Answer first
What this calculator tells you
Estimate the posts, pickets and rails a fence needs from its length and layout. Turn a fence length into a shopping list before you price lumber. Formula: Sections = ⌈length ÷ post spacing⌉; posts = sections + 1; pickets = ⌈length × 12 ÷ (picket width + gap)⌉; rails = sections × rails per section. At the worked-example inputs, the posts needed is 14. Holding every other input steady, moving fence length (ft) from 80 to 120 moves the result from 11 to 16.
Transparent method
The formula
Turn a fence length into a shopping list before you price lumber.
Worked example
Example inputs
How to interpret the result
A fence is a run of sections with a post at each end, so posts always outnumber sections by one. A 100-foot run at 8-foot spacing needs 13 sections and 14 posts. Pickets depend on width plus gap: 5.5-inch boards with half-inch gaps need 200 across the run. Rails multiply by the number of sections, which is where lumber counts climb.
At the worked-example inputs the posts needed is 14. It rises with fence length (ft) and falls as post spacing (ft) increases; picket width (in), gap between pickets (in) and rails per section do not move it.
These are material-quantity estimates. Always add a waste and overage margin, and confirm actual coverage with your specific product and supplier before purchasing.
Before you rely on it
What to check
Count gates and corners separately. Each adds posts and changes the section count, so the total here is for a straight run only.
The common error
Where people go wrong with fence calculator
Dividing length by spacing and stopping there. That counts the sections, not the posts, and leaves you one post short at the end.
Sensitivity evidence
How fence length (ft) changes the posts needed
Holding every other input at the worked-example value, moving fence length (ft) from 80 to 120 moves the posts needed from 11 to 16: a spread of 5, or 36% of the worked-example result.
| Fence length (ft) | Posts needed | Pickets needed | Rails needed |
|---|---|---|---|
| 80 | 11 | 160 | 20 |
| 90 | 13 | 180 | 24 |
| 100worked example | 14 | 200 | 26 |
| 110 | 15 | 220 | 28 |
| 120 | 16 | 240 | 30 |
Every input, tested
Which input moves the posts needed most
Of the 5 inputs, post spacing (ft) moves the posts needed most (3 across the range tested) and fence length (ft) moves it least (2). Picket width (in), gap between pickets (in) and rails per section do not change it at all.
| Input | Tested from | To | Posts needed at each end | Swing |
|---|---|---|---|---|
| Post spacing (ft) | 7 | 9 | 16 to 13 | 3 (21%) |
| Fence length (ft) | 90 | 110 | 13 to 15 | 2 (14%) |
| Picket width (in) | 5 | 6.1 | 14 to 14 | none |
| Gap between pickets (in) | 0.45 | 0.55 | 14 to 14 | none |
| Rails per section | 1 | 3 | 14 to 14 | none |
Two variables at once
Posts needed by fence length (ft) and post spacing (ft)
Across the grid the posts needed runs from 9 to 21. Moving fence length (ft) from 80 to 120 shifts it by 5 at the middle column, and moving post spacing (ft) from 6 to 10 shifts it by 7 at the middle row, so post spacing (ft) is the bigger lever here.
| Fence length (ft) \ Post spacing (ft) | 6 | 8 | 10 |
|---|---|---|---|
| 80 | 15 | 11 | 9 |
| 90 | 16 | 13 | 10 |
| 100 | 18 | 14 | 11 |
| 110 | 20 | 15 | 12 |
| 120 | 21 | 16 | 13 |
The highlighted cell is the worked example: 14.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Fence length (ft) | 100 | Enter the fence length (ft) used in this calculation. |
| Post spacing (ft) | 8 | Distance between posts. 6 to 8 feet is common. |
| Picket width (in) | 5.5 | Enter the picket width (in) used in this calculation. |
| Gap between pickets (in) | 0.5 | Use 0 for a solid privacy fence. |
| Rails per section | 2 | Two for a fence up to 4 feet, three for taller fences. |
| Posts needed | 14 | |
| Pickets needed | 200 | |
| Rails needed | 26 | |
Inputs, definitions and assumptions
Fence length (ft)
Enter the fence length (ft) used in this calculation. The prefilled worked-example value is 100.
Post spacing (ft)
Distance between posts. 6 to 8 feet is common. The prefilled worked-example value is 8.
Picket width (in)
Enter the picket width (in) used in this calculation. The prefilled worked-example value is 5.5.
Gap between pickets (in)
Use 0 for a solid privacy fence. The prefilled worked-example value is 0.5.
Rails per section
Two for a fence up to 4 feet, three for taller fences. The prefilled worked-example value is 2.
How to use this calculator
- 1Verify the inputs. Gather fence length (ft), post spacing (ft), picket width (in), gap between pickets (in) and rails per section from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the posts needed at 14. Store your own version of it as Scenario A.
- 3Test one change. Start with post spacing (ft), the input with the biggest effect here: moving post spacing (ft) from 7 to 9 takes the posts needed from 16 to 13, a swing of 21% of the worked-example figure.
- 4Check the extremes. At half the example post spacing (ft) (4) the posts needed is 26; at double (16) it is 8.
People also ask
Frequently asked questions
How do you calculate fence?
Sections = ⌈length ÷ post spacing⌉; posts = sections + 1; pickets = ⌈length × 12 ÷ (picket width + gap)⌉; rails = sections × rails per section. At the worked-example inputs the posts needed is 14.
What does the fence result mean?
Turn a fence length into a shopping list before you price lumber. At the worked-example inputs the posts needed is 14. It rises with fence length (ft) and falls as post spacing (ft) increases; picket width (in), gap between pickets (in) and rails per section do not move it.
How much does fence length (ft) change the posts needed?
Holding every other input at the worked-example value, moving fence length (ft) from 80 to 120 moves the posts needed from 11 to 16, a spread of 5.
What are the limits of this fence calculator?
These are material-quantity estimates. Always add a waste and overage margin, and confirm actual coverage with your specific product and supplier before purchasing. The tables on this page test fence length (ft) only from 80 to 120; a value outside that range is not tabulated here.
Which input moves the posts needed most in the fence calculator?
Ranked by how far each moves the posts needed across the range tested: post spacing (ft) (3, 21%) and fence length (ft) (2, 14%). Picket width (in), gap between pickets (in) and rails per section do not change it.
If I double post spacing (ft) in the fence calculator, does the posts needed double?
Doubling it from 8 to 16 takes the posts needed from 14 to 8, which is 0.57 times the worked-example figure. So it falls instead of rising. Halving it to 4 gives 26.
How much does post spacing (ft) matter in the fence calculator?
The worked example uses 8. With the other inputs left at the worked example, moving post spacing (ft) from 7 to 9 takes the posts needed from 16 to 13, a swing of 21% of the worked-example figure.
How much does picket width (in) matter in the fence calculator?
The worked example uses 5.5. The posts needed does not depend on picket width (in); it moves the pickets needed from 221 to 184 instead when picket width (in) goes from 5 to 6.1.
How much does gap between pickets (in) matter in the fence calculator?
The worked example uses 0.5. The posts needed does not depend on gap between pickets (in); it moves the pickets needed from 202 to 199 instead when gap between pickets (in) goes from 0.45 to 0.55.
How much does rails per section matter in the fence calculator?
The worked example uses 2. The posts needed does not depend on rails per section; it moves the rails needed from 13 to 39 instead when rails per section goes from 1 to 3.
Which inputs change the pickets needed in the fence calculator?
At the worked-example inputs it is 200. Fence length (ft) takes it from 180 to 220, picket width (in) takes it from 221 to 184 and gap between pickets (in) takes it from 202 to 199.
Which inputs change the rails needed in the fence calculator?
At the worked-example inputs it is 26. Fence length (ft) takes it from 24 to 28, post spacing (ft) takes it from 30 to 24 and rails per section takes it from 13 to 39.
How much extra should I buy for tile waste?
10% is a common baseline for a straightforward rectangular room in a simple grid layout. Diagonal layouts, rooms with lots of cuts around fixtures, or a pattern that requires matching increase that. 15-20% is common for more complex jobs.
Sources and evidence
Free Calculators Online is independent and is not affiliated with or endorsed by the source organizations. Educational estimates only.