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.

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.

At $3.50 per 6-foot dog-ear picket: $861.

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.

For 14 posts: 28 bags. At $6 per bag: $168 in concrete.

Some people skip concrete entirely and tamp gravel around the posts instead. It drains better and the posts arguably last longer because water doesn’t pool against the wood at the concrete line. But concrete is standard practice and most inspectors expect it for permitted fences.

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 ($8-15 per pair) and one latch ($10-20). Self-closing hinges if code requires it (pool fences always require self-closing, self-latching gates per IRC G105.2).

Post caps: one per post, $2-8 each depending on style. Purely decorative but they prevent water from soaking into the end grain of the post.

Screws: figure 8-10 screws per picket (2-3 per rail attachment point). For 246 pickets: roughly 2,000-2,500 screws. A 5-pound box of #8 x 1-5/8” exterior deck screws (about 700-800 screws per box) costs $30-40. You’ll need three boxes.

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:

Material	Quantity	Unit Price	Total
4x4 x 8’ posts	12	$12	$144
6x6 x 8’ gate posts	2	$28	$56
2x4 x 8’ rails	39	$6	$234
1x6 x 6’ pickets	246	$3.50	$861
80lb concrete bags	32	$6	$192
Exterior screws (5lb box)	3	$35	$105
Gate hardware (hinges + latch)	1 set	$25	$25
Post caps	14	$4	$56
Total materials			$1,673

Labor to install a wood privacy fence runs $15-35 per linear foot in most markets. Our 100-foot fence: $1,500-3,500 for labor plus $1,673 for materials.

Running the Numbers

SiteCalc has a fence calculator that takes linear footage, height, spacing, and style, then gives you posts, rails, pickets, concrete bags, and hardware. It handles privacy, picket, and board-on-board styles. The budget calculator tells you how much fence a dollar amount buys, and you can export the list as a PDF for the supply house.

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