Fence Calculator — posts, rails and pickets

How it works

A straight fence is a line of equal sections between posts, so the counts follow simple geometry:

• Sections = fence length ÷ post spacing, rounded up (a partial last section still needs its own posts). Posts are commonly spaced 6 to 8 feet apart.
• Posts = sections + 1. This is the fencepost principle: a run of n sections has a post at the start of each, plus one more to close the far end — the classic reason it is easy to be off by one.
• Rails (the horizontal stringers) = sections × rails per section. Solid privacy fences use 3; ranch-rail fences use 2.
• Pickets = fence length ÷ (picket width + gap), rounded up. A solid fence uses no gap; a spaced or shadowbox fence leaves a gap between boards.

Enter your dimensions and the calculator gives the materials for the run. Order a little extra to cover cuts and corners, and remember that gate openings and corners may need additional posts.

Worked example

For a 100 ft fence with posts every 8 ft: 100 ÷ 8 = 12.5, rounded up to 13 sections, so 14 posts (13 + 1). With 3 rails per section that is 13 × 3 = 39 rails. Using 5.5-inch pickets with a 0.25-inch gap, the pitch is 5.75 inches, so pickets = 100 × 12 ÷ 5.75 ≈ 209.

Frequently asked questions

How many fence posts do I need?

Divide the fence length by the post spacing and round up to get the sections, then add one: posts = sections + 1. For a 100-foot fence with 8-foot spacing that is 13 sections and 14 posts.

Why is it sections plus one post?

Because each section has a post at its start, and one more is needed to finish the far end — like the posts along a fence, there is always one more post than the number of gaps between them.

How many pickets do I need?

Divide the fence length (in inches) by the picket width plus the gap between pickets, and round up. A 100-foot fence with 5.5-inch pickets and a 0.25-inch gap needs about 209 pickets.

What post spacing should I use?

Most wood fences use 6 to 8 feet between posts; 8 feet is standard for pre-cut rails. Closer spacing makes a sturdier fence, wider spacing uses fewer posts but longer rails.

How many rails per section?

Fences 6 feet and taller usually use 3 rails; shorter or ranch-style fences often use 2. Enter the number your design calls for and the calculator multiplies it by the sections.

Should I add extra for waste?

Yes — order a little more than the estimate to allow for cuts, damaged boards, gates and corners, which need extra posts. A 5–10% cushion on pickets is common.