Fence Calculator
Works out panels or bays, posts, rails, gravel boards and post concrete for a fence, and the cost. Covers panel, closeboard and post-and-rail fences.
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The formula
bays = ceiling((run − gates × 1 m) ÷ bay width) posts = bays + 1 + gates + extra corner posts post length = height + max(600 mm, height ÷ 3)
A fence has one more post than it has bays, the way a wall has one more stud than spaces. A gate takes the place of a bay but adds two posts of its own, which is why gates increase the post count rather than reducing it.
What to watch out for
What the count assumes, and where it goes wrong on site:
- Posts to the panel, not to a tape. Set the end posts, then space the intermediates by offering up a panel. Measuring at 1.83 m centres leaves every panel a few millimetres too tight.
- A third in the ground. 600 mm minimum. A 1.8 m fence takes a 2.4 m post. Solid fences on exposed sites need more, or concrete posts.
- Gravel boards. They keep the timber off the ground, and they are the difference between a panel lasting eight years and fifteen.
- Gates need their own posts. Heavier section, deeper hole, more concrete — a gate hangs off one post and slams against the other.
Frequently Asked Questions
How many fence panels do I need?
Divide the run by the panel width and round up, after taking off a bay for each gate. A 30 m run with one gate in 1.83 m panels is 16 bays — 17 panels with a spare — and 18 posts.
How deep should fence posts be?
About a third of the fence height, and never less than 600 mm. A 1.8 m fence takes a 2.4 m post with 600 mm buried; on an exposed site with solid panels, go to 750 mm.
How many bags of post mix per fence post?
Two 20 kg bags for a 600 mm hole around a 75 or 100 mm post, three for a larger or deeper hole. Gate posts usually take three or four.