Offcut WorksFence panel and post calculator
Work out how many panels, posts and gravel boards a run of panel fencing needs, and how long the posts should be.
How the quantities are worked out
Panels = fence length ÷ post spacing, rounded up. Posts = panels + 1 for a straight run. A gate splits the run in two, so each gate adds one more post. Gravel boards = one per panel. Corner posts are shared, so a corner does not add a post.
Post length = panel height + gravel board height + depth in the ground. For a 1.8 m panel, a 150 mm gravel board and 600 mm in the ground that is 2.55 m, so you would choose the next length up that your supplier sells.
Panel width and post spacing
A standard UK fence panel is described as 6 ft, which is 1.83 m wide. The post spacing depends on how the panel is held, and that is why sources quote different figures:
- Slotted posts: panel width + post width − twice the slot depth. A 1.83 m panel in 100 mm posts with 25 mm slots gives 1.83 + 0.10 − 0.05 = 1.88 m. This matches the 1880 mm capping rail Jacksons sells for intermediate slotted posts.
- Between posts, on clips or brackets: panel width + post width, so 1.83 + 0.10 = 1.93 m.
- Fixed to the face of the posts: the panel overlaps the posts, so the spacing is the panel width, 1.83 m.
Many calculators and guides simply say 1.83 m, which is only right for the third case. Measure your panel and post, check the slot depth if your posts are slotted, and use the spacing that matches how yours fit together.
Height limits
Under Part 2, Class A of the planning Order in England, a fence can be up to 2 metres high without planning permission. Where it is adjacent to a highway used by vehicles, the limit is 1 metre. The Order gives no distance for "adjacent", and councils judge it case by case, so ask yours if the fence is near a road. The rules do not apply within the curtilage of a listed building, and planning conditions or Article 4 directions can remove them. If the fence sits on a retaining wall, the height of both together counts. This page does not cover who owns a boundary.
Quick check: does my fence need planning permission?
England only. Based on Part 2, Class A of the planning Order.
Gravel boards
A gravel board is a board at the bottom of the fence that keeps the panel off wet ground, which can cause rot, and protects it from mowers and strimmers. Concrete gravel boards slot between slotted concrete posts, and timber gravel boards are fixed to timber posts.
Slopes and gates
- Sloping ground. Panels step down the slope, or are raked, and posts and gravel boards stack to suit. Work out the number of bays along the slope and ask your supplier about stepped panels.
- Gates. Gate posts are usually heavier than line posts and need a bigger concrete base. Order them separately.
- The last panel. The calculator shows how much of the last bay is left over. You may need to cut a panel to fit, or adjust the spacing.
Questions
How many fence panels do I need?
Divide the fence length by the post spacing and round up. For 12 m at 1.88 m spacing that is 7 panels.
How many posts do I need?
One more than the number of panels for a straight run. Each gate adds one more post.
How far apart should fence posts be?
It depends on how the panel is held. For a 1.83 m panel it is about 1.88 m in 100 mm slotted posts with 25 mm slots, 1.93 m between 100 mm posts on clips, and 1.83 m if the panel is fixed to the face of the posts.
Do I need gravel boards?
They are recommended because they keep the panel off wet ground, which helps it last longer.
How much concrete do I need for the posts?
Use the fence post concrete calculator. It works from your hole size instead of a fixed number of bags.
Where the defaults come from
- Jacksons Fencing, Good Fencing Guide: fence posts (PDF). Slotted posts are 100 x 100 mm with two 54 x 25 mm slots for panels. Panels are 1.83 m wide (Jacksons panel spec sheets).
- Planning Order, Schedule 2 Part 2 Class A, gates, fences and walls (text reproduced by Peak District National Park Authority, PDF). The wording of the height limits: 1 metre next to a highway used by vehicles, 2 metres elsewhere.
- Barker Gillette: do I need planning permission to make changes to my garden?. A law firm's summary of the fence height limits. Checked against the wording of the Order above.
- GOV.UK: permitted development rights for householders, technical guidance. Confirms that gates, fences and walls are covered by Part 2 of the Order rather than the householder classes.
- UK Calculator: fence calculator. Panel width of 1.83 m, posts equal panels plus one, and 2.4 m posts with 600 mm buried for a 1.8 m fence.
- Squote: concrete posts and gravel boards. Standard slotted concrete post lengths, 1.83 m centres for 6 ft panels, and a 2.55 m post for a 6 ft fence with a 150 mm gravel board.
- Bendrey Bros: concrete gravel boards. Concrete gravel board size and contractors' bag allowances.
- The 1.88 m and 1.93 m spacings are geometry (panel + post, less the slots) rather than a published table. The 1.88 m result matches the 1880 mm capping rail length Jacksons lists for intermediate slotted posts. Other makers' slot depths may differ, so check yours.
- The 600 mm depth in the ground is a common figure. It is consistent with Jacksons selling slotted posts 0.6 m longer than the fence height they are for (for example a 1.8 m post for a 1.2 m fence), but ground and fence type can change it.
- How close counts as adjacent to a highway is not defined in the Order. Some council decisions have treated a fence set back around 3 m as not adjacent, but that is a judgment for your local planning authority.