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Fence post depth chart
Put your fence height and frost depth in: 6 ft in 24 inches of frost needs posts 24 inches deep — 1 bag per post.
★ CONSTRUCTION · FENCE · POST DEPTH
Fence Post Depth Chart: Height, Frost & Concrete Bags
On the defaults — a 6 ft fence in 24 inches of frost — the required post depth is 24 inches, and the concrete estimate is 1 bag per post. The fence rule alone gives 24 inches too (6 ft divided by 3), so the two rules tie on the defaults — the fence-post-size-chart page is the SIZE version — post dimensions and length for a given height; the fence-post-spacing-chart page is the SPACING version — how far apart posts go and how many a run needs; and the concrete-calculator page is the VOLUME version — yards and bags for a slab or footing.
What does 24 inches mean?
It means the post must be buried 24 inches deep — two feet underground for a six-foot fence. Two rules compete for that number and the deeper one wins: the fence rule says bury one third of the fence height, which for a 6 ft fence is 2 ft, or 24 inches; the frost rule says bury below the frost line, which on the defaults is also 24 inches. When the two rules tie, either gives the same answer, but they diverge fast — the lookup table below shows an 8 ft fence in 12 inches of frost needing 32 inches by the fence rule, and a 6 ft fence in 36 inches of frost needing 36 inches by the frost rule. The part people get wrong is using only one rule: a tall fence in warm ground fails by leverage if the posts are shallow, and a short fence in cold ground fails by frost heave if the posts stop above the frost line.
Three things to check before you trust the number
Is your frost depth really 24 inches? Frost depth varies by location and even by winter: 12 inches in a mild climate, 36 inches or more in a cold one. Your local building department publishes the design frost depth, and that figure — not a neighbour's guess — belongs in the frost field. The 36-inch branches show how much the answer moves.
Is the one-third rule enough for your fence? One third of the height is the common rule of thumb for a standard fence in ordinary soil. Tall fences, fences in soft or sandy soil, and fences carrying gates need deeper posts or wider footings, and a gate post is not a line post — gate posts should go deeper and wider than this chart suggests.
What does the concrete estimate assume? The bag count assumes 0.5 bag of concrete per foot of post depth, rounded up — a planning estimate stated plainly, not a product fact. Hole diameter and post size change the real volume: a wide hole around a 6 by 6 post needs considerably more than a narrow hole around a 4 by 4. Use the estimate for ordering a rough count, then check the concrete product label for your hole size.
Worked example
On the defaults: 6 ft fence, 24 inches of frost.
- Fence rule: 6 ft ÷ 3 is 2 ft, which is 24 inches.
- Frost rule: 24 inches.
- Required depth: the larger of 24 inches and 24 inches is 24 inches.
- Concrete: 0.5 bag × 2 ft is 1.0, so 1 bag per post.
With a deeper frost: a 6 ft fence in 36 inches of frost needs 36 inches, because the frost rule beats the 24-inch fence rule, and 0.5 × 3 ft is 1.5, rounded up to 2 bags per post. And the other way: the frost depth at which the frost rule takes over from the fence rule for a 6 ft fence is 24 inches, the fence height at which the fence rule takes over from the frost rule at 24 inches of frost is 6 ft, and the smallest whole frost depth needing 2 bags of concrete per post for a 6 ft fence is 25 inches.
Post depth by fence height and frost depth
| Fence height | Frost depth | Required post depth | Rule that sets it | Concrete per post |
|---|---|---|---|---|
| 3 ft | 12 inches | 12 inches | fence rule | 1 bag per post |
| 3 ft | 24 inches | 24 inches | frost rule | 1 bag per post |
| 3 ft | 36 inches | 36 inches | frost rule | 2 bags per post |
| 4 ft | 12 inches | 16 inches | fence rule | 1 bag per post |
| 4 ft | 24 inches | 24 inches | frost rule | 1 bag per post |
| 4 ft | 36 inches | 36 inches | frost rule | 2 bags per post |
| 5 ft | 12 inches | 20 inches | fence rule | 1 bag per post |
| 5 ft | 24 inches | 24 inches | frost rule | 1 bag per post |
| 5 ft | 36 inches | 36 inches | frost rule | 2 bags per post |
| 6 ft | 12 inches | 24 inches | fence rule | 1 bag per post |
| 6 ft | 24 inches | 24 inches | fence rule | 1 bag per post |
| 6 ft | 36 inches | 36 inches | frost rule | 2 bags per post |
| 7 ft | 12 inches | 28 inches | fence rule | 2 bags per post |
| 7 ft | 24 inches | 28 inches | fence rule | 2 bags per post |
| 7 ft | 36 inches | 36 inches | frost rule | 2 bags per post |
| 8 ft | 12 inches | 32 inches | fence rule | 2 bags per post |
| 8 ft | 24 inches | 32 inches | fence rule | 2 bags per post |
| 8 ft | 36 inches | 36 inches | frost rule | 2 bags per post |
Concrete assumes 0.5 bag per foot of post depth, rounded up — a planning estimate, not a product specification. Wider holes and larger posts need more concrete; check your concrete product label and your local code before ordering.
Every branch, checked
- 4 ft fence, 12 inches of frost: 4 ft fence, 12 inches of frost: Post depth 16 inches (fence rule 16 inches, frost depth 12 inches); 1 bag of concrete per post.
- 8 ft fence, 36 inches of frost: 8 ft fence, 36 inches of frost: Post depth 36 inches (fence rule 32 inches, frost depth 36 inches); 2 bags of concrete per post.
- 6 ft fence, 36 inches of frost: 6 ft fence, 36 inches of frost: Post depth 36 inches (fence rule 24 inches, frost depth 36 inches); 2 bags of concrete per post.
- 3 ft fence, 12 inches of frost: 3 ft fence, 12 inches of frost: Post depth 12 inches (fence rule 12 inches, frost depth 12 inches); 1 bag of concrete per post.
- 8 ft fence, 12 inches of frost: 8 ft fence, 12 inches of frost: Post depth 32 inches (fence rule 32 inches, frost depth 12 inches); 2 bags of concrete per post.
- 5 ft fence, 24 inches of frost: 5 ft fence, 24 inches of frost: Post depth 24 inches (fence rule 20 inches, frost depth 24 inches); 1 bag of concrete per post.
The reverses, worked backwards
- Frost depth at which the frost rule takes over from the fence rule for a 6 ft fence: 24 inches — at 24 inches the figures are Post depth 24 inches (fence rule 24 inches, frost depth 24 inches); 1 bag of concrete per post.; at the lower neighbor the figures are Post depth 24 inches (fence rule 24 inches, frost depth 18 inches); 1 bag of concrete per post.; at the upper neighbor the figures are Post depth 30 inches (fence rule 24 inches, frost depth 30 inches); 2 bags of concrete per post..
- Fence height at which the fence rule takes over from the frost rule at 24 inches of frost: 6 ft — at 6 ft the figures are Post depth 24 inches (fence rule 24 inches, frost depth 24 inches); 1 bag of concrete per post.; at the lower neighbor the figures are Post depth 24 inches (fence rule 20 inches, frost depth 24 inches); 1 bag of concrete per post.; at the upper neighbor the figures are Post depth 28 inches (fence rule 28 inches, frost depth 24 inches); 2 bags of concrete per post..
- Smallest whole frost depth needing 2 bags of concrete per post for a 6 ft fence: 25 inches — at 25 inches the figures are Post depth 25 inches (fence rule 24 inches, frost depth 25 inches); 2 bags of concrete per post.; at the lower neighbor the figures are Post depth 24 inches (fence rule 24 inches, frost depth 24 inches); 1 bag of concrete per post.; at the upper neighbor the figures are Post depth 26 inches (fence rule 24 inches, frost depth 26 inches); 2 bags of concrete per post..
What the defaults assume
The defaults are a 6 ft fence and 24 inches of frost, so the headline is a standard privacy fence in a moderate frost climate where the two rules happen to tie. The model is one division, one comparison and one round-up: the fence rule is height in feet divided by 3 converted to inches, the required depth is the larger of that and the frost depth, and concrete bags are 0.5 per foot of depth rounded up, because a part-bag cannot be bought. The frost reverse is checked at 6 inches either side, because 6 inches of frost is half a foot of depth; the height reverse is checked at 1 ft either side, because 1 ft of fence height is 4 inches of fence-rule depth. The model does not account for soil type, wind exposure, gate loads, or local code requirements — those belong to your local building department, not to the depth arithmetic.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Fence rule depth | fence height in feet ÷ 3, in inches | 24 inches — 6 ft × 12 ÷ 3. |
| Required post depth | the larger of the fence rule and the frost depth | 24 inches — larger of 24 inches and 24 inches. |
| Concrete per post | 0.5 bag per foot of post depth, rounded UP | 1 bag — 0.5 × 2 ft, exact. |
| 4 ft fence, 12 inches of frost | same model | Post depth 16 inches (fence rule 16 inches, frost depth 12 inches); 1 bag of concrete per post. |
| 8 ft fence, 36 inches of frost | same model | Post depth 36 inches (fence rule 32 inches, frost depth 36 inches); 2 bags of concrete per post. |
| 6 ft fence, 36 inches of frost | same model | Post depth 36 inches (fence rule 24 inches, frost depth 36 inches); 2 bags of concrete per post. |
| 3 ft fence, 12 inches of frost | same model | Post depth 12 inches (fence rule 12 inches, frost depth 12 inches); 1 bag of concrete per post. |
| 8 ft fence, 12 inches of frost | same model | Post depth 32 inches (fence rule 32 inches, frost depth 12 inches); 2 bags of concrete per post. |
| 5 ft fence, 24 inches of frost | same model | Post depth 24 inches (fence rule 20 inches, frost depth 24 inches); 1 bag of concrete per post. |
| Frost depth at which the frost rule takes over from the fence rule for a 6 ft fence | reverse | 24 inches: Post depth 24 inches (fence rule 24 inches, frost depth 24 inches); 1 bag of concrete per post. |
| Fence height at which the fence rule takes over from the frost rule at 24 inches of frost | reverse | 6 ft: Post depth 24 inches (fence rule 24 inches, frost depth 24 inches); 1 bag of concrete per post. |
| Smallest whole frost depth needing 2 bags of concrete per post for a 6 ft fence | reverse | 25 inches: Post depth 25 inches (fence rule 24 inches, frost depth 25 inches); 2 bags of concrete per post. |
| 0 inches of frost | same model | Post depth 24 inches (fence rule 24 inches, frost depth 0 inches); 1 bag of concrete per post. |
| 0 ft fence height | same model | Fence height — more than 0 ft. |
| Negative fence height | same model | Fence height — more than 0 ft. |
| Negative frost depth | same model | Frost depth — 0 or more inches. |
Who uses this
- A homeowner setting a privacy fence — 6 ft in 24 inches of frost is 24 inches deep and 1 bag per post, straight from the lookup table.
- A contractor quoting a fence in a cold climate — the 36-inch frost column shows why northern jobs need deeper holes and more concrete.
- A DIYer replacing a single leaning post — the 8 ft branch shows a tall fence needing 32 inches even in mild frost.
- A renter building a short garden fence — the 3 ft branch shows 12 inches deep and 1 bag per post, a much smaller job.
Accuracy & limitations
Figures are the model’s own arithmetic on the numbers you type, computed from unrounded values and displayed rounded HALF_UP to 2dp, with concrete bags always rounded UP to the next whole bag. The main limitation is the inputs, not the arithmetic: the true local frost depth and the real soil conditions come from your building department and your site, and the 0.5-bag-per-foot concrete figure is a planning assumption, not a measured volume for your hole size. Local code may require deeper posts than either rule gives, and this page is a calculation tool, not building advice.
Cite this calculator
A 6 ft fence in 24 inches of frost needs posts 24 inches deep — 1 bag of concrete per post. Source: Bright Side Kit, https://brightoolbox.com/fence-post-depth-chart/, accessed 28 October 2026.
Frequently asked questions
How deep should fence posts be?
The deeper of one third of the fence height and the local frost depth. For a 6 ft fence in 24 inches of frost both rules give 24 inches.
Why bury one third of the fence height?
Because a post is a lever. Wind pushes the top of the fence, and the buried third resists that leverage. A shallower post leans over time, especially on a tall fence.
What happens if posts stop above the frost line?
Frost heave lifts them. Freezing ground expands and pushes shallow posts upward, leaving a wavy fence line by spring. Posts must reach below the frost depth.
How many bags of concrete per post?
About 0.5 bag per foot of post depth, rounded up. A 24-inch post needs 1 bag; a 36-inch post needs 2 bags. This is a planning estimate — hole diameter and post size change the real volume.
Do gate posts need to go deeper?
Yes. Gate posts carry a swinging load that line posts never see. Go deeper and wider than this chart suggests for any post that holds a gate.
How do I find my local frost depth?
Ask your local building department. It publishes the design frost depth for your area; that figure belongs in the frost field, not a neighbour’s guess.
Does this chart replace local building code?
No. Local code may require deeper posts than either rule gives. Check the code before digging, and call before you dig to mark buried utilities.