Fence post depth calculator
Get the right embedment depth for a fence post from frost line and above-ground height, so corner posts hold and winter frost cannot heave them.
Fence calculator
Enter your specs. Every result updates live and shows its formula.
Overview
A fence post stays put when its embedment covers two rules at once: the frost line where frost heave applies, and at least one third of the above-ground height by the traditional rule of thumb.
This calculator takes your local frost depth and your post height and returns a single recommended depth that satisfies the larger of the two requirements, so a shallow-set post cannot uplift in winter.
Common mistakes
- Setting every post to the same depth regardless of height, when a 4 foot post and an 8 foot post need different third-of-height embedment.
- Ignoring the frost line entirely and using a fixed 24 inches everywhere, which fails in frost-prone ground.
- Undersizing corner and gate posts, which take tension and gate load and commonly want a deeper, wider footer.
Related calculators
- Fence post calculator Calculate line posts, corner posts and end posts from run length and spacing, with concrete for each post included in the take-off.
- Fence post concrete calculator Work out how many bags of concrete mix each fence post footer needs, accounting for the post itself, based on hole and post dimensions.
- Fence cost calculator Estimate total fence cost from run length, height and your local material and labor rates, with the budget math shown.
Related reading
In-depth guides
Frequently asked questions
How deep should a 6 foot fence post be?
By the one-third rule, about 24 inches of embedment for the above-ground portion, and deeper where the frost line goes below that. This calculator returns the greater of the rule and your entered frost depth, so the number can be 30 inches or more in deep-frost regions.
What is the formula for post depth?
Take the larger of two values: your local frost depth, and one third of the post height above grade. That is the recommendation this calculator displays, because meeting only one of the two still lets the other cause movement.
How do I find my local frost line?
US county and city frost-depth tables are published by building departments and engineering agencies; a common quoted rule is that frost depth increases with latitude and elevation. This calculator takes the number as an input, so look it up locally with the building official for your address.
Does post height change the depth?
Yes, by the one-third rule it does. An 8 foot post should embed around 32 inches before frost is considered, versus 24 inches for 6 feet, which is why the height input drives the depth recommendation here.
Is deeper always better for a fence post?
Practically, deeper is safer for hold, but cost rises with every extra bag of concrete and length of post, and over-deepening can run into utilities or rock. Use the frost and third-of-height rules as the floor, and go deeper at corners and gates intentionally.
Do posts need to go below the frost line even with concrete?
Yes. Concrete does not cancel frost: when the surrounding soil freezes and heaves, a footer set above the frost depth can lift with the soil. The frost floor in this calculator exists for exactly that reason.
Is the one-third rule safe for gates?
It is a good start, not the end of the story. Gate posts carry a swinging, hinged load plus tension, and gate manufacturers commonly want deeper embedment, sometimes with a concrete collar, so treat the rule as a minimum and upsize at gate and corner posts.
What is the difference between depth and hole diameter rules?
Depth is how far the concrete footer goes below grade; diameter is how wide it is, and a common rule of thumb is three post widths for the hole diameter. This calculator sets the depth floor; the post concrete calculator handles the volume that the diameter and depth produce.