Fence Post Calculator
Post depth is a third of the height above ground.
Post depth is a third of the height above ground. Posts for a fence run, from the panel width and the run length, with the count, linear metres and cost.
Set by the span table for your load, timber grade and section
Across 20 m at 1,800 mm centres
13 posts
12 gaps, so 13 members
Posts go at the panel width plus the post thickness, so the spacing is set by the panel rather than chosen. A run that does not divide evenly needs a cut panel, and putting that at the least visible end is worth planning. Post depth in the ground should be at least a third of the above-ground height, and corner and gate posts want more again because they carry sideways load.
How the Fence Post Calculator works
Posts for a fence run, from the panel width and the run length, with the count, linear metres and cost. Spacing is set by the panel rather than chosen, and a run that does not divide evenly needs a cut panel.
Also known as: how many fence posts do i need · fence panel and post calculator · how deep should a fence post go · fencing quantity calculator
Panel width sets the spacing
Fence posts are not spaced by preference. The panel fits between them, so the spacing is the panel width plus the post thickness — for a 1,800 mm panel with 100 mm posts, 1,900 mm centre to centre.
A run that does not divide evenly needs a cut panel, and where that goes is worth deciding before the first post is set rather than at the last one. The least visible end is usually the answer.
Post count is gaps plus one, the same arithmetic as studs and joists. A 20 m run at 1,900 mm centres takes eleven gaps and twelve posts, and the closing post is the one people forget.
Depth, and where posts actually fail
A common rule is at least a third of the above-ground height in the ground — 600 mm for a 1.8 m fence, 800 for a 2.4 m one. Corner and gate posts want more, because they carry sideways load rather than just vertical.
Timber posts fail almost universally at ground level, where wet and air meet. Below ground it is too wet for the fungi that cause decay and above ground too dry; the interface is where both conditions coexist.
That is why the detail at ground level matters more than the depth. Concrete shaped to shed water away from the post, or a free-draining gravel setting, both address the actual failure mechanism. Concrete that forms a bowl around the post makes it worse.
Slopes, gates and whose fence it is
On a slope a fence is either stepped, with each panel level and posts at different heights, or raked with panels following the ground. Stepped is easier with standard panels and leaves triangular gaps at the base that a gravel board can close.
Gate posts are a different problem. A gate applies constant twisting load that a panel does not, so they are usually a size up, set deeper, and often given a larger footing. A gate that drops within a year is almost always an under-specified post.
The question of which side is whose has no universal rule, despite the widespread belief about which side the posts face. It is determined by the deeds and the boundary plan, and assuming otherwise is behind a great many neighbour disputes.
Where to go next
The Fence Post question rarely arrives on its own. These are the ones that usually come with it:
- Deck Board Calculator — The gap between boards is not optional.
- Stud Spacing Calculator — Twelve gaps need thirteen studs.
- Concrete Slab Calculator — Dry materials occupy 54% more than the wet mix.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How far apart should fence posts be?
At the panel width plus the post thickness, since the panel fits between them. For a 1,800 mm panel with 100 mm posts that is 1,900 mm centre to centre.
How deep should a fence post go?
At least a third of the height above ground, so a 1.8 m fence wants 600 mm in the ground and a 2.4 m fence wants 800. Corner and gate posts want more, because they carry sideways load.
How many posts for a 20 metre fence?
Divide by the spacing and add one for the closing post. At 1,900 mm centres a 20 m run takes eleven gaps and twelve posts.
Should posts be concreted in?
For most fences yes, with the concrete shaped to shed water away from the post. Postcrete sets fast; a wetter mix is stronger but needs bracing while it cures.
What about a run that does not divide evenly?
One panel gets cut. Putting the cut panel at the least visible end is worth planning before starting rather than discovering at the last post.
Do gate posts need to be bigger?
Yes, noticeably. A gate applies a constant twisting load that a fence panel does not, so gate posts are usually a size up and set deeper, often with a larger concrete footing.
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