How to measure for a fence
Measure each straight stretch of fence along the ground, from the outside of the first post to the outside of the last. Each straight stretch is a run: a fence down one side of the garden is one run, and a fence that turns a corner is two. Use Add another run for each one.
If the runs meet at corners, switch on The runs meet at corners and share a post, and each corner gets one post instead of two. Leave it off for separate fences, such as one down each side of the garden.
Measure a sloping run along the ground too. Panels are usually stepped down a slope, so the posts at the low end of each step stand taller; the calculator doesn’t allow for that, so check those posts by hand. If you measure in feet, switch the units at the top.
Recommended fence post sizes and depths
The posts hold the fence up, so their size and how deep they go matter more than anything else on the list. These are the typical figures from UK fence makers and merchants, including Jacksons Fencing, Forest Garden, Wickes and Jewson.
| Post | Hole width | In the ground | Post cap | Apply |
|---|---|---|---|---|
| Timber post75 mm square, the usual size | 225 mm | 525–600 mm | Yes | |
| Heavier timber post100 mm square | 300 mm | 525–600 mm | Yes | |
| Slotted concrete postAbout 100 mm; the panels slide into the slots | 300 mm | 525–600 mm | Not needed |
How long should fence posts be? Add the panel height, the gravel board if you fit one, and the depth in the ground, then round up to the next length sold, as Jacksons Fencing does. Fence makers bury 525–600 mm of each post; the calculator picks the shortest standard post that leaves at least the lower figure in the ground, adds 140–150 mm for a gravel board, and shows in the list how deep the posts actually go. That is why a 6 ft fence takes 2.4 m posts on its own but 2.7 m posts with gravel boards. You can saw a post down, but you can’t stretch one.
These figures are a guide, not a design. On exposed sites, in soft ground or for taller fences, check the post size and depth with your supplier or a fencing contractor.
How many fence posts do I need?
One more than the number of panels in each run. A run of five panels has six posts: one at each end and one between each pair of panels. Where two runs meet at a corner they can share the corner post, so a fence round two sides of a garden needs one post fewer than two separate runs.
The number of panels comes from the length of the run. Each panel takes up its own width plus one post, so the calculator works out (run − one post) ÷ (panel + post) and rounds up. That is the arithmetic behind any fencing calculator. UK panels are almost all 1.83 m wide, so the post width matters too: over a long run, 100 mm posts take up more room than 75 mm ones and can tip the count over by a panel.
Using it as a fence panel calculator
A run is rarely an exact number of panels, so the last one usually has to be cut down. The calculator counts it as a whole panel, because that is what you buy. If you would rather finish on a full panel, try a slightly shorter or longer run and watch the count change: that way it works as a fence panel calculator for planning the line, not just for ordering.
Spare panels adds an extra allowance to the panels only. Posts, caps and gravel boards are counted exactly.
Gravel boards and post caps
A gravel board is a plank about 140–150 mm high that sits under each panel and keeps it off the soil, and it is much easier to replace than a panel. The calculator lists one per panel. It raises the fence, so the posts must be longer by the same amount, usually one post length more than the same fence without gravel boards, cut to height once the fence is up.
Timber posts get a cap each to keep rain off the end grain; slotted concrete posts don’t need one. With concrete posts the gravel boards are concrete too and slot in between the posts, and the list shows them that way.
Post-fixing concrete
Fence posts are often set in bags of rapid-setting post-fixing concrete, which Tarmac sells as Postcrete, instead of concrete mixed by hand. Tarmac and Hanson say to buy at least 2 bags per post for a 6 ft panel, but that is a minimum: filling a round hole about three times the post’s width takes more. The calculator works out each post’s concrete from that hole, 225 mm across for a 75 mm post and 300 mm for a 100 mm or concrete post, dug to full depth, and never goes below the makers’ figure.
If your holes will be a different size, work out the bags with the postcrete calculator, which uses the same sums, and buy that instead.
Worked example: a fence round a corner
Here is the full calculation for two runs that meet at a corner and share the corner post, using 1.83 m high panels on 75 mm timber posts, with gravel boards. Every line comes from the same code that runs the calculator above.
- Run 1 (9.5 m − 75 mm post) ÷ (1.83 m panel + 75 mm post), rounded up = 5 panels, 6 posts
- Run 2 (6 m − 75 mm post) ÷ (1.83 m panel + 75 mm post), rounded up = 4 panels, 5 posts
- Shared corner posts 1 corner where runs meet = 10 posts in total
- Post length 1.83 m panel + 0.15 m gravel board + at least 0.525 m in the ground = 2.505 m → 2.7 m posts
- Concrete per post π × (225 mm ÷ 2)² × 600 mm hole, less the post, ÷ 10 litres a bag = 2.05 bags
- Post-fixing concrete 10 posts × 2.05 bags, rounded up = 21 bags
On the list:
- 9fence panels (6 ft (1.83 m) wide, 6 ft high (1.83 m))
- 10posts (75 × 75 mm timber, 2.7 m long)600 mm in the ground
- 9gravel boards (150 × 22 × 1830 mm)one under each panel
- 10post caps
- 21bags of post-fixing concrete (Post-fixing concrete, 20 kg bag)2.05 per post
Notice the shared corner post: the same two runs built as separate fences would need one post more. Notice too how much longer the posts are than the panels are high. The gravel board and the depth in the ground both add to the length, and the total is rounded up to the next post sold, so expect to cut the tops off once the panels are in.
UK fence panel and post sizes
Almost every fence panel sold in the UK is 1.83 m wide, in heights from 0.91 m to 1.83 m. Timber posts come 75 mm or 100 mm square, and slotted concrete posts in a few set lengths. Gravel boards are made to the width of the panels, and post-fixing concrete comes in 20 kg bags.
| Product | Size | Good for |
|---|---|---|
| 6 ft (1.83 m) wide | 1.83 m wide | The standard UK fence panel width. |
| 6 ft high (1.83 m) | 1.83 m high | Panel height. |
| 5 ft high (1.52 m) | 1.52 m high | Panel height. |
| 4 ft high (1.22 m) | 1.22 m high | Panel height. |
| 3 ft high (0.91 m) | 0.91 m high | Panel height. |
| 75 × 75 mm timber | 75 mm wide | Common timber post for panels. |
| 100 × 100 mm timber | 100 mm wide | Stronger timber post for exposed sites. |
| Slotted concrete, about 100 mm | 100 mm wide | Panels slide into the slots. |
| 1.8 m post | 1.8 m long | |
| 2.1 m post | 2.1 m long | |
| 2.4 m post | 2.4 m long | |
| 2.7 m post | 2.7 m long | |
| 3.0 m post | 3 m long | |
| 2.36 m post | 2.36 m long | |
| 2.665 m post | 2.665 m long | |
| 150 × 22 × 1830 mm | 0.15 m high | Sits under a panel to keep it off the soil. |
| Post-fixing concrete, 20 kg bag | 20 kg | Rapid-setting concrete for fence posts. |
Panel sizes are nominal. A panel can be a few millimetres over or under, and gravel boards from different makers differ slightly in length, so a run rarely comes out to the exact millimetre. Set the posts out to the panels you actually buy, not to the plan.
Common mistakes when ordering a fence
- Counting panels but not posts. Every run needs one more post than it has panels. Runs that meet at a corner can share a post; separate runs can’t.
- Posts too short. Add the panel, the gravel board and the depth in the ground before you choose a length. A post can be cut down, but not lengthened.
- Forgetting the part panel. Most runs end with a panel cut to fit, and you still have to buy the whole panel.
- Leaving gravel boards out of the post length. They lift the panel by 140–150 mm, so the posts must be that much longer too.
- No caps on timber posts. The open top of a post soaks up rain. A cap on each one helps it last.
- Buying only the makers’ minimum of concrete. 2 bags per post is the least the makers suggest. A full hole three times the post’s width takes more, and wider holes more still.
What else you'll need
- Post-fixing concrete for your own hole size: the postcrete calculator works out the bags for each post.
- Fence treatment to protect the panels and posts: the fence paint calculator gives litres and tins for one side or both.
- A gravel strip along the base of the fence, if you want an edge that’s easy to keep clear: the gravel calculator covers it.
- Fence clips or brackets and screws to fix the panels to timber posts. Slotted concrete posts don’t need them.
- A post-hole spade or auger, a spirit level and a string line to keep the posts upright and in line.
Last checked 28 October 2026