Fence Calculator
Full material list for a run of fence.
Landscaping & Outdoor
Fence Post Depth Calculator
How deep to set a fence post, and how much concrete each hole takes, from the fence height and your frost depth.
Your result
Sample values3.50
Post depth, ft
12
Hole diameter, in
9.50
Post length, ft
6
Bags of concrete per post
6 ft fence, 4 × 4 posts, 36 in frost depth, firm soil.
Per post
| Measure | Value |
|---|---|
| Depth below grade | 3.50 ft |
| Hole diameter | 12 in |
| Post length to buy | 9.50 ft — order 10 ft |
| Concrete, 60 lb bags | 6 per post |
| Total for 14 posts | 84 bags |
How to use this calculator
Enter the height of fence you want above ground and the frost depth published for your area. The calculator takes whichever governs: a third of the post above ground, frost depth plus six inches, or a two foot minimum.
These are the rules of thumb the trade works to, not a code requirement. Some jurisdictions specify post embedment directly, and a fence over a certain height may need a permit and an engineered detail. Check before you dig.
Formula
Depth = the greatest of:
Fence height ÷ 3
Frost depth + 6 in
24 in minimum
Hole diameter = Post width × 3 (× 3.5 in loose soil)
Concrete = (Hole volume − Post volume) per hole
- ÷ 3
- a third of the post buried is the common rule for a fence
- + 6 in
- the footing sits below the frost line, not on it
Concrete at 0.45 ft³ per 60 lb bag. Gravel at the hole base is not included.
Worked example
A 6 ft fence on 4 × 4 posts, 36 in frost depth, firm soil.
- A third of 6 ft = 2.00 ft (24 in)
- Frost 36 in + 6 = 42 in (3.50 ft)
- Minimum 24 in — the frost rule governs, so 3.50 ft
- Hole: 3.5 in post × 3 = 10.5 in, rounded to a 12 in auger
- Hole volume: π × 0.5² × 3.5 = 2.75 ft³
- Less the post: 2.75 − 0.30 = 2.45 ft³, or 6 bags
3.50 ft deep, 12 in hole, 6 bags per post
Understanding your results
Frost is what governs in most of the country, and the reason is heave rather than strength. Water in the soil expands as it freezes and lifts anything with a footing above the frost line. A post set two feet deep where frost reaches three will rise and drop every winter until the fence is visibly out of line.
A wider hole does more than a deeper one for wind resistance, because the concrete plug resists overturning across its diameter. That is why loose soil calls for a wider hole rather than simply a deeper post.
Questions
About this calculator
The usual rule is a third of the post above ground, so a 6 ft fence gets 2 ft buried and a 9 ft post. Where the ground freezes, the hole must reach below the frost line instead, which often governs and can push a 6 ft fence to three and a half feet or more.
Several inches of compacted gravel under the post lets water drain away from the end grain rather than sitting against it, which is the single biggest cause of rot in a set post. It is worth doing and is not included in the concrete figure here.
Not always. Posts tamped in with compacted gravel drain better and are far easier to replace, and for light rail fences they hold perfectly well. Concrete is worth it for gate posts, corners, ends and tall privacy fences, which take the real load.
No. These are common trade rules of thumb. Some jurisdictions publish specific embedment depths, and fences above a certain height often need a permit. Check locally before setting anything, particularly on a boundary.
Keep going
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Estimates, not specifications.
Results are based on the values you enter and on typical material
properties. Densities, coverage rates and product sizes vary between
suppliers, so confirm quantities against your supplier's data before
ordering. Structural work must be signed off by a qualified
professional and meet your local building code.