Fence Calculator
Posts, rails, pickets and the concrete to set them, for a run of fence. The post count includes both ends of the run, which is where most fence calculations go wrong.
Posts needed
14posts
- Posts
- 14
- Bays
- 13
- Rails
- 39
- Pickets
- 219
- Concrete
- 16.1 cu ft
- 50lb bags
- 43
- Posts include both ends of the run. A 24-foot fence at 8-foot spacing has three bays but four posts — the mistake worth avoiding here.
- Concrete assumes a 4x4 post in a round hole of the diameter and depth given, and subtracts the post itself.
- A gate needs heavier posts than a line post and usually a deeper hole. Price that separately rather than counting it as another bay.
About the fence calculator
Dividing a hundred feet by eight-foot spacing gives twelve and a half, so thirteen bays — but a fence with thirteen bays has fourteen posts, because there is one at each end as well as between every pair of bays. It is a classic bit of arithmetic and getting it wrong means discovering you are a post short with the last hole already dug.
Pickets work the other way round. Each occupies its own width plus whatever gap sits beside it, so a solid fence in 5.5-inch boards fits a picket every 5.5 inches while the same boards with a two-inch gap fit one every 7.5. Note that a nominal six-inch board actually measures 5.5.
The concrete figure treats each post hole as a cylinder and subtracts the post itself at its real 3.5-inch dimension. Gate posts carry a swinging load and need to be heavier and deeper, so price those separately rather than counting a gate as another bay.
What it works out
- Posts, bays, rails and pickets
- Concrete volume and 50lb bag count
- Any spacing, picket width and gap
The formula
Posts = (Run ÷ Spacing) + 1 Pickets = Run ÷ (Width + Gap)
The post count is where fence calculations go wrong, and the reason is a classic bit of arithmetic known as the fencepost problem. Dividing a hundred feet by eight-foot spacing gives you twelve and a half, so thirteen bays — but a fence with thirteen bays has fourteen posts, because there is a post at both ends as well as between every pair of bays.
Get that wrong and you are one post short, which you discover at the end of the run with the hole already dug.
Pickets work the other way. Each one occupies its own width plus whatever gap sits beside it, so a solid fence in 5.5-inch boards fits a picket every 5.5 inches, while the same boards with a 2-inch gap fit one every 7.5. Note that a nominal "six inch" board is actually 5.5 inches, and using the nominal figure leaves you short.
- Run
- Total length of fence. Measure each straight section separately if it turns corners.
- Spacing
- Centre to centre between posts. Six to eight feet is standard.
- Width
- Actual picket width, not nominal. A "6 inch" board measures 5.5.
- Gap
- Zero for a solid privacy fence, two to three inches for a picket fence.
A worked example
A hundred-foot privacy fence, posts at eight feet, three rails a bay, solid 5.5-inch boards.
That works out to 14 posts.
- Posts
- 14
- Bays
- 13
- Rails
- 39
- Pickets
- 219
- Concrete
- 16.1 cu ft
- 50lb bags
- 43
Questions
How many posts do I need for a 100 foot fence?
Fourteen at eight-foot spacing. A hundred feet divided by eight is 12.5, which rounds up to thirteen bays, and thirteen bays need fourteen posts because both ends of the run get one.
How far apart should fence posts be?
Six to eight feet centre to centre for most fences. Eight is common because rails come in eight-foot lengths, but go closer in exposed or windy positions, and closer again for heavy solid panels that catch the wind.
How deep should fence posts be?
Roughly a third of the post above ground, so a six-foot fence wants about two feet in the ground, and always below the local frost line. Frost heave will lift a shallow post out over a few winters regardless of how much concrete is around it.
How much concrete per fence post?
About one cubic foot for a 4x4 in a 10-inch hole 30 inches deep, once you subtract the post itself. That is roughly three 50lb bags per post.
What size hole should I dig for a fence post?
Around three times the post width, so a 10 to 12 inch hole for a 4x4. Wider than that just costs concrete; narrower leaves too thin a collar to hold the post steady.
Does this include gates?
No. Gate posts carry a swinging load and need to be heavier and set deeper than line posts, so price them separately rather than counting a gate as another bay.