Fence Post Depth Calculator
Enter the fence height and your frost depth to get the hole depth and the post length to buy.
The Fence Post Depth Calculator works out how deep to set a fence post in the ground, how deep to dig the hole, and what length of post to buy. Enter your fence height and a depth rule, and the result updates as you type. It runs in your browser, is free, and needs no sign-up.
Use the results for planning and ordering. Local building codes and the fence manufacturer’s instructions decide the final post depth and spacing, so treat each figure as an estimate until you have checked those sources.
It also shows the hole depth including any gravel base and the hole diameter to dig, so you can plan the concrete next.
How to use the Fence Post Depth Calculator
- Enter the Fence height above ground, meaning the height of the top of the post above the ground.
- Choose a Depth rule: one-third of the post in the ground (common builder’s guideline) or half of the post in the ground (taller or exposed fences).
- Enter the Post width. For example, 3½ in for a 4×4 or 5½ in for a 6×6.
- Optional: enter the Local frost depth from your local building department. The tool has no frost map.
- Optional: enter a Gravel base under the post. It is added to the hole depth. Leave it blank for none.
- Optional: enter the Post length you have to check that it is long enough.
- Enter your Stock post lengths at your supplier as a comma-separated list, and pick Stock lengths are in feet or metres.
- Read the results: Post depth in the ground, Hole depth (with gravel base), Post length needed, Post length to buy, Hole diameter (3× post width), and Depth by the rule alone.
The example buttons at the top fill in a realistic case: 6 ft fence, 4 ft fence, 6 ft with frost 42 in, or 1.8 m metric. Reset restores the starting example, and some fields are under More options. Copy result copies the figures. Copy link keeps all inputs in the page address, so opening it restores the same result. Print / save as PDF prints the inputs, result, calculation steps and date.
How deep should a fence post be in the ground?
A widely published builder’s guideline is to bury about one third of the total post length. This is the same as burying half the above-ground height: a 6 ft fence on a 9 ft post goes 3 ft into the ground. For tall or exposed fences, the tool also offers half the total post length in the ground, which makes the in-ground depth equal to the above-ground height. Both are builder’s guidelines, not building code.
Gate posts and corner posts carry more load than line posts. Many installers set them deeper or in larger holes, so check the depth you choose against the load they will take.
How do you calculate the hole depth for a fence post?
Add the gravel base to the in-ground depth. The hole is deeper than the post goes, so a 3 ft in-ground depth with a 4 in gravel base needs a hole 3 ft 4 in deep.
The post length needed is the fence height plus the in-ground depth. For a 6 ft fence with a 3 ft in-ground depth, that is 9 ft. The post length to buy is the shortest stock length that is at least that long. If your yard sells 8 ft and 10 ft posts, that means a 10 ft post.
Hole diameter is commonly about three times the post width (for example Long Fence’s guide, which also puts the depth between a third and half of the post). A 4×4 post, at 3½ in wide, suits a hole about 10½ in across.
In cold climates, set the bottom of the post, or its concrete, below the local frost depth to limit frost heave. Get that figure from your local building department. Before digging, call 811 (US) or your local utility locator. Local codes and the fence manufacturer’s instructions take priority over any guideline.
Sources
- Long Fence: How deep should I dig wood fence posts? — depth between a third and half of the post, hole about three times the post width (builder’s guideline, not code).
- Frost depth comes from your local building department; call 811 before you dig (call811.com).
Worked examples
| Input | Result |
|---|---|
| 6 ft fence above ground, 1/3 of post in ground | 3 ft (0.914 m) in the ground, buy 9 ft posts |
| 4 ft fence above ground, 1/3 of post in ground | 2 ft (0.61 m) in the ground, buy 6 ft posts |
| 6 ft fence above ground, 1/2 of post in ground | 6 ft (1.83 m) in the ground, buy 12 ft posts |
| 6 ft fence above ground, 1/3 of post in ground, frost depth 42 in | 3 ft 6 in (1.07 m) in the ground, buy 10 ft posts |
| 1.8 m fence above ground, 1/3 of post in ground, frost depth 60 cm | 0.9 m (90 cm) in the ground, buy 2.7 m posts |
Calculated by the tool above with these values.
Frequently asked questions
Is the fence post depth calculator free to use?
Yes. It is free, needs no sign-up, and runs in your browser. Nothing you type is uploaded.
Does the result show that my fence meets building code?
No. The results are estimates for planning and ordering. Local building codes and the manufacturer’s instructions decide post depth, spacing, and other details.
Why is the hole deeper than the post goes into the ground?
Adding a gravel base under the post means the hole has to be deeper than the in-ground depth. Enter the gravel depth in the optional field, and the tool adds it to the hole depth.
What if my yard sells posts in metres?
Enter your stock post lengths as a comma-separated list, then set Stock lengths are in to metres. The tool also has a 1.8 m metric example you can load to see how it works.
What do the warnings mean?
A warning appears when the post you have is too short for the fence, when it is not even as long as the fence height, or when the length you need is longer than the longest stock length you listed. Each warning explains what to change, such as using a longer post or lowering the fence.
Can I use the result for a quote or material list?
Yes. Results can be used in commercial work such as quotes, material lists, and job sheets, with no attribution needed.
Related tools
- Fence post concrete calculator — concrete bags per post hole.
- Fence post spacing calculator — the number of posts and an equal spacing.
- Fence cost calculator — the materials list and total cost with your local prices.
- Fence panel calculator — panels, posts and the cut size of the last panel.
- Chain link fence calculator — the full chain link materials list.
- Deck railing calculator — posts, balusters and rails for a deck.
- Split rail fence calculator — sections, rails and line, end and corner posts.
- Barbed wire fence calculator — rolls of wire, line posts, braces and cost for a farm fence.
- Gate brace calculator — diagonal brace length, angle and saw cuts for a gate.