The "One-Third" Rule for Fence Posts
A stable, long-lasting fence starts below the ground. The general rule of thumb for setting fence posts is that at least one-third of the total post length should be buried in the ground. For a 6-foot tall fence, this means you need a 9-foot long post, with 3 feet of it set underground.
This provides the necessary leverage to resist wind, leaning, and pressure. For cold climates, there's an even more important rule: the bottom of your post hole must be below the local frost line.
Depth Adjustments for Common Situations
| Situation | Recommendation |
|---|---|
| Gate Posts | Go 6-12 inches deeper than standard posts and use a wider hole with more concrete to support the extra weight and movement. |
| Cold Climates | The bottom of the hole MUST be below your local frost line to prevent frost heave, even if this is deeper than the 1/3 rule. |
| Sandy or Soft Soil | Increase embedment depth by another 6-12 inches and use a wider hole to create a larger, more stable concrete footing. |
| Fences Over 8 ft | For very tall fences, a safer rule is to bury half the height of the fence. A 10ft fence should have 5ft buried. |
How to Use the Calculator
- Enter the desired height of the fence as it will appear above the ground.
- Enter the width or diameter of your post. For a 4x4 post, enter 4. For a 6x6, enter 6.
- Input the price for one bag of post concrete from your local store (e.g., a 50lb bag).
- Click "Calculate Depth" to see the minimum depth, total post length to buy, and concrete estimate.
Frequently Asked Questions
Why is the frost line so important?
In cold climates, moisture in the soil freezes and expands, pushing the ground upward. This process, called "frost heave," can slowly lift fence posts right out of the ground if they are not set deep enough. Placing the bottom of the post below the frost line ensures it sits on stable, unfrozen ground.
Do I really need concrete for every post?
While some people set posts in compacted gravel, using concrete is highly recommended for maximum stability and longevity. It is considered essential for corner posts and gate posts, which are under the most stress.