How this is worked out
Each bay is one panel plus one post. Panels = fence length ÷ (panel width + post width), rounded up. Posts = panels + 1, plus any extras you add for corners or gates.
Postmix defaults to one 20kg bag per post, which is Tarmac’s guidance for Blue Circle Postmix in a standard fence post hole.
Worked example
A 20m fence with 1.83m panels and 100mm posts: 20 ÷ 1.93 = 10.4, so 11 panels, 12 posts and 12 bags of postmix. The last bay is about 0.7m, so one panel gets cut down.
Before you start
- Check where the boundary is and who owns the fence. Your deeds or the Land Registry title plan may show it with a “T” mark.
- In England a fence up to 2m high doesn’t usually need planning permission, or up to 1m next to a road used by vehicles. Listed buildings and conservation areas have tighter rules.
- Call before you dig if there could be cables or pipes along the line.
Questions people ask
How many fence panels do I need for 20 metres?
With 1.83m (6ft) panels and 100mm posts each bay is 1.93m, so 20m needs 11 panels and 12 posts. The last panel will need trimming slightly.
How many bags of postmix per fence post?
Tarmac says one 20kg bag of Blue Circle Postmix per standard fence post. Gate posts and posts in loose soil often get two.
How deep should fence posts go?
A common rule is a quarter of the post’s length in the ground, so 600mm for a 2.4m post holding a 1.8m panel. Exposed sites benefit from going deeper.
Do I need gravel boards?
They’re optional but worth it. A concrete or treated timber board at the bottom keeps the panel off wet soil, and it’s far cheaper to replace than a rotten panel.
Sources
Trade review pending. A named tradesperson will check this calculator, and their name and qualifications will appear here.
Figures last checked 25 October 2026. Read how we calculate and our disclaimer. Found a mistake? Tell us.