Fencing Calculator

How This Calculator Works

A straight fence run needs one more post than the number of spans between posts. Enter your fence’s total length and how far apart you want the posts (a common range is 6-8 feet), and this calculator returns exactly how many posts, rail sections, and rails to buy.

The Formula

$$\vD{\text{Spans}} = \left\lceil \frac{\vA{\text{Fence Length}}}{\vB{\text{Post Spacing}}} \right\rceil$$

Rounded up, since a fence can’t use a fractional span.

$\vE{\text{Posts Needed}} = \vD{\text{Spans}} + 1$ — the same reason a fence line with 3 gaps between posts needs 4 posts, not 3: each interior post is shared by the span on either side of it, but the two end posts each belong to only one span.

$$\vF{\text{Rail Sections}} = \vE{\text{Posts Needed}} - 1$$

$\text{Total Rails Needed} = \vF{\text{Rail Sections}} \times \vC{\text{Rails per Section}}$ (2 rails is common for a simple rail fence, 3 for a taller privacy fence)

$\text{Recommended Posts to Buy} = \vE{\text{Posts Needed}} + 1$ — a small one-post buffer for a damaged post or a miscount, the same waste-margin philosophy used elsewhere on this site, just as a fixed spare rather than a percentage since posts are large discrete units.

Worked Example

A 100 ft fence with posts every 8 ft, using 3 rails per section:

1. Spans: $\vA{100} \div \vB{8} = 12.5$, rounded up to $\vD{13}$ spans.
2. Posts needed: $\vD{13} + 1 = \vE{14}$ posts.
3. Rail sections: $\vE{14} - 1 = \vF{13}$ sections.
4. Total rails needed: $\vF{13} \times \vC{3} = 39$ rails.
5. Recommended posts to buy (with a spare): $\vE{14} + 1 = 15$ posts.