kg premix or 1.5 bags of 60 lb mix per post. Larger 300 mm holes take about 3.5 bags of 25 kg.
How deep should fence posts be set?
One-third of the post's above-ground height, with 600 mm as a practical minimum. A 1.8 m (6 ft) fence: at least 600 mm deep in concrete. Loose or wet soils: 750 mm or more.
What post spacing should I use?
1.8 m for panel fencing (panels come 1.8 m wide), 2.4 m for post-and-rail and picket fences. Don't exceed 2.4 m on fences over 1.5 m tall — wind load bends the rails.
Wet-set or dry-pour concrete for posts?
Dry-pour (pour dry mix in the hole, add water) is fine for garden fences and is much faster. Wet-set gives higher, more predictable strength — use it for gates, tall fences and exposed sites.
Should I put gravel under fence posts?
Yes — 100–150 mm of compacted gravel under the post lets water drain away from the end grain, which is where timber posts rot first. Set the concrete on top of the gravel layer.