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Fence Calculator: Posts, Rails, Pickets, and Concrete
The Formula
Number of Posts = Fence Length (ft) / Post Spacing (ft) + 1
The +1 accounts for both end posts.
A 100-foot fence at 8 feet on center: 100 / 8 + 1 = 13.5, round up to 14 posts.
That’s the starting point. Add posts for every corner and every gate.
Post Spacing
Wood fences: 8 feet maximum between posts. Six feet is better if you’re using 4x4 posts, especially for 6-foot tall privacy fences where wind load is significant. The post flex on a 6-foot tall, 8-foot span in a 40 mph wind is visible and unsettling.
Chain link: 10 feet between line posts. Terminal posts (ends, corners, gates) need bracing.
Vinyl: follows manufacturer specs, usually 6-8 feet. Vinyl panels are sold in fixed lengths, so the post spacing must match the panel size.
Rails
Wood fences need horizontal rails between posts to attach the pickets.
- Fences under 5 feet: 2 rails (top and bottom)
- Fences 5-6 feet: 2 or 3 rails. Two works structurally, three reduces picket warping.
- Fences over 6 feet: 3 rails
Rail count: Number of Sections x Rails per Section
Sections = posts minus 1. Our 14-post fence has 13 sections. At 3 rails per section: 39 rails.
Rails are typically 2x4 lumber (for flat-rail construction) or 2x3 (for standard picket fence). An 8-foot section uses one 8-foot rail per slot. If your post spacing is 6 feet, a standard 8-foot 2x4 gets trimmed to 6 feet with 2 feet of waste. That waste adds up. Consider 10-foot or 12-foot lumber for non-standard spacing to minimize cutoffs. If you’re buying in volume and the yard quotes by board feet instead of per piece, the board feet calculator handles the conversion.
Pickets
Privacy Fence (No Gap)
Pickets per section = Section Width (inches) / Picket Width (inches)
Standard picket is 5-1/2 inches wide (a 1x6 actual). An 8-foot section:
96 inches / 5.5 inches = 17.5 pickets. Round up to 18 per section.
In practice, you overlap each picket by about 1/4” so there are zero gaps. That means slightly more pickets per section. Budget 18-19 per 8-foot span.
For 13 sections: 13 x 18 = 234 pickets. Add 5% waste: 246 pickets.
Picket Fence (With Gaps)
A traditional picket fence with 3.5” wide pickets and 3.5” gaps (equal to the picket width):
Pickets per section = Section Width / (Picket Width + Gap Width)
96 / (3.5 + 3.5) = 13.7, round to 14 per section.
Thirteen sections x 14 = 182 pickets. Plus waste: 191.
Board-on-Board
Both sides get pickets, offset so the gap on one side is covered by a picket on the other. Uses roughly 1.5 to 1.8 times the material of a flat privacy fence.
For 13 sections at 18 pickets per side per section: 13 x 18 x 1.7 = 398 pickets. This is why board-on-board fences cost 70-80% more in materials.
Concrete Per Post
Most fence posts set in concrete. Post holes are typically 10-12 inches in diameter and one-third to one-half the total post length deep.
For a 6-foot fence with 4x4 posts (8 feet long, 2 feet buried): Hole: 10” diameter, 24” deep.
Volume per hole: Pi x (5”)^2 x 24” = 1,885 cubic inches = 1.09 cubic feet.
An 80-pound bag of Quikrete makes 0.6 cubic feet. Two bags per hole. The same per-bag math applies to any small concrete pour — see how to calculate concrete for a slab for the bags-vs-truck breakpoint on larger pours.
For 14 posts: 28 bags.
Some people skip concrete and tamp gravel around the posts instead. It drains better, and water doesn’t sit against the wood at a concrete collar. Concrete is more common. Check whether your area has a fence permit or post-depth rule, and call 811 before you dig.
Gate Posts
Gate posts need to be bigger. A 4x4 post flexing under the weight of a gate sags within a year. Use 6x6 posts for gates, set 6 inches deeper than line posts, with more concrete.
Every gate needs two posts. A 100-foot fence with one single gate: 14 line posts minus 2 (replaced by gate posts) plus 2 gate posts = 14 total. But the gate posts are 6x6 instead of 4x4, and each gets 3 bags of concrete instead of 2.
Hardware
Gate hardware per gate: two hinges and one latch. Pool barriers need self-closing, self-latching gates under the International Swimming Pool and Spa Code (ISPSC), which most jurisdictions use for residential pools.
Post caps: one per post. Besides looks, they keep water out of the end grain.
Screws: 2 per rail per picket, so 6 per picket with 3 rails. For 246 pickets: about 1,500 screws. Check the count per box; it varies with screw size.
Nails work too (two 8d galvanized ring-shank nails per attachment), and a pneumatic nailer is much faster. But screws hold better and don’t pop out as the wood shrinks.
Full Material List
100-foot privacy fence, 6 feet tall, 8-foot post spacing, 3 rails, one gate:
| Material | Quantity |
|---|---|
| 4x4 x 8’ posts | 12 |
| 6x6 x 8’ gate posts | 2 |
| 2x4 x 8’ rails | 39 |
| 1x6 x 6’ pickets | 246 |
| 80 lb concrete bags | 32 |
| Exterior screws | about 1,500 |
| Gate hinges and latch | 1 set |
| Post caps | 14 |
Running the Numbers
Trussly has a Fence Posts calculator (posts, sections, and concrete per hole) and a Picket Fence calculator (pickets by width and gap, rails, posts, and fasteners). Board-on-board isn’t built in; double the picket count and adjust for overlap.