A fence run always needs one more post than it has sections: divide the length by the post spacing, round up for the sections, then add one. A 100 ft run at 8 ft spacing is 13 sections and 14 posts — plus an extra post at every corner and either side of every gate.
The off-by-one is the cheap mistake. The expensive one is the hole, because a fence post fails by rocking rather than by pulling out.
What spacing does the fence type allow?
Spacing is set by the material, not by preference, and going wider than the timber wants produces a fence that sags within a season.
| Fence type | Usual spacing |
|---|---|
| Panel fencing | Fixed at the panel width, usually 1.83 m |
| Rail and picket | Up to about 2.4 m |
| Closeboard with arris rails | Up to about 3 m |
Panel fencing is unforgiving: the posts must land exactly at the panel width or the panel does not fit, so the run is set out from one end and any short section is taken at the far end.
How deep should the hole be?
About a third of the post height, in a straight-sided hole roughly three times the post width across. For a standard 100 mm post standing 1.8 m above ground, that is a hole around 300 mm across and 600 mm deep, taking about 0.03 cubic metres of concrete.
The rule exists because a fence post is a lever. Wind pushes at the top and the buried section resists in two directions at once, so shallow posts do not pull out — they rock until the hole widens, and then the fence leans.
That is also why the hole should be a straight-sided cylinder rather than a cone. A cone-shaped hole gives the post somewhere to move as it works, and a tapered plug of concrete is easier to lever out of the ground than a parallel one.
Concrete or gravel?
Both work and they fail differently. Concrete gives the most resistance to movement and traps water against the timber at the collar, which is where posts rot. Compacted gravel drains, so the post lasts longer, and it gives less lateral support — which is fine for a picket fence and marginal for a 1.8 m closeboard in an exposed garden.
The compromise most people land on is concrete with the collar finished above ground level and sloped away, so water runs off rather than sitting in the joint. A post set flush in a dished collar is a post that will rot at the line.
What about the deck?
Board count is the easy half of a deck and the joists underneath are where it succeeds or fails. A 4 × 3 m deck with 150 mm boards laid at 155 mm centres over 3.6 m lengths needs 23 boards after a 10 per cent cutting allowance.
Joists want 400 mm centres for standard timber decking, dropping to 300 mm if the boards run diagonally, and often 300 mm for composite because it is less stiff than wood. Every joist needs support within its span, and the frame wants a slight fall away from the building so water does not stand.
The 5 mm gap between boards is drainage rather than expansion allowance, and it is why board coverage is calculated at 155 mm rather than 150. Leaving it out gives a deck that holds water and a board count that is 3 per cent short.
How do gates change the count?
A gate needs its own posts at both sides, and they should be heavier than the run posts and set deeper. A gate post carries a swinging load that the rest of the fence never sees, and it is the single most common point of failure in an otherwise sound fence.
Corners work the same way. A corner post takes tension from two directions and is not simply the last post of one run and the first of the next — it wants the same upgrade in size and depth.
Add both to the count before ordering. A 100 ft run with two corners and one gate is 14 posts plus two corner posts plus two gate posts, and the two gate posts are a different specification.
Questions people ask
How many bags of postcrete per post? Usually one to two for a standard 100 mm post in a 300 mm hole, depending on the product. Postcrete is poured dry and watered rather than mixed, which is why it is quoted per hole rather than by volume.
Can I use metal post spikes instead? For a light fence in firm ground, yes. For anything tall or exposed they are a compromise — they resist the lever far less well than a buried post, and they are difficult to drive plumb.
What gap between decking boards? Around 5 mm for timber, and follow the manufacturer for composite, which moves more with temperature and often wants more.
Should decking run across or along the house? Across, usually — running the boards perpendicular to the house means the joists run away from it, which is the direction you want the fall to go.
Count the sections, add the closing post, then add the corners and gates separately. The fence calculator handles the run and the spacing, the fence post calculator sizes the holes and the concrete, and the decking calculator covers boards, gaps and the cutting allowance.