How to Calculate Fence Panels, Posts and Postcrete
Work out fence panels, posts, gravel boards, feather-edge boards and post concrete for a UK garden fence, with clear formulas and examples.
Start With the Actual Fence Line
Measure the proposed fence along the ground, not from a plan or an old panel count. Record each straight run separately and mark:
- corners and changes of direction;
- gates and their clear opening widths;
- existing walls or posts that can genuinely form an end support;
- changes in level; and
- trees, drains, services and other obstructions.
That sketch is more useful than one total length because each run usually needs its own end post. The Fencing Calculator can calculate either panel fencing or close-board/feather-edge quantities once the runs are known.
Panel Fence Formula
For standard panels:
Panels = fence run ÷ panel width, rounded up
Posts = panels + 1 for a freestanding straight run
A common nominal panel width is 1.83m (6ft), but measure or check the ordered product. Concrete posts, slotted systems and proprietary composite fencing can create different centre-to-centre dimensions.
Worked Example: 18m Straight Run
For an 18m run using 1.83m panels:
18 ÷ 1.83 = 9.84, so order 10 panels
A freestanding straight run needs:
10 + 1 = 11 posts
If gravel boards run beneath every bay, order 10 gravel boards. Add caps, clips or cleats to match the post and panel system.
The final bay will be narrower than a full panel. Decide before ordering whether it will use a cut panel, a custom panel or adjusted bay positions. Do not spread the discrepancy across the whole run unless the system allows it and the appearance has been agreed.
Posts Around Gates and Corners
The simple “panels plus one” rule only covers one straight run. Add or separate posts for:
- both sides of a gate;
- every corner where one post cannot carry both directions;
- a change in height or fence system;
- a stepped section on sloping ground; and
- any free end not fixed to a suitable structure.
Gate posts are often larger or set deeper than ordinary intermediate posts. Use the gate manufacturer's requirements and account for hinges, latch clearance and the finished ground level.
Post Length and Concrete
Post length is the visible fence height plus the designed embedment, while also allowing for a gravel board or clearance where relevant.
Post length = height above ground + depth below ground
Builder Tools uses a 600mm below-ground allowance for its first-pass fence calculation. That is an estimating default, not a site design rule. Exposed sites, tall or solid fences, weak ground, retaining conditions and gate posts may require a different post, hole or foundation. Confirm the system instructions and site conditions before digging.
Bagged post concrete coverage varies with the product and hole size. The calculator uses a simple purchasing allowance of two bags per timber post or one per concrete post, but the reliable method is to calculate the hole volume and check the bag yield:
Hole volume = π × radius² × depth
Subtract the post volume if you need a closer quantity. Never reduce a foundation simply to match a convenient bag count.
Feather-Edge Board Formula
For feather-edge fencing, the overlap changes the effective cover of every board.
Effective board cover = board width − overlap
Boards = fence length ÷ effective cover, rounded up
For 150mm boards with a 25mm overlap:
Effective cover = 150 − 25 = 125mm, or 0.125m
For a 12m run:
12 ÷ 0.125 = 96 boards
Then calculate the bays and rails:
Bays = run length ÷ chosen post spacing, rounded up Posts = bays + 1 Rails = bays × rails per bay
If the design uses 2.4m bays and three rails per bay:
12 ÷ 2.4 = 5 bays Posts = 6 Rails = 5 × 3 = 15 rails
Add a small board allowance for rejected pieces and awkward cuts. Keep treatment class, fixings and cut-end treatment consistent with the specification.
Planning and Boundary Checks
In England, planning rules often treat a fence next to a highway differently from one elsewhere. Current local-authority guidance commonly identifies 1m next to a highway or footway and 2m elsewhere as important height thresholds, with further restrictions for listed buildings, Article 4 directions and planning conditions. Check the Planning Portal guidance and your own local planning authority before promising the work.
Planning permission does not decide who owns a boundary. Check title information, existing agreements and the customer's instructions. Avoid removing a neighbour's structure or fixing into it without clear authority.
Before digging, identify underground services and use the relevant safe-digging process. The HSE's HSG47 underground-services guidance covers planning the work, locating services and safe excavation. A tidy fence estimate is no substitute for locating electric, gas, water, drainage or communications routes.
Complete Ordering Checklist
- Panels or feather-edge boards
- Intermediate, corner, end and gate posts
- Gravel boards or kickboards
- Arris rails or cant rails
- Post concrete to the specified hole sizes
- Caps, clips, cleats and brackets
- Exterior-grade nails or screws
- Gate, hinges, latch and drop bolt
- Timber treatment for cut ends
- Disposal and excavation allowance
Use the Fencing Calculator for the quantity schedule, then carry the materials and labour into the Quote Generator.