How many fence panels and posts do I need?

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.

Worked example: a 12 m fence run

A 12 m run of 1830 mm panels on 100 mm posts, 2 bags of Postcrete per hole.

How we got there

1. 12 m ÷ 1830 mm panels = 7 bays — Seven bays fit the run; the last bay is a part-panel of 0.42 m.
2. Posts: 8 — One more post than there are bays — a post at each end and between every bay.
3. Arris rails: 21 — Three rails per bay across 7 bays.
4. Postcrete: 16 bags — 2 bags per post × 8 posts = 320 kg.

What you need to buy

The order for the worked example above, straight from this calculator's own shopping list.

• Fence panels, 1,830 mm wide — 7 panels (7 bays of 1 panel each — the last bay is 420 mm, so cut one panel down.)
• Fence posts, treated timber (100 mm) — 8 posts (At 1,930 mm centres: 7 bays plus one post at each end.)
• Gravel boards — 7 boards (One per bay, cut to the 1,830 mm clear span between posts.)
• Arris rails (3 per bay) — 21 rails (Carry feather-edge boards. Not needed if you are screwing ready-made panels to the posts.)
• Feather-edge / clout nails (galvanised) — 360 nails (about 2 boxes of 250) (Rule of thumb: about 30 nails per metre of fence — one or two per board at each rail. Galvanised, or the fixings streak the timber within a year.)
• Coach screws for rails to posts — 42 screws (Rule of thumb: two per rail per post, so 21 rails take about 42. Stainless or galvanised, with a washer — a black screw into wet-treated timber does not last.)
• Postcrete (20 kg bags) — 16 bags (≈ 320 kg — 2 bags per post hole (8 holes).)
• Post hole digging / spoil removal — as needed (Holes want to be 600 mm deep and about 200 mm across — deeper in clay or on an exposed site.)

Figures for the worked example only. Change the measurements above and the list recalculates.

Waste and over-ordering

Panels and posts are bought as whole units, so the waste is the part-panel at the end of the run — here 0.42 m. Rotten or rocky ground can need a deeper hole than planned, which uses more Postcrete.

Regs & planning for this calculator

Fence heights are set by planning rules, and fence posts are where the Party Wall Act usually bites.

• Planning — fences, walls and gates 2 metres generally, and 1 metre adjacent to a highway used by vehicular traffic. Read the requirement.
• Party Wall etc. Act 1996 — boundary excavations Excavating within three metres (or six, on the 45-degree rule) of a neighbour's structure, and the one month's notice. Read the requirement.