Ten panels need eleven posts
Every bay is a panel with a post beside it. Then the run has to be closed at the far end, which takes one more. So an open run always needs one more post than it needs panels, and the arithmetic never comes out even.
It sounds obvious written down and it is missed constantly, because panels and posts are sold side by side and it feels natural to buy them in matching numbers. The moment you find out is the end of the day, with the last panel up against the boundary and nothing to fix it to.
The exception is a run closed at both ends, between a house and a garage for instance, where the two end posts already exist. Then the count goes the other way. Be certain both ends really are closed before ordering on that basis, because a free-standing end you had forgotten about puts you right back to being a post short.
A post is not as long as the fence is tall
A third of the post goes in the ground. That is the rule, and there is a hard floor of 600mm underneath it for a domestic fence, because a short post in a shallow hole levers out in the first proper wind however the arithmetic works out.
That matters because the load on a fence is not weight pressing down, it is wind against a sail. The post is a lever and the concrete is the fulcrum, so depth does far more work than thickness.
So a 1.8 metre fence wants 900mm in the ground and a 2.7 metre post. Add a 150mm gravel board and the panel sits higher again, so either the fence gets taller or the post goes deeper. This is precisely why the rack carries 2.4 and 2.7 metre posts rather than 1.8s, and buying a post the height of the fence is the mistake this page exists to prevent.
Dig the holes square
A round hole the width of a spade holds far less concrete around the post than it looks like it does, and the concrete is the part doing the work. Three post widths square, to the depth above, is the shape to aim for.
The tool works the postcrete from your actual hole size less the volume the post itself takes up, rather than quoting a flat bags-per-post figure, because the hole depth changes with the fence height and the flat figure is only right for one of them.
It also totals the spoil, which surprises people. A dozen holes does not sound like much until it is in a pile on the lawn, and excavated soil swells once it is out of the ground.
Gravel boards and post caps, the two cheap things that decide how long it lasts
A gravel board keeps the bottom of the panel out of wet soil, and wet soil is where a fence rots from. A concrete gravel board under a timber panel is the combination that lasts, because the part in permanent contact with the ground becomes the part that does not care. They also cost a fraction of a panel and are far easier to replace.
A post cap keeps water off the end grain, and end grain is where a timber post drinks fastest. It is the cheapest life extension available on the whole fence and it is the thing most often left off, usually because the caps are the last item on the list and the day has run out.
Where to put the cut panel
Almost no real boundary divides evenly into 1.83 metre panels, so one of them gets cut. Put the cut one at the least visible end, next to the house rather than at the bottom of the garden where you look at it from the kitchen.
On a close board panel, cut from the plain side rather than the framed side, so the cut edge is the one nobody sees. And if the tool tells you the run comes out in whole panels, measure again on site anyway: a boundary 100mm out either way needs a cut panel, and it is much better to know that before the delivery than after it.
Two things to check before anything is ordered
Whose fence is it. The convention about which side the posts are on is a convention, not a rule of law, and it is wrong often enough to matter. Check the deeds before taking anything down, because replacing a neighbour's fence without asking is an expensive way to start a conversation.
How high you are allowed to go. 2 metres above ground level without planning permission, and only 1 metre where the fence adjoins a highway, which includes a footpath along the road. A 1.8 metre panel on a 150mm gravel board is 1.95 metres, which is closer to the limit than most people expect.
Common questions
How many posts do I need for 10 fence panels?
Eleven, on an open run. Every bay is a panel with a post beside it, and then the run has to be closed at the far end, so there is always one more post than panel. Buying them in matching numbers is the commonest fencing mistake there is, and you find out about it at the end of the day with a panel and nothing to hang it on. The exception is a run closed at both ends, between a house and a garage, where both end posts already exist.
What length posts do I need for a 6ft fence?
Not 6ft. A third of the post goes in the ground, so a 1.8 metre fence needs a 2.7 metre post, and that is before a gravel board raises the panel another 150mm. This is exactly why the rack carries 2.4 and 2.7 metre posts. Buying a post the same height as the fence is the single commonest mistake on the job.
How deep should fence posts go?
A third of the post length, and never less than 600mm on a domestic fence whatever the arithmetic says. A short post in a shallow hole levers straight out in the first proper wind, because the load on a fence is wind against a sail rather than weight pressing down. On anything 1.8 metres tall, 900mm in the ground is the working figure.
How many bags of postcrete per fence post?
Usually one to two for a hole three post widths square at the right depth, and the tool works it from your actual hole size less the volume the post takes up. Dig the holes square rather than round if you can: a round hole the width of a spade holds far less concrete around the post than it looks like it does, and the concrete is the part doing the work.
Do I need gravel boards?
They are the cheapest thing on the list and they are what stops the bottom of the panel sitting in wet soil, which is where a fence rots from. A concrete gravel board under a timber panel is the version that lasts, because the part touching the ground is then the part that does not care about water. Remember they raise the fence by 150mm, so the posts get longer too.
How high can a garden fence be without planning permission?
2 metres above ground level, and only 1 metre where the fence is next to a highway, which includes a footpath along the road. Worth checking before ordering 1.8 metre panels and putting them on a 150mm gravel board, because that is 1.95 metres and closer to the limit than most people realise.