Fence Post Depth Calculator
A fence post depth calculator takes the greatest of post height ÷ 3, frost depth, or 2 ft. A 6 ft post where frost reaches 36 in should be set 3.00 ft deep.
How deep should a fence post be set?
A fence post depth calculator recommends the deepest of three limits: one third of the post’s height above ground, your local frost depth, or a 2 ft minimum. Depth = max(height ÷ 3, frost depth in inches ÷ 12, 2 ft). A 6 ft fence post in an area where frost reaches 36 inches should be set 3.00 ft deep, so you need a 9 ft post.
Most fence failures start below ground. A post that is too shallow leans in the first big wind, or rises a little every winter until the gate no longer latches.
The rule in plain words
- Leverage check: height above ground ÷ 3. Wind pushes on the fence panel, and the buried part of the post resists it like a lever.
- Frost check: frost depth ÷ 12, in feet. The bottom of the post should be below the frozen soil.
- Minimum: 2 ft, so short posts are not set in a shallow hole.
- Answer: the largest of the three.
The one-third rule is the low end of common practice. Many fence builders set posts as deep as half the above-ground height for tall, solid fences in soft or sandy soil.
Worked examples
6 ft privacy fence in a cold climate, frost depth 36 in
- Height ÷ 3: 6 ÷ 3 = 2.00 ft
- Frost: 36 ÷ 12 = 3.00 ft
- Minimum: 2.00 ft
- Recommended: 3.00 ft. Buy 9 ft posts.
8 ft privacy fence in a mild climate, frost depth 12 in
- Height ÷ 3: 8 ÷ 3 = 2.67 ft
- Frost: 12 ÷ 12 = 1.00 ft
- Minimum: 2.00 ft
- Recommended: 2.67 ft, about 32 in. Buy at least 11 ft posts, or 12 ft to allow for a gravel base.
4 ft picket fence with no frost
- Height ÷ 3 = 1.33 ft, frost = 0
- Recommended: 2.00 ft, the minimum. A 6 ft post works.
Set depth by fence height and frost depth
| Fence height | No frost | 12 in frost | 24 in frost | 36 in frost | 48 in frost |
|---|---|---|---|---|---|
| 3 ft | 2.00 ft | 2.00 ft | 2.00 ft | 3.00 ft | 4.00 ft |
| 4 ft | 2.00 ft | 2.00 ft | 2.00 ft | 3.00 ft | 4.00 ft |
| 6 ft | 2.00 ft | 2.00 ft | 2.00 ft | 3.00 ft | 4.00 ft |
| 7 ft | 2.33 ft | 2.33 ft | 2.33 ft | 3.00 ft | 4.00 ft |
| 8 ft | 2.67 ft | 2.67 ft | 2.67 ft | 3.00 ft | 4.00 ft |
Frost wins quickly in cold regions. Below about 24 in of frost, the fence height usually decides.
Hole size and setting tips
- Diameter: about three times the post width is common, so roughly 10 to 12 in for a 4 × 4 post.
- Base: put a few inches of gravel in the bottom for drainage. Add that to the hole depth, not to the embedment.
- Bell the bottom: a hole slightly wider at the bottom than the top resists frost lifting.
- Corner, end, and gate posts carry more load. Set them deeper and wider than line posts.
Size the concrete for each hole with the fence post concrete calculator, and count posts along the run with the fence post calculator.
How to measure
- Measure fence height from finished grade to the top of the post, not the top of the panel.
- Get frost depth from your building department, not a general map.
- On slopes, measure height on the downhill side, where the post is tallest.
Common mistakes
- Stopping just at the frost line. Go a few inches below it.
- Burying a flat-bottomed plug. Concrete that mushrooms at the top can give frost something to lift. Keep the top flush or slightly below grade.
- Using one depth for every post. Gate posts need extra.
- Ignoring buried lines. Call 811 before digging post holes.
For a full material list, the fence calculator adds rails and pickets. Fences over certain heights often need a permit, so check local code before you buy posts.
Frequently asked questions
How deep should a 6 ft fence post be?
At least 2 ft by the one-third rule, but deeper where the ground freezes. With a 36 in frost depth the calculator recommends 3.00 ft, so a 6 ft fence needs a 9 ft post. In mild climates, many builders still go 2½ to 3 ft deep for tall privacy fences that catch a lot of wind.
Why compare three numbers instead of using one rule?
Each one guards against a different failure. Height ÷ 3 resists leaning from wind and leverage. Frost depth stops freeze-thaw from jacking the post up. The 2 ft floor keeps short posts from being set too shallow. The calculator returns whichever is deepest.
What is frost heave and why does it matter for post depth?
When wet soil freezes it expands and grips the post, lifting it a little each winter. A post that stops above the frost line can creep upward over several seasons and lean. Setting the bottom below the frost line, often with an extra few inches, keeps it anchored.
Where do I find my local frost depth?
Your local building department sets it, often as the minimum footing depth in the code. It ranges from almost nothing in warm southern climates to 48 in or more in the far north. Do not borrow a number from another region.
How long a post should I buy?
Add the set depth to the height above ground, plus a few inches for a gravel base you will not count as embedment. A 6 ft fence set 3.00 ft deep needs at least a 9 ft post, and a 4 ft fence set 2.00 ft deep needs a 6 ft post.
Estimates only. Verify quantities with your supplier before purchasing.