Worked example: a 12 m fence run
Twelve metres of 1,830 mm panels on 100 mm posts at 1,930 mm centres, with 2 bags of Postcrete per hole.
| Item | Quantity | Why |
|---|---|---|
| Bays | 7 | Six full bays plus a part bay of 420 mm at the end of the run |
| Panels | 7 | One per bay — the last is cut down to 420 mm |
| Posts | 8 | One at each end and one between every bay: bays + 1 |
| Gravel boards | 7 | One per bay, cut to the 1,830 mm clear span |
| Arris rails | 21 | Three per bay for feather-edge boards |
| Postcrete | 16 bags | 2 bags per hole × 8 posts = 320 kg |
Why the post count is not the panel count
Panels sit between posts, so the posts always outnumber the panels by one: 7 bays need 8 posts, with one at each end of the run. Order the posts with the panels, or the fence stops one post short.
The part panel at the end
Twelve metres does not divide into whole 1,830 mm panels, so the last bay is 420 mm. That panel has to be cut down, which is why the calculator returns 7 panels and not fewer — the part bay is a whole panel you cut.
Setting the posts
Postcrete at 2 bags per hole is 16 bags, or 320 kg, for the 8 posts. Holes want to be about 600 mm deep and 200 mm across, and deeper in clay or made ground — a deeper hole takes more bags, so buy the Postcrete for the ground you actually have.
How the calculator works
- Enter the run length and the panel width.
- The bays are the run divided by the panel, rounded up.
- Posts are the bays plus one; gravel boards are one per bay.
- Rails are three per bay if you are building the panels up from boards.
- Postcrete is the bags per hole multiplied by the number of posts.
Where to go next
Fence FAQs
How many fence panels do I need for a 12 m run?
7. Six full 1,830 mm panels fill the run up to the last bay, which is 420 mm — so one panel is cut down to finish it.
How many posts does a 12 m fence need?
8. Posts outnumber panels by one: a post at each end of the run and one between every bay.
How many bags of Postcrete per fence post?
2 bags is the working figure, so 16 bags — about 320 kg — for a 12 m fence with 8 posts. Deeper holes in clay or made ground take more.
Do I need gravel boards on a panel fence?
One per bay is the calculator's default — 7 for a 12 m fence. They stop the bottom of the panel sitting in damp soil, which is what rots it.
How deep should fence posts be set?
600 mm is the working depth, and deeper in clay or on ground that has been made up. The hole should be about 200 mm across, with the post packed and braced while the Postcrete sets.
Guidance only. The quantities above are planning estimates from the ProjectCalc calculators, not a quote and not a specification. Check your own measurements and your supplier's current product data before ordering.