What this calculator does
The usual rule buries a third of the above-ground height. A 1.8 metre fence therefore needs 0.59 metres in the ground, and with a 100 mm gravel base the hole is 0.69 metres deep and the post must be 2.49 metres long.
In a freezing climate the frost line overrides the rule. A post founded above it gets pushed upward by frost heave each winter and works loose over a few seasons. Where the frost line is deeper than the burial ratio would give, the hole must go deeper regardless of what the ratio says.
The formula
The buried length is the above-ground height multiplied by the burial ratio. The hole depth is that plus a gravel drainage base, or the frost depth plus the gravel base if that is deeper. The total post length is the finished height plus the hole depth, which is the figure to order against.
| Term | Meaning |
|---|---|
| Burial ratio | Fraction of the above-ground height set below grade. One third is common, two fifths for exposed sites. |
| Frost line | The depth to which ground freezes locally. Posts must be founded below it. |
| Frost heave | Ground lifting as water in it freezes, which pushes shallow posts upward. |
| Gravel base | A drainage layer under the post, so water does not sit against the end grain. |
The inputs explained
| Field | What to enter |
|---|---|
| Height above ground (finished) (m) | Finished height above ground in metres. |
| Burial ratio | Burial ratio. 0.33 is the common rule; use 0.4 for gates, corners and exposed sites. |
| Gravel base below the post (mm) | Gravel drainage base below the post. |
| Local frost depth (0 if none) (mm) | Local frost depth. Enter 0 where the ground does not freeze. |
When to use it
Ordering fence posts
Posts are bought at fixed lengths, and the total length needed determines which to buy.
Setting gate posts
Gate posts carry a swinging load and are usually set deeper than the run posts.
Building in a cold climate
The frost line is the governing dimension wherever it exceeds what the ratio gives.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How deep for each burial ratio?
The same fence at a range of burial ratios.
| Burial ratio | Hole depth | Buried post length | Total post length to buy |
|---|---|---|---|
| 25% | 0.55 m | 0.45 m | 2.35 m |
| 33% | 0.69 m | 0.59 m | 2.49 m |
| 40% | 0.82 m | 0.72 m | 2.62 m |
Questions
How deep should a fence post go?
About a third of the above-ground height as a general rule, so roughly 600 mm for a 1.8 m fence. Go deeper for gate posts, corner posts and exposed sites, and deeper again wherever the frost line demands it.
Why does the frost line matter?
Because water in the ground expands as it freezes and lifts anything founded above the freeze depth. A post heaved upward each winter gradually works loose and the fence racks. Setting below the frost line puts the post base in ground that never freezes.
Why put gravel under the post?
So water drains away from the end grain rather than sitting against it. A post set directly into concrete with no drainage path holds water at its base and rots there first. The gravel gives that water somewhere to go.
Should the post be concreted in?
Usually, for a fence of any height, though gravel-set posts are also used and are easier to replace. What matters more than the concrete is the depth and the drainage: a shallow post in a large concrete plug still heaves and still works loose.
For the concrete volume, see the post hole concrete calculator. For the fence materials above ground, see the fence calculator.