Show the formula and working
This fence panel calculator works out how many fence panels, posts and gravel boards you need for a garden fence, how long the posts should be and how many bags of post mix to buy. It is built for UK and European fences using standard panels about 1.83 m (6 ft) wide, with a feet and inches switch, and it tells you how much to cut off the last panel so you are not surprised on the day.
Quick answer: Take off one post width from the fence length, then divide by the panel width plus one post width and round up. An 18 m run of 1.83 m panels with 100 mm posts needs 10 panels and 11 posts, with the last panel cut to about 0.43 m. The fence panel calculator also sizes posts and post mix.
What Is a Fence Panel Calculator?
A fence panel calculator is a tool that turns the length of a fence line into the number of panels and posts needed to build it. Each panel sits between two posts, so one bay takes up a panel width plus a post width, and the run needs one extra post at the end. Panels = (fence length minus one post width) / (panel width + post width), rounded up. Posts = panels + 1 for a continuous fence, because corner posts are shared.
Post length = panel height + gravel board height + the depth set into the ground. The tool then picks the nearest common stock length. Timber fence posts are widely sold in 75 mm and 100 mm sections at lengths such as 1.8 m, 2.1 m and 2.4 m, as listed in the Forest Garden fence post range, with longer posts available from many merchants.
How Do You Use the Fence Panel Calculator?
- Measure the whole fence line and enter it as Fence length.
- Set Number of straight sections: 1 for a straight fence, plus 1 for every corner.
- Enter the Panel width and Panel height from the product listing, and the Post width of your posts.
- Tick Gravel boards under the panels if you are using them and check the Gravel board height.
- Set Post depth in the ground. About a quarter of the post length is a common rule of thumb.
- Enter Bags of post mix per post from the instructions on the bag you plan to buy.
- Read the panels, posts and post mix, then open “Show the formula and working” to see each step with your numbers.
How Many Fence Panels Do I Need for 10 Metres?
A straight 10 m fence needs 6 panels of 1.83 m and 7 posts of 100 mm, but the sixth panel only needs to be about 0.15 m wide. Five full panels and six posts cover 9.75 m, so many people adjust the line slightly or fit a short custom panel. This is why counting bays, not dividing the length by 1.83, gives the right answer.
How Long Should Fence Posts Be?
Fence posts should be the fence height above ground plus the depth in the ground, and about a quarter of the post is usually buried. For a 1.83 m panel on a 150 mm gravel board with 600 mm in the ground, that is 2.58 m, so buy 2.7 m posts. Without a gravel board, a 2.4 m post leaves about 570 mm in the ground.
Panels and Posts by Fence Length
Straight runs with 1.83 m panels, 100 mm posts and 2 bags of post mix per post.
| Fence length | Panels | Posts | Last panel width | Post mix bags |
|---|---|---|---|---|
| 5 m | 3 | 4 | 0.94 m | 8 |
| 10 m | 6 | 7 | 0.15 m | 14 |
| 15 m | 8 | 9 | 1.29 m | 18 |
| 20 m | 11 | 12 | 0.50 m | 24 |
| 25 m | 13 | 14 | 1.64 m | 28 |
| 30 m | 16 | 17 | 0.85 m | 34 |
How Many Bags of Post Mix Do I Need per Post?
The number of bags of post mix per post depends on the hole size and depth, and the bag tells you how many to use, so enter that figure in the calculator. The default of 2 bags per post is an example for planning only. Wider or deeper holes, often needed in soft ground or for taller fences, take more. For holes in concrete instead of post mix, the concrete calculator in our building tools works out the volume.
Do You Need Planning Permission for a Fence?
In England you need planning permission for a fence over 2 m high from ground level, or over 1 m next to a road used by vehicles or its footpath. The Planning Portal guidance on fences, gates and garden walls also covers listed buildings and other exceptions. A 1.83 m panel on a 150 mm gravel board stands about 1.98 m high, just under the limit.
Before you put the fence up, our fence paint calculator works out how much paint or stain the new panels will need, and the paint and decorating calculators hub has tools for sheds and decking too.
Worked Example: An 18 m Garden Fence
Example: a straight 18 m fence is being replaced with 1.83 m by 1.83 m panels, 100 mm posts and 150 mm gravel boards. Panels = (18 m minus 0.1 m) / (1.83 m + 0.1 m) = 9.27, rounded up to 10 panels, so you need 11 posts and 10 gravel boards. Ten bays and eleven posts would cover 19.4 m, so the last panel is cut to 1.83 m minus 1.4 m = 0.43 m. Each post needs 1.83 m + 0.15 m + 0.6 m = 2.58 m, so buy 2.7 m posts, and at 2 bags each you need 22 bags of post mix. These are the default inputs.
Fence Panel Calculator FAQs
How wide is a standard fence panel?
A standard UK fence panel is 6 ft, sold as about 1.83 m wide, and heights commonly range from 3 ft (0.91 m) to 6 ft (1.83 m). Some contemporary slatted panels are 1.8 m wide instead, so always enter the exact width from the listing.
How many posts do I need for a fence?
You need one more post than the number of panels for a continuous fence, because every panel sits between two posts and corner posts are shared. Ten panels need eleven posts. A fence that forms a closed loop needs the same number of posts as panels.
Should I use gravel boards under fence panels?
Gravel boards keep the bottom of the panel off wet soil, so the panel lasts longer and a rotten board is cheaper to replace than a whole panel. They add height, usually about 150 mm, so you may need longer posts and should check the 2 m planning limit.
How deep should fence posts go in the ground?
A common rule of thumb is to bury about a quarter of the post, which is roughly 600 mm for a 6 ft fence. Go deeper in soft ground, on exposed sites or for taller fences. Concrete posts often come with the maker’s recommended depth, so follow that where given.
What size fence posts should I use?
For 6 ft panels, 100 mm square timber posts are widely used because a thicker post is stiffer in the wind, while 75 mm posts are common for lower fences. Slotted concrete posts let panels slide in and out for replacement. Enter your post width so the calculator spaces the panels correctly.
How do I fit a fence with corners?
Treat each straight stretch as a section and set the number of sections to the number of corners plus one. The calculator works out panels for each section and shares the corner posts. If your sections are very different lengths, run each one separately for the most accurate cut sizes.
Checked October 2026. Sources: Planning Portal: Fences, gates and garden walls, Forest Garden: 6ft fence panels (panel sizes), Forest Garden: 8ft fence posts (post sizes).