How many fence panels and posts do I need?

Two errors account for most fence material mistakes: forgetting the extra post at the end of the run, and measuring panel width rather than post centres. Both are avoided by working from the post centres.

Step by step

1. Measure the run, not the boundary.Tape along the actual fence line, corner to corner, and note each separate straight run. A boundary with a return is two runs and needs the maths done twice.
2. Work in post centres.A standard 1830mm (6ft) panel plus a 100mm post gives 1930mm centres for face-fixed timber posts, or 1880mm for slotted concrete posts where the panel sits inside the slot. Confirm against the actual products.
3. Divide and round down.Run length divided by the post centre spacing, rounded down, is the number of full panels. The remainder is a cut panel at one end.
4. Add one post.Every run needs one more post than it has panels. A ten-panel run takes eleven posts. This is the single most common ordering error.
5. Match gravel boards to panels.One gravel board per bay, same length as the clear gap between posts, plus one cut board for the odd bay.
6. Work out the concrete.A 300mm square hole 600mm deep is 0.054 cubic metres per post. Postcrete covers roughly one hole per one to two bags depending on hole size — check the bag. Mixed concrete needs about 130kg of ballast and cement per hole.
7. Add the fixings and caps.Six panel clips per panel is a sensible allowance, one post cap per post, and a box of exterior screws. Add a litre or two of end-grain preserver for cuts.
8. Add a contingency.One spare panel, two spare posts and 10% extra fixings. Returning surplus is easier than a second trip mid-job.

Tips

• Sketch the run with the posts numbered before ordering. It takes five minutes and catches the missing end post every time.
• Put the cut panel at the least visible end of the run, usually against the house or a corner.
• Check the delivery vehicle can reach the site. Concrete posts and boards are not carried far by hand.

Common mistakes

• Ordering posts equal to the number of panels — A fence needs a post at each end as well as between panels, so it is always panels plus one. This is the mistake that stops the job on delivery day.
• Using panel width as the post spacing — The post itself takes up space between panels. Set out on post centres — panel width plus post width for face-fixed, panel width plus slot depth for concrete.

If it doesn't work

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