Bays plus one
Divide 100 feet by 8-foot spacing and you get 12.5. Round up to 13 and order 13 posts, and you will be one short — because 13 is the number of bays, and a run of 13 bays has 14 posts. One at each end and one between every pair of panels.
The calculator applies bays + 1 automatically. Two adjustments remain yours. Corners are shared between two runs, so calculate each straight run separately and subtract one post for every corner where they meet, or you will double-count. Gates need a post on each side, and both should be heavier than the line posts — a gate hangs its full weight off one post and swings it back and forth for years.
Depth and diameter
Two rules cover almost every domestic fence. The hole should be about three times the width of the post, so a 100 mm post wants a 250–300 mm hole. The depth should be about a third of the post's above-ground height, which for a 1.8 m fence means roughly 600 mm in the ground — so you are buying 2.4 m posts.
Where the ground freezes, the frost line overrides the rule of thumb entirely. Water in the soil expands as it freezes and will lift a shallow post over a few winters no matter how much concrete surrounds it. Required depth varies enormously by region and is a question for your local building department.
Volume grows with the square of the diameter, so widening the hole costs more concrete than deepening it. Going from a 250 mm to a 300 mm hole is 44% more concrete, not 20%.
| Post | Hole diameter | Depth for a 1.8 m fence | Concrete each |
|---|---|---|---|
| 75 mm | 200 mm | 600 mm | about 18 L |
| 100 mm | 250 mm | 600 mm | about 28 L |
| 100 mm | 300 mm | 750 mm | about 50 L |
| 150 mm | 400 mm | 900 mm | about 110 L |
Wind is what actually breaks fences
A solid fence is a sail. The load on each post is the panel area times the wind pressure, applied as leverage at the top of a post held only at the bottom. That is why fences fail at the post, at ground level, almost every time.
On an exposed site, closer post spacing and deeper holes do far more for survival than extra concrete in a hole that is too shallow. Slotted or louvred panels that let some air through reduce the load substantially and are worth considering where wind is a real factor.
Once you are past about twenty bags of concrete, price a ready-mix delivery or hire a mixer — a 100-foot fence is over a tonne of material to move by hand, twice.
Common questions
How many fence posts for 100 feet?
At 8-foot spacing, 14 posts — 13 bays plus one. Dividing length by spacing alone always leaves you one short.
How deep should a fence post go?
About a third of the above-ground height, so roughly 600 mm for a 1.8 m fence — deeper if your ground freezes below that.
How wide should the hole be?
About three times the post width. A 100 mm post wants a 250–300 mm hole.