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Fence Post Depth Calculator

Construction

Calculate how deep to bury a fence post using the 1/3-rule and your local frost line depth. Get the required burial depth and total post length to buy.

215
072

Required Burial Depth

42
Total Post Length to Buy
114

This calculator computes your Required Burial Depth, Total Post Length to Buy from the values you enter.

Inputs
Post Height Above GroundLocal Frost Line Depth
Outputs
Required Burial DepthTotal Post Length to Buy

What is a Fence Post Depth?

A Fence Post Depth Calculator determines how far underground a fence post needs to be set to remain stable and resist frost heave. The calculation weighs two competing requirements: a stability rule based on the post's total height (commonly one-third of the post buried underground), and a frost-protection rule that requires the post to extend at least 6 inches below the local frost line. The calculator takes whichever requirement demands the deeper hole.

Getting post depth right is one of the most overlooked steps in fence, deck, and pergola construction — a post that's too shallow can heave or lean within a season or two in colder climates. If you're planning the rest of your fence layout, pair this with the Fence Calculator for post and rail counts, or the Rectangle Fence Perimeter Calculator for total fence length.

How to use this Fence Post Depth calculator

  1. Enter the Post Height Above Ground you want for your finished fence in feet.
  2. Enter your Local Frost Line Depth in inches — check your local building code or extension office if you don't know this figure.
  3. Review the Required Burial Depth result, which is the deeper of the 1/3-rule and frost-line-plus-6-inch requirements.
  4. Review the Total Post Length to Buy, which combines your above-ground height and burial depth.
  5. Use these figures to mark your post holes and purchase posts of adequate length before beginning installation.

Formula & Methodology

The calculator compares two depth requirements and uses the larger value:

Depth by 1/3 Rule = Post Height Above Ground × 12 × 0.33

Depth by Frost Line = Frost Line Depth + 6 inches

Required Burial Depth = the greater of the two values above

Total Post Length = Post Height Above Ground × 12 + Required Burial Depth

Worked example: For a 6 ft post in an area with a 36 in frost line:

Depth by 1/3 Rule = 6 × 12 × 0.33 = 23.8 in

Depth by Frost Line = 36 + 6 = 42 in

Required Burial Depth = 42 in (frost line requirement governs)

Total Post Length = 6 × 12 + 42 = 114 in (9.5 ft) — so you'd need to buy a 10 ft post to allow for this depth.

Frequently Asked Questions

A common rule of thumb is to bury about one-third of the post's total length underground, but the post should also go at least 6 inches below your local frost line to prevent frost heave from pushing it out of the ground over winter. The calculator uses whichever of these two requirements results in the deeper hole.
The frost line is the maximum depth to which the ground freezes in winter in your area, and it varies significantly by climate — from 0 inches in warm regions to over 60 inches in northern climates. If a post isn't set below the frost line, seasonal freezing and thawing of the soil can gradually heave the post upward, loosening or tilting your fence over time.
Local building departments, county extension offices, and regional frost depth maps published by building code authorities are the most reliable sources for your area's frost line. Many US regions publish this as part of their residential building code requirements for footings.
The 1/3 rule is a reasonable general guideline for typical fence posts in stable soil, but very tall posts, loose or sandy soil, or high-wind areas may need proportionally deeper footings for stability. Always check local building codes for fence height requirements that may mandate specific depths.
Total post length should equal your desired height above ground plus the required burial depth. The calculator's Total Post Length to Buy output already adds these together so you know the minimum post length to purchase before cutting.
Concrete footings are common for taller or load-bearing fence posts (like gate posts) since they add stability and resist frost heave better than tamped soil alone, though many residential fences use tamped gravel or soil successfully. The required depth calculated by this tool applies whether you're using concrete, gravel, or compacted soil.
A post buried too shallow, or above the frost line, is more likely to lean, heave, or become loose over a few freeze-thaw cycles, especially in colder climates. This often shows up as a fence that starts leaning or gates that no longer swing correctly after a winter or two.
Burial depth is primarily driven by the frost line and the 1/3-rule stability requirement rather than post material, though wood posts in direct ground contact should be pressure-treated for rot resistance regardless of depth. Steel and vinyl posts follow the same depth guidelines but don't face the same rot concerns.
This tool tells you how deep to set each post, while a separate tool like the [Fence Calculator](/fence-calculator/) tells you how many posts and rails you need based on your total fence length and post spacing.
Also known as
how deep to bury fence postsfence post burial depth calculatorfrost line fence post calculatorfence post hole depth calculatordeck post depth calculator