How to Calculate Fence Post Spacing
Getting post spacing right determines how strong your fence stands and how professional it looks. Space posts too far apart and rails sag; too close and you waste material. Here's how to work it out precisely.
Step 1 — Choose your post spacing
Post spacing depends on fence type, height, and wind exposure. These are the industry-standard spacings used by professional fencing contractors:
Step 2 — Calculate number of posts
The formula for a straight fence run is:
The ⌈ ⌉ symbol means round up to the nearest whole number. Always round up — never down — or your last section will be too wide.
Example: 100 ft fence, 8 ft spacing:
If your fence includes gates, add 2 posts per gate opening — one on each side.
Step 3 — Adjust to even bay spacing
After calculating post count, work backwards to get perfectly even spacing. This prevents an awkward short bay at the end of your run.
Example: 100 ft fence, 14 posts:
Step 4 — Calculate post length and burial depth
Every post needs to be buried deep enough to stay rigid. The rule is:
Quikrete's post-setting instructions put the hole depth at 1/3 of the overall post length. If a third of the post is underground, the two-thirds above ground is the fence height — so burial depth is half the fence height. A 6 ft fence needs posts buried 3 ft, and a 9 ft post (cut from a 10 ft length).
In cold climates, burial depth must go below the local frost line even if that is deeper than the 1/3 rule. Frost lines range from 24 inches in mild regions to 48+ inches in northern states and Canada. Check your local building code.
Step 5 — Count rails
Rails are the horizontal boards that run between posts. The number you need per section depends on fence height:
Step 6 — Estimate concrete
Each post needs a concrete footing. Work out the concrete volume around the post, then divide by the yield printed on the bag and round up:
Example: a 4×4 post (3½ × 3½ in actual) in a 12-inch hole with 24 inches of concrete around it needs (π × 0.5² − 0.292²) × 2 ≈ 1.40 ft³ per post. Using the yields on Quikrete's data sheets:
Concrete depth is not always the same as hole depth. With Fast-Setting mix, leave out the gravel layer at the bottom and the top 3–4 inches that Quikrete says to fill with soil. With standard Concrete Mix, add the 6-inch layer of dry mix that Quikrete puts in the bottom of the hole (see below). As a cross-check, Quikrete says two 50 lb bags of Fast-Setting mix set a 3-inch square post in a 9-inch hole 30 inches deep. If you're using standard (not fast-setting) mix, see how to mix concrete by hand for water amounts and mixing steps.
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Use the Fence Calculator →FAQ
How many fence posts do I need for 100 feet?
For 100 ft with standard 8 ft spacing: divide 100 by 8 (= 12.5), round up to 13 sections, then add 1 post = 14 posts total. Add 2 extra posts per gate opening.
How far apart should fence posts be?
8 feet on centre is standard for most wood and vinyl fences. Use 6 ft for fences over 6 ft tall or in high-wind areas. Chain-link fences typically use 10 ft spacing.
How deep should fence posts be set?
Quikrete says to dig the hole 1/3 of the overall post length deep — half the above-ground height. So a 6 ft fence needs 3 ft buried and a 9 ft post (cut from a 10 ft length). In cold climates, always go below the local frost line (36–48 inches in northern regions).
Why do I get an uneven last bay?
Because fence length rarely divides evenly by your desired spacing. Fix it by using adjusted spacing: divide your total length by the number of bays (posts minus 1). This spreads the difference evenly across all sections.
How many rails do I need per section?
Use 2 rails for fences under 5 ft, 3 rails for 5–7 ft fences, and 4 rails for 8 ft fences. Rails run horizontally between posts and carry the pickets or boards.