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How the fence calculator works
Everything follows from the post centres, because that is what fixes the geometry. Divide the run by the centres and round up — that gives the number of bays, and it guarantees the spacing is never wider than you asked for. Then add one post for the end of each bay, so a run of six bays needs seven posts. For a 12 m run at 1930 mm centres that is 7 bays and 8 posts, with the last bay cut down to 420 mm.
The panel count is where most fence estimates go wrong. A panel does not span the post centres — it spans the clear opening between two posts, which is the centres less the post thickness. With 1930 mm centres and a 100 mm post, the clear span is 1830 mm, so one standard panel fits each bay exactly. Set the centres at 3 m and the same panel needs two per bay, each cut down; the calculator does that division for you and rounds up.
Panel and post, or slotted concrete post?
The two styles are built differently and the material list changes with them. With timber panel and post, the posts sit between the panels, and each bay is carried by three arris rails screwed to the posts with feather-edge boards fixed to them. Ready-made panels can be screwed straight to the posts instead — in that case ignore the rail line on the shopping list.
With slotted concrete posts, both the panel and the gravel board drop into slots moulded into the post, so there are no rails, no screws into end grain, and nothing to rot at panel height. Concrete posts cost more and need a bigger hole, but they outlast the panels and let you swap a panel out in minutes if it is damaged. A concrete gravel board under the panel is what stops the bottom edge of the panel sitting in wet soil and rotting within a decade.
Post holes and postcrete
Post holes want to be around 600 mm deep and 200 mm across for a standard garden fence, and deeper on clay or on an exposed site. Postcrete is the fast option: pour it dry around the post, add water, and it sets in about ten minutes. One 20 kg bag fills roughly a 600 mm × 200 mm hole around a 100 mm post, which is why the default here is 2 bags per hole — a single bag often leaves the post short of the ground level. If you are mixing your own concrete instead, set the bags-per-hole box to 0 and use the concrete calculator for the footings.
Two last practicalities. Order the panels after you have set the posts, not before — hole positions drift when you hit a root, and a run that is 60 mm short at the far end is a cut panel. And leave a small gap at the bottom of any solid fence for air to move, or the panels will stay damp and the bottom rail will go first.
Fence FAQs
How many fence panels do I need for a 12 m run?
With 1830 mm panels at 1930 mm centres: 7 panels and 8 posts. The run divides into 7 bays of 1.93 m and the final bay is cut to 420 mm. If you are using slotted concrete posts at 1830 mm centres the same 12 m run needs 7 bays too, but at 1830 mm centres the bays come out slightly narrower.
Should posts be at 1830 mm or 1930 mm centres?
It depends on how the panel is held. 1830 mm centres suit slotted concrete posts, where the panel slots into the post and its true width is what spans the gap. 1930 mm centres suit timber posts that sit between the panels — a 1830 mm panel plus a 100 mm post. Measure the panel you are actually buying: some "6 ft" panels are 1800 mm.
How many bags of postcrete per fence post?
Allow two 20 kg bags for a standard 600 mm deep × 200 mm hole around a 100 mm post, and three for a heavier concrete post or a hole dug deeper on clay. Do not mix it in a bucket — postcrete is designed to be poured dry into the hole and watered from the top, then left alone to set.
Do I need gravel boards?
Yes, if the fence sits on soil or grass. A gravel board lifts the bottom edge of the panel clear of the ground, so the panel is not permanently damp and does not rot from the bottom rail up. It also gives you something to replace cheaply later. Concrete gravel boards last far longer than timber ones and cost little more over the life of the fence.
What are arris rails and do I need them?
Arris rails are the horizontal rails screwed between posts that carry feather-edge boards. A bay takes three: top, middle and bottom. If you are fitting ready-made panels, the panel has its own battens and the rails are not needed — that is why the calculator lists them separately.
How deep should fence posts be set?
A rule of thumb is a third of the post's above-ground height, with a minimum of 600 mm. A 1.8 m fence on a 2.4 m post should have 600 mm in the ground — and 750–900 mm on clay, on a slope, or on an exposed corner. Concrete the post so the top of the concrete is slightly proud and sloped away from the post, so water runs off rather than sitting against the timber.