How many fence posts do I need?
Split each straight run into equal sections no longer than your post spacing, then add one post. A 100-foot run with posts at most 8 feet apart is 13 sections (each 7' 8 5/16"), so it takes 14 posts. Where two runs meet at a corner they share a post, and every gate needs a post on each side.
Example: a backyard with two 60-foot sides and a 100-foot back with one 4-foot gate. Each side is 8 sections and 9 posts; the back is two 48-foot stretches of 6 sections each, either side of the gate. Sharing the two corners, that's 30 posts.
Post spacing
| Fence | Typical post spacing |
|---|---|
| Wood privacy or picket | 6 to 8 ft on center (8 ft suits 8-ft rails) |
| Board-on-board, tall or windy sites | 6 ft on center |
| Vinyl panels | Set by the panel: about 6 ft or 8 ft on center |
With vinyl, the panel width fixes the spacing, so the last section of a run is usually cut down. The calculator counts that cut panel separately so you know which section to measure last.
How deep should fence posts be?
A common rule is to bury a third to a half of the post's height above ground, and never less than 24 inches. A 6-foot fence with posts 24 inches deep needs 8-foot posts; go to 30 or 36 inches and you need 10-foot posts. Where the ground freezes, the bottom of the hole must sit below your local frost line, or frost will lift the posts. Your building department can tell you the depth.
Make the hole about three times the post's width — roughly 10 to 12 inches for a 4x4 and 16 to 18 inches for a 6x6 — and put a few inches of gravel in the bottom so water drains away from the end of the post.
How much concrete per fence post?
Work out the volume of the hole above the gravel, then subtract the post. A 10-inch hole with 20 inches of concrete around a 4x4 holds about 0.77 cu ft. Fast-setting concrete yields about 0.375 cu ft per 50-lb bag, so that's just over two bags — plan on three. An 80-lb bag of regular concrete mix yields about 0.60 cu ft. The calculator rounds bags up for each post, because a part bag left over from one hole rarely gets used in the next.
Rails and pickets
Fences up to about 5 feet tall use 2 rails per section, 6-foot fences use 3, and 8-foot fences use 4. Pickets are counted from the fenced length (gates are left out, since a gate is built or bought separately):
- Privacy: boards butt together — fence length ÷ board width.
- Spaced picket: fence length ÷ (board width + gap). A 2 to 3 inch gap is typical.
- Board-on-board: boards on both sides of the rails, each overlapping its neighbors by about a quarter of its width. That uses roughly 30% more boards than a privacy fence.
Each picket gets two fasteners at every rail, so a 3-rail fence uses 6 per picket.
Before you dig
Call 811 a few days before you dig so buried utility lines get marked — it's free. Confirm your property line, and check your town's height limits and any HOA rules. Many places cap backyard fences at 6 feet and front-yard fences at 3 to 4 feet.
Building a deck next to it? Try the deck board calculator. Pouring a patio slab? Use the concrete calculator.