Key Takeaways
- Post count is fence length divided by spacing, rounded up, plus one
- A 100 foot fence at 8 foot spacing needs 14 posts, not 13
- Setting concrete is the hole volume minus the volume the post displaces
- Hole depth should follow local frost line rules, not a fixed guess
Post count is the one number that has to be right before a fence project starts, because a missing post means a return trip to the supplier, and an extra one means wasted material and an unplanned hole.
The plus-one rule
A fence run divides into sections by post spacing, and every section needs a post at each end. That means the post count is always one more than the number of sections, not equal to it. A 100 foot fence at 8 foot spacing has 100 / 8 = 12.5, rounded up to 13 sections, and 13 + 1 = 14 posts. It is a small detail that is easy to get wrong by counting sections instead of posts.
Concrete accounts for the post itself
The concrete needed to set a post is not the full volume of the hole: the post itself takes up some of that space, so the actual concrete volume is the hole volume minus the post’s volume. For a round hole and a square post, that is pi times the hole radius squared times depth, minus the post’s cross-section squared times depth. Skipping this subtraction overestimates concrete needs, sometimes significantly for a wide post in a tight hole.
Hole depth is a code question
How deep a post hole needs to be depends on the local frost line, the fence height, and the load it carries, and is set by local building code rather than a single rule of thumb. Many residential fence posts are set 24 to 36 inches deep, but this varies significantly by region; check with your local building department before digging, and always check for buried utility lines first.
Use the fence post calculator to work out post count and setting concrete for your own fence length and spacing. Once the posts are planned, the fence picket calculator works out how many pickets and how much rail the visible fence face needs.