Fence Calculator

Ten panels need eleven posts, and the depth rule barely moves below 2.4 m

A run of fence has a post at each end, so the post count is always panels plus one. Twenty metres at two-metre panels is ten panels and eleven posts — order ten and the fence stops a panel short. The burial depth has a similar quiet rule: it is a quarter of the height but never less than 600 mm, and for anything under 2.4 m tall the 600 mm floor is what actually applies.

How it works

• Divides the run by the panel width and rounds up, then adds the closing post.
• Sets the burial depth as the greater of 600 mm and a quarter of the fence height.
• Sizes the post hole at three times the post width, which is what the concrete fills.

panels = ⌈run ÷ panel width⌉
posts = panels + 1
depth = max(0.6 m, height × 0.25)
hole width = post width × 3

Worked example

A twenty-metre fence, 1.8 m high, in two-metre panels.

1. panels = 20 ÷ 2 = 10
2. posts = 10 + 1 = 11, because both ends need one
3. a quarter of 1.8 m is 0.45 m, which is below the 0.6 m minimum
4. so the depth is 0.60 m, set by the floor rather than the ratio
5. the ratio only takes over above 2.4 m, where a quarter of the height exceeds 600 mm

Eleven posts, each 600 mm in the ground. The extra post is the classic off-by-one in fencing, and it is the item most often short on delivery.

Reading the result

• A quarter of the height buried is a common rule of thumb, not a structural calculation. Exposed sites, soft or waterlogged ground, and tall close-boarded fences that act as sails all need more, and a gate post needs more again because it carries a moving load.
• The hole at three times the post width is what the concrete occupies, so it drives the volume you need to order. Widening the hole costs concrete quickly, since volume grows with the square of the width.
• Panel widths are nominal and posts have thickness. Over a long run those millimetres accumulate, so setting out from a string line and adjusting as you go beats trusting the arithmetic to the last post.
• Boundary fences are a legal matter as much as a practical one. Height limits, who owns which side, and whether permission is needed vary by country and often by street, so check before digging.

Common questions

How many fence posts do I need for a 20 m fence?

12 posts with standard 1.83 m (6 ft) panels. 20 ÷ 1.83 = 10.9, so you need 11 panels, and a run of 11 panels needs one more post than panels. The last panel will have to be trimmed, since 11 panels cover slightly more than 20 m.

How long should fence posts be for a 1.8 m fence?

2.4 m. The calculator buries a quarter of the fence height, but never less than 600 mm, and a quarter of 1.8 m is only 0.45 m, so the 600 mm minimum applies. A 2 m fence would need 2.6 m posts; only above 2.4 m does the quarter rule make the hole deeper than 600 mm. This is a rule of thumb, and exposed or soft ground calls for more.

How many bags of postcrete per fence post?

About five 20 kg bags per 100 mm post on a 1.8 m fence, on the calculator's assumptions. The hole is 300 × 300 mm and 600 mm deep, minus the post itself, which leaves 0.048 m³, and a 20 kg bag makes roughly 0.01 m³. Twelve posts come to 58 bags; a narrower hole needs noticeably less, since the volume grows with the square of its width.