How to Calculate Fence Materials

A fence has three structural components — posts, rails, and pickets — plus the concrete to set the posts. Getting the count right before you go to the lumber yard saves two extra trips.

// Post count
Posts = CEIL(fence length ÷ post spacing) + 1

// Sections (spans between posts)
Sections = Posts - 1

// Rails (per section)
Rails = Sections × rails per section

Pickets = CEIL(fence length × 12 ÷ (picket width + gap))

The Post Depth Rule — Critical for Longevity

The #1 cause of fence failure is posts that aren't buried deep enough. Follow this rule:

Fence Height	Post Length Needed	Bury Depth	Above Ground
4 ft fence	7 ft post	2.5–3 ft	4–4.5 ft
6 ft fence	9 ft post	3 ft minimum	6 ft
8 ft fence	12 ft post	3.5–4 ft	8 ft

In frost regions: Bury posts at least 6 inches below the local frost line. Frost heave lifts posts that aren't deep enough, and a fence that shifts in winter won't shift back in spring.

Post Spacing: 6 ft vs. 8 ft

6 ft spacing — Stronger, better wind resistance, required for gates and heavy-panel fences. Uses ~33% more posts.
8 ft spacing — Standard for most residential privacy fences. Rails span the full 8 ft between posts; works well with standard 8-ft 2×4 rails.

Use 6-ft spacing wherever you have a gate, at corners, and at both ends of the fence run.

Installation Order That Saves Time

Set corner and end posts first. These anchor the layout.
Run a string line between them at the desired fence height.
Mark post hole positions along the string at your chosen spacing.
Dig all holes before setting any posts. Digging is the hardest part — get it done in one pass.
Set posts in concrete. Plumb each post before the concrete sets. Brace tall posts.
Let concrete cure 24–48 hours before hanging rails and pickets.
Attach rails, then pickets. Snap a chalk line for consistent picket height.

Frequently Asked Questions

How many fence posts do I need for 100 feet of fence?

At 8-foot spacing: 100 ÷ 8 = 12.5, rounded up to 13 sections, which means 14 posts. At 6-foot spacing: 100 ÷ 6 = 16.7, so 17 sections = 18 posts. Always add extra posts for gate openings and corners. This calculator handles all of that automatically.

How deep should fence posts be set?

The general rule is 1/3 of the total post length, or a minimum of 2 feet for any fence. For a 6-foot privacy fence, use 9-foot posts buried 3 feet deep. In frost-prone regions, always go at least 6 inches below the frost line — call your local building department or check a frost map for your area's frost depth.

How many bags of concrete per post?

For a 4×4 post in an 8–10 inch diameter hole, 2 bags of 80 lb concrete is standard. For larger post diameters (12+ inch holes or 6×6 posts), use 3 bags. Fast-setting concrete (Quikrete Fast-Set, Sakrete Fast-Setting) can be poured dry directly into the wet hole and set in 20–40 minutes — no mixing required.

Do I need a permit to build a fence?

In most US cities and counties, fences over a certain height (typically 6 feet in front yards, 8 feet in backyards) require a permit. HOA communities often have strict fence rules on material, color, and height. Always check with your local building department and HOA before purchasing materials — getting this wrong means tearing it down.

What wood is best for fence pickets?

Pressure-treated pine is the most economical and durable choice for ground-contact fence posts. For pickets and rails above ground, cedar is preferred — it's naturally rot-resistant, takes stain beautifully, and won't warp as badly as untreated pine. Redwood is excellent but expensive and hard to find outside the West Coast. All wood fences should be sealed or stained within 6 months of installation.

🚧 Fence Calculator

How to Calculate Fence Materials

The Post Depth Rule — Critical for Longevity

Post Spacing: 6 ft vs. 8 ft

Installation Order That Saves Time

Frequently Asked Questions