What Is the Fence Post Depth Calculator?
This tool tells you how deep to dig the hole for a fence post using the classic carpenter's "one-third rule." The rule of thumb says that for a stable post, you should bury about one-third of the total post length below ground, leaving two-thirds visible above ground. Burying enough of the post resists wind loading, frost heave, and leaning over time.
How to Use It
Enter the total length of the post you bought (not the finished above-ground height) in feet. The calculator instantly divides that length by three to give the recommended hole depth, and also shows how much post will remain above the ground. Buy posts long enough to account for the buried third — a 6 ft fence typically needs a 9 ft post.
The Formula Explained
The math is simple: $$\text{Depth} = \frac{\text{Post Height}}{3}$$ The above-ground portion is the remainder, \(\text{Post Height} - \text{Depth}\), which works out to two-thirds of the total. For example, a 9 ft post gives a 3 ft hole and 6 ft of visible fence.
Worked Example
Suppose you have a 9-foot post. $$\text{Depth} = \frac{9}{3} = 3 \text{ feet}$$ buried, leaving \(9 - 3 = 6\) feet above ground. Dig your hole 3 feet deep, add gravel for drainage, set the post, and backfill with concrete.
FAQ
Should I always use one-third? It is a reliable guideline for residential fences. For tall, heavy, or windy gate posts, dig deeper.
Does frost matter? Yes. In cold climates, set the hole below the local frost line even if that is deeper than one-third.
What about hole diameter? A common rule is a hole about three times the post width; this calculator focuses on depth only.
