Fence Post Depth Calculator
Set post depth from fence height and frost line with safety margin.
What it does
Fence posts fail two ways: wind push pulls shallow posts, frost heave lifts posts set above the freeze line. Depth must beat both the height rule and the frost rule, plus a safety margin.
The tool above takes fence height and frost line and returns depth, height third, and frost plus margin. The defaults 6 ft with 20 in frost match the worked example, so the page opens showing 2.17 ft. Switch to metric for meters.
The height rule buries one third of the above ground height: a 6 ft fence needs 2 ft. The frost rule adds 6 in (15 cm) below the local frost line: 20 in frost needs 26 in. Depth is the larger of the two, with a 2 ft (0.6 m) floor for stability in no frost zones.
Soil, wind, and gate loads shift real specs. Sandy or wet soils, tall privacy panels, and wide gates want deeper or wider holes with concrete bells. Always call utility locate before digging and crown concrete to shed water.
This page shows the formula, one full hand checked example at 6 ft with 20 in frost, a reference table, and practice checks that run through the tool in both unit systems.
How to use
- Pick Imperial (ft, in) or Metric (m, cm).
- Type fence height above ground.
- Type local frost line depth, 0 where ground never freezes.
- Read Height third rule as height over 3.
- Read Frost plus margin as frost plus 6 in or 15 cm.
- Dig to Depth, the larger of the two with minimum floor.
Formula
- height/3
- One third of above ground height.
- frost+margin
- Frost line plus 6 in or 15 cm safety.
- depth
- Larger of the two, at least minimum.
Gravel base sits below the post in the same hole and does not reduce depth. In rock or high water, follow local code over this guide.
Worked example
Example: 6 ft fence, 20 in frost
Privacy fence 6 ft high where frost reaches 20 in. Find post depth.
- Height rule: 6 / 3 = 2 ft.
- Frost rule: (20 + 6) / 12 = 26 / 12 = 2.17 ft.
- Minimum: 2 ft.
- Max: larger of 2, 2.17, 2 is 2.17 ft.
- Metric mirror: 1.8 m with 50 cm frost gives 0.65 m.
- Dig 2.17 ft plus gravel base, crown concrete.
Reference table
| Height | Height third | Depth |
|---|---|---|
| 4 ft | 1.33 ft | 2.17 ft |
| 6 ft | 2 ft | 2.17 ft |
| 8 ft | 2.67 ft | 2.67 ft |
| 1.8 m, 50 cm frost | 0.6 m | 0.65 m |
Limits
- Guide only. Wind, gates, and soil can demand more.
- Frost lines vary by town. Confirm with local code.
- No hole width or concrete volume here.
- Rock, water, and utilities may force other methods.
Common mistakes
- Setting to frost line exactly with no margin below.
- Burying less than one third on tall wind catching panels.
- Digging shallow in no frost zones below the 2 ft floor.
- Pouring flat concrete that ponds water at the post.
- Skipping utility locate before augering.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 ft fence, 12 in frost?
Height 1.33 ft, frost 1.5 ft, depth 2 ft.
8 ft fence, 30 in frost?
Height 2.67 ft, frost 3 ft, depth 3 ft.
Questions
How deep for a 6 ft fence?
At least 2 ft by height thirds, deeper with frost. Here 2.17 ft with 20 in frost.
Below frost how far?
This guide adds 6 in or 15 cm below the line.
No frost?
Use height thirds with 2 ft or 0.6 m minimum.
Gates deeper?
Often yes. Wide gates lever posts hard, so go deeper and wider.
Concrete needed?
Common for fences. Crown it to shed water away from wood.
Call before dig?
Yes. Locate utilities first in most regions.