Fence Post Depth Calculator
Fence post depth calculator: enter fence height to get how deep to bury the post, hole size, gravel base and total post length. Frost-aware.
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How the Fence Post Depth Calculator Formula Works
Bury half of the height that shows above ground. That is the same as burying one third of the whole post, and it is the rule on the QUIKRETE data sheet for setting posts. A post that is 6 feet above ground goes 36 inches down, so it is 9 feet long.
Corner and gate posts carry more load, so the calculator multiplies the line depth by 4/3, which turns Penn State Extension's 36 inches into 48. If your minimum depth or your local frost depth is deeper than the rule, the deepest of the three wins, and any extra depth for soft soil is added on top.
The hole diameter is 3 times the post width. The hole depth is the buried depth plus any gravel base, because the post stands on the gravel. The post length to buy is the height above ground plus the buried depth.
buried depth = above-ground height / 2 (line post); post length = above-ground height + buried depth- above-ground heightftHow tall the fence stands over the soil.
- buried depthinHalf the above-ground height (line post), 4/3 of that for corner and gate posts, or your minimum or frost depth if deeper.
- hole diameterin3 times the post width.
- gravel baseinOptional gravel under the post. It deepens the hole, not the post.
- above-ground height
- ft
- How tall the fence stands over the soil.
- buried depth
- in
- Half the above-ground height (line post), 4/3 of that for corner and gate posts, or your minimum or frost depth if deeper.
- hole diameter
- in
- 3 times the post width.
- gravel base
- in
- Optional gravel under the post. It deepens the hole, not the post.
Using the Fence Post Depth Calculator
Enter units
Choose from Feet and inches, Metres, mm and cm.
Enter fence height above ground (feet or metres)
Type the figure you already have, then press Calculate.
Enter post width (inches or mm)
Type the figure you already have, then press Calculate.
Enter which post is it?
Choose from Line post, Corner, end or gate post.
Enter minimum buried depth (inches or cm)
Type the figure you already have, then press Calculate.
Enter local frost depth (inches or cm, 0 if unknown)
Type the figure you already have, then press Calculate.
Enter extra depth for soft or wet soil (inches or cm)
Type the figure you already have, then press Calculate.
Enter gravel base under the post (inches or cm)
Type the figure you already have, then press Calculate.
Read the result
Press Calculate to update the result. Until you do, the last result is dimmed and marked as out of date, so you always know which numbers belong to the values in the boxes.
A Real Worked Example
A 6 foot privacy fence uses 4x4 posts (3.5 inches actual). The line post rule gives half of the 72 inch height above ground, which is 36 inches. That is above the 24 inch minimum, so the rule sets the depth, and 33.3 percent of the post is underground.
The hole diameter is 3 × 3.5 = 10.5 inches. With the default 6 inch gravel base the hole is 42 inches deep. The total post is 6 + 3 = 9 feet, so buy a 9 foot post.
A 4 foot fence, where the minimum applies
A 4 foot fence is 48 inches above ground. Half of that is 24 inches, which equals the default minimum, so the buried depth is 24 inches. The post is 4 + 2 = 6 feet long. The hole is 30 inches deep with a 6 inch gravel base and 10.5 inches wide.
A 3 foot fence would give only 18 inches by the rule, so the minimum of 24 inches would take over. Lower the minimum if you know your soil and code allow it.
A 6 foot corner or gate post
A corner post carries more load, so the line depth of 36 inches is multiplied by 4/3 to give 48 inches. That matches the 48 inches Penn State Extension gives for corner and gate posts, against 36 inches for line posts.
The post is 6 + 4 = 10 feet long, and 40 percent is underground. With 6 inches of gravel the hole is 54 inches deep. Use a 6x6 post if your plans call for one, which would make the hole 16.5 inches wide.
A 5 foot fence with a 48 inch frost depth
A 5 foot fence is 60 inches above ground, so the rule gives 30 inches. The minimum is 24 inches. The local frost depth is 48 inches, which is the deepest of the three, so the frost depth sets the buried depth at 48 inches. The post is 5 + 4 = 9 feet long, and 44.4 percent is underground.
The hole is 54 inches deep with the gravel base. If you did not enter the frost depth, you would get only 30 inches, so enter it whenever your area freezes.
Metric: a 1.8 m fence with 75 mm posts
With units set to metric, a 1.8 m fence uses 75 mm posts, a 60 cm minimum and a 15 cm gravel base. Half of 1.8 m is 0.9 m, so the buried depth is 90 cm, above the 60 cm minimum.
The hole is 3 × 75 = 225 mm (22.5 cm) wide and 105 cm deep. The post is 1.8 + 0.9 = 2.7 m long, so buy a 2.7 m post. The share underground is 33.3 percent.
A 6 foot post in soft soil with 6 extra inches
In sandy or wet soil, Penn State Extension says posts are set deeper, but it gives no figure, so you choose an extra depth. Adding 6 inches to the 36 inch line depth makes the buried depth 42 inches.
The post is 6 + 3.5 = 9.5 feet long, so buy a 10 foot post. The hole is 48 inches deep with the gravel base. A concrete collar, as the same publication suggests for wet soils, is also worth considering.
What Else to Know
How deep should a fence post be?
The depth of a fence post comes from a simple rule of thumb: bury half of the height that shows above ground. The QUIKRETE Fast-Setting Concrete data sheet states it directly, saying that the depth of the post hole should be one half of the above-ground post height. A fence that stands 6 feet above the soil therefore has a post buried 36 inches, and the whole post is 9 feet long.
That is the same as the common one third rule. If half of the above-ground height is buried, the buried part is one third of the total post length. For a 6 foot fence, 3 of the 9 feet are underground. Both statements describe the same post.
These are rules of thumb. No building code says to bury a fence post one third of its length, and a fence that is short, heavy, tall, or gated can behave very differently. The calculator keeps a minimum depth, 24 inches by default, so short fences do not end up with posts that barely reach into the soil. The minimum is your own choice, and you can lower it or raise it.
The table below shows the buried depth for fence heights from 3 to 8 feet for line posts and for corner and gate posts.
Wood, chain link, concrete and metal posts
The geometry is the same for every material: part of the post stays above ground, part is buried, and the buried part resists wind and leaning loads. A wood post, a metal post and a concrete post all start from the same depth rule in this calculator. What changes is the product's own instruction sheet, which can give a depth, a hole size and a concrete footing for that particular post, and the sheet comes first.
Chain link fences are a special case. Terminal, corner and gate posts carry the tension of the mesh. Penn State Extension notes that fences under tension, such as wire strand or mesh, need deeply set posts to offer long-term resistance. Use the corner and gate post option for those, and consider more depth than the minimum.
Some posts are driven instead of set in a dug hole. The same Penn State publication says T-posts can be used in some fences, and that 4 inch recycled plastic posts need a small-bore pilot hole before driving. Driven posts suit light fences with little load. For heavy gates and tall solid fences, a dug hole and a firm footing is the safer plan.
How wide should the hole be?
The QUIKRETE data sheet says the hole diameter should be 3 times the post diameter. A 4x4 post is 3.5 inches wide in reality, so the hole should be about 10.5 inches wide. A 6x6 post is 5.5 inches, so the hole is about 16.5 inches. A 75 mm post gets a 225 mm hole.
A wider hole does not make the fence stronger. It means more digging, more concrete, and more soil to replace. A hole that is too narrow leaves no room to pack concrete or gravel around the post. The 3 times rule is a good middle ground, and it is the one the calculator uses.
The diameter is for the hole at the bottom as well as the top. A cone-shaped hole that is narrower at the bottom than the top loses much of its grip, so aim for straight sides. If you plan to use concrete, the number of bags follows from the hole size, and you can work it out with a post hole concrete calculator.
Frost line, soil and deeper corner or gate posts
In cold regions the soil freezes to some depth in winter. The [frost line](https://en.wikipedia.org/wiki/Frost_line) is the depth to which groundwater in soil is expected to freeze, and building codes take it into account because frost heaving can lift and shift foundations. Frost heaving can lift a shallow fence post out of line over several winters, and Penn State Extension notes that some soils heave with frost and loosen posts that are not driven deep enough.
So if your local frost depth is deeper than the rule, enter it. The calculator takes the deepest of the rule, your minimum and the frost depth. For a 6 foot fence, the rule gives 36 inches, so a 24 inch frost depth changes nothing, but a 48 inch frost depth makes the post go 48 inches deep. Your building department can give you the local frost depth.
The same publication says a typical line post depth is 36 inches, and that corner and gate posts handle greater loads, are set deeper, and are often set to 48 inches. It also says that posts in sandy or chronically wet soil will need to be set deeper and perhaps supported by a collar of concrete. The calculator multiplies the line depth by 4/3 for corner and gate posts, and gives you an extra depth box for soft soil.
Gravel base and the length of post to buy
Many installers put about 6 inches of gravel in the bottom of the hole and stand the post on it, so water can drain away from the bottom of the post. This is a common practice, not a code rule, and advice differs by soil: in clay, a pocket of gravel can hold water. The calculator uses 6 inches as a default and lets you change it to 0.
The gravel changes the hole, not the post. The post stands on the gravel, so the buried depth is measured to the bottom of the post, and the hole is deeper by the thickness of the gravel. For the 6 foot fence the hole is 42 inches deep while the post is buried 36 inches. The post length to buy is the above-ground height plus the buried depth: 6 feet plus 3 feet is 9 feet.
Posts are sold in set lengths, so the calculator also shows the length rounded up to the next whole foot, or the next 0.1 m in metric. You can trim the top once the rails are on. If the gravel is for drainage under a base layer, the gravel calculator turns a depth and area into cubic yards and tons.
Setting posts without concrete, and when concrete helps
Concrete is common but not required. Some fences use compacted gravel or tamped soil around the post. In firm, well-drained soil that can hold a light fence, and it lets the wood dry out better than concrete does. Concrete holds posts firmly, which helps with gates, corners and tall solid fences that catch the wind, and for loose or wet soil.
If you use concrete, the hole diameter rule still applies, and the concrete is poured around the post from the bottom of the hole up to a few inches below grade. The QUIKRETE data sheet says to pour the mix until it is about 3 to 4 inches from the top of the hole, then fill the rest with soil from the hole. Keeping the top of the concrete below grade also lets the soil slope water away.
If you need to know how many bags of mix to buy, use the post hole concrete numbers from the hole diameter and depth that this calculator gives you. A 4x4 line post in a 10.5 inch hole that is 42 inches deep is a good starting point for a 6 foot fence.
Using the result, and what to read next
Enter the height above ground and the post width, pick line or corner, and add your own minimum, frost depth, extra depth and gravel base. The calculator returns the buried depth, hole depth, hole width, post length to buy and the share of the post underground, in inches and in centimetres.
Use the numbers to plan the lumber too. If you are costing the posts, the board foot calculator turns the post size and length into board feet. For rock or decorative stone around the posts, the stone calculator gives you the amount by area and depth.
This is an estimate based on rules of thumb. Your local building code, your local frost depth, your soil and the fence maker's instructions come first. Many places need a permit for taller fences. Have an engineer or your building inspector confirm the footings for anything that carries a deck, a heavy gate or a structure, and call before you dig so buried utilities are marked.
Reference Tables
| Fence height above ground | Half of height (in) | Line post buried (in) | Corner or gate post buried (in) | Line post, one third of total length? |
|---|---|---|---|---|
| 3 ft | 18 | 24 | 24 | no, minimum applies |
| 4 ft | 24 | 24 | 32 | yes |
| 5 ft | 30 | 30 | 40 | yes |
| 6 ft | 36 | 36 | 48 | yes |
| 7 ft | 42 | 42 | 56 | yes |
| 8 ft | 48 | 48 | 64 | yes |
| Fence height above ground | Buried (in) | Total length (ft) | Length to buy (ft) | Gravel base 6 in: hole depth (in) |
|---|---|---|---|---|
| 3 ft | 24 | 5.00 | 5 | 30 |
| 4 ft | 24 | 6.00 | 6 | 30 |
| 5 ft | 30 | 7.50 | 8 | 36 |
| 6 ft | 36 | 9.00 | 9 | 42 |
| 7 ft | 42 | 10.50 | 11 | 48 |
| 8 ft | 48 | 12.00 | 12 | 54 |
| Post | Width (in) | Hole diameter (in) | Hole diameter (mm) |
|---|---|---|---|
| 4x4 (3.5 in actual) | 3.50 | 10.5 | 267 |
| 4x6 (3.5 in thick) | 3.50 | 10.5 | 267 |
| 6x6 (5.5 in actual) | 5.50 | 16.5 | 419 |
| 8x8 (7.25 in actual) | 7.25 | 21.8 | 552 |
| 4 inch round | 4.00 | 12.0 | 305 |
| 75 mm round or square | 2.95 | 8.9 | 225 |
| Frost depth (in) | Buried depth (in) | What sets the depth | Total post (ft) | Length to buy (ft) |
|---|---|---|---|---|
| 0 | 36 | the one half rule | 9.00 | 9 |
| 12 | 36 | the one half rule | 9.00 | 9 |
| 24 | 36 | the one half rule | 9.00 | 9 |
| 36 | 36 | the one half rule | 9.00 | 9 |
| 42 | 42 | the frost depth | 9.50 | 10 |
| 48 | 48 | the frost depth | 10.00 | 10 |
| 60 | 60 | the frost depth | 11.00 | 11 |
| Post | Penn State Extension (in) | Calculator (in) | Difference (in) |
|---|---|---|---|
| Line post | 36 | 36 | 0 |
| Corner or gate post | 48 | 48 | 0 |
| Fence height above ground | Buried depth (cm) | Hole depth (cm) | Post length (m) | Length to buy (m) |
|---|---|---|---|---|
| 0.9 m | 60 | 75 | 1.50 | 1.5 |
| 1.2 m | 60 | 75 | 1.80 | 1.8 |
| 1.5 m | 75 | 90 | 2.25 | 2.3 |
| 1.8 m | 90 | 105 | 2.70 | 2.7 |
| 2.1 m | 105 | 120 | 3.15 | 3.2 |
| 2.4 m | 120 | 135 | 3.60 | 3.6 |
| Rule | Buried depth (in) | Post length (ft) | Share buried |
|---|---|---|---|
| One quarter of the post | 24 | 8.00 | 25 % |
| One third of the post (calculator rule) | 36 | 9.00 | 33 % |
| Two fifths of the post | 48 | 10.00 | 40 % |
| One half of the post | 72 | 12.00 | 50 % |
| Gravel base (in) | Buried depth (in) | Hole depth (in) | Post length (ft) |
|---|---|---|---|
| 0 | 36 | 36 | 9.00 |
| 4 | 36 | 40 | 9.00 |
| 6 | 36 | 42 | 9.00 |
| 8 | 36 | 44 | 9.00 |
| 12 | 36 | 48 | 9.00 |
| Extra depth (in) | Buried depth (in) | Share of post buried | Length to buy (ft) |
|---|---|---|---|
| 0 | 36 | 33.3 % | 9 |
| 3 | 39 | 35.1 % | 10 |
| 6 | 42 | 36.8 % | 10 |
| 9 | 45 | 38.5 % | 10 |
| 12 | 48 | 40.0 % | 10 |
| Post width (mm) | Hole width (mm) | Hole width (cm) | Hole width (in) |
|---|---|---|---|
| 50 | 150 | 15.0 | 5.9 |
| 75 | 225 | 22.5 | 8.9 |
| 100 | 300 | 30.0 | 11.8 |
| 125 | 375 | 37.5 | 14.8 |
| 150 | 450 | 45.0 | 17.7 |
| Example | Buried depth (in) | Hole depth (in) | Hole diameter (in) | Post length to buy |
|---|---|---|---|---|
| 6 ft line post | 36 | 42 | 10.5 | 9 ft |
| 4 ft line post | 24 | 30 | 10.5 | 6 ft |
| 6 ft corner post | 48 | 54 | 10.5 | 10 ft |
| 5 ft, frost 48 in | 48 | 54 | 10.5 | 9 ft |
| 1.8 m, 75 mm post | 35.4 | 41.3 | 8.9 | 2.7 m |
| 6 ft, soft soil | 42 | 48 | 10.5 | 10 ft |
Frequently Asked Questions About the Fence Post Depth Calculator
How deep do I put my fence post in the ground?
A common rule is to bury half of the above-ground height, which is one third of the whole post. A 6 foot fence gives 36 inches, and the post is 9 feet long in total. The QUIKRETE data sheet states the half rule, and Penn State Extension gives 36 inches for a typical line post. Frost depth, soil and local code can require more.
Is it possible to install a fence post without digging a hole?
Yes, for some posts. Driven posts need no dug hole: Penn State Extension notes that solid recycled plastic posts need a small-bore pilot hole before driving, and that T-posts are used in some fences. Ground spikes and post anchors also exist. They suit light fences and sheds, not tall solid fences, gates or corners. Follow the maker's directions and local rules.
How wide should a post hole be for a 75mm fence post?
Use 3 times the post width, so a 75 mm post needs a hole about 225 mm wide, which is 22.5 cm or 8.9 inches. That is the hole rule on the QUIKRETE data sheet for setting posts. A wider hole needs more concrete and more digging, and a narrower one leaves too little concrete around the post.
How deep should concrete fence posts be buried?
Concrete posts are heavy and often bought in set lengths, but the burial rule is the same as for other posts: about half of the above-ground height, with corner and gate posts deeper. A 6 foot fence is about 36 inches. Follow the maker's instructions for the post, because concrete posts can be much heavier than wood and may be specified by length.
What is the rule for how deep a wood fence post goes?
For wood, bury about one third of the total post length, which equals half of the height above ground. A 6 foot fence uses a 9 foot post with 36 inches in the ground, and an 8 foot fence uses a 12 foot post with 48 inches. Pressure-treated wood suited to ground contact is the usual choice. The rule is a rule of thumb, not a code.
How deep should chain link fence posts be?
Start with the same rule: half of the above-ground height. A 4 foot chain link fence gives 24 inches with no minimum. Terminal, corner and gate posts carry the tension of the mesh, and Penn State Extension says fences under tension need deeply set posts, so go deeper there. Check the fence maker's instructions for the exact hole size.
How deep should metal fence posts be set?
Metal posts follow the same geometry as wood. The half-height rule gives 24 inches for a 4 foot fence and 36 inches for a 6 foot fence. Light driven T-posts are an exception because they go in without a dug hole. Post-and-rail systems, tubes and gate posts usually specify a depth and a concrete footing, so read the instructions.
How deep should timber fence posts be?
Timber follows the same rule as any wood post: about one third of the total length underground. The calculator works in metres too: a 1.8 m fence needs 90 cm buried, and the post is 2.70 m long. The 60 cm minimum is your own setting. Enter your frost depth if you have one, and follow the fence maker's or your local authority's directions.
Why bury a post one third of its length?
Burying a third keeps the post stable against wind and leaning loads, because the buried part acts like the base of a lever. With the half-height rule, the underground share is 33.3 percent of a 6 foot fence post. For short fences it is not enough, so the calculator applies a minimum depth, 24 inches by default, which you can change.
Do corner and gate posts need to be deeper?
Yes. They carry more load than line posts. Penn State Extension says corner and gate posts are set deeper, often to 48 inches, compared with 36 inches for a typical line post. The calculator multiplies the line depth by 4/3 for these posts. A 6 foot fence goes from 36 to 48 inches, so the post is 10 feet long.
What is the frost line and does it change post depth?
The frost line is the depth to which groundwater in the soil is expected to freeze. Frost heave can lift posts that are not set deep enough, so in cold regions the post must reach below the local frost depth. Enter your frost depth and the calculator uses it when it is deeper than the rule: a 5 foot fence with 48 inches of frost goes 48 inches deep.
How wide should the hole be for a fence post?
The hole diameter should be 3 times the post width, according to the QUIKRETE data sheet. A 4x4 post (3.5 inches actual) needs a hole about 10.5 inches wide, and a 6x6 (5.5 inches) about 16.5 inches. That leaves enough room to pack concrete or gravel around the post without a huge hole to dig.
How much gravel should go in the bottom of the hole?
The common practice is about 6 inches of gravel under the post, and that is the calculator's default. It is a common habit, not a code rule, and advice differs in clay soils where gravel can hold water. Enter 0 if you skip it. A 6 inch base on a 6 foot fence makes the hole 42 inches deep, even though the post is buried 36 inches.
How long a post do I buy for a 6 foot fence?
Add the above-ground height to the buried depth. A 6 foot fence with a post buried 36 inches needs a 9 foot post, and that is what the calculator shows. If the number is not a whole foot, buy the next full length, since posts are sold in set lengths and you can trim the top once the rails are on.
How deep for an 8 foot fence post?
An 8 foot fence gives 48 inches buried by the half-height rule, so the post is 12 feet long. A corner post on the same fence goes 64 inches. Tall solid fences catch wind like a sail, so check local code and consider a deeper hole or concrete footing for gates and corners.
What is the minimum depth for a 4 foot fence post?
By the half-height rule a 4 foot fence needs 24 inches, and the calculator keeps a 24 inch minimum by default, so the answer is 24 inches. Total post length is 6 feet. The minimum is your own choice, not a code figure. In frost country the local frost depth may be deeper than 24 inches.
Should I set fence posts in concrete?
Concrete holds posts firmly, which helps with gates, corners and tall solid fences. It can trap moisture against wood, so many installers keep it below grade and slope the top away from the post. Light fences in firm soil often use tamped gravel or soil. The calculator for bags of mix is a separate tool, and your plans, soil and code decide.
Does soft or wet soil change the depth?
Yes. Penn State Extension says posts in sandy or chronically wet soil will need to be set deeper and perhaps supported by a collar of concrete. It gives no number, so use the extra depth box for your own allowance. Adding 6 inches to a 6 foot line post makes it 42 inches deep and the post 9.5 feet long.
Does the calculator replace local code or an engineer?
No. It gives an estimate from rules of thumb. Local code, the local frost depth, soil conditions and the fence maker's instructions come first, and a fence over a certain height may need a permit. Posts that support decks, gates on heavy loads or retaining structures should be checked by an engineer or your building inspector.
How do I find the local frost depth?
Ask your local building department. Frost depth is set by local code and varies widely, from near zero in warm regions to several feet in cold ones. Wikipedia gives sample frost lines for various places, but your building office has the figure that applies to your address. Enter it in the frost depth box and leave it at 0 if you do not need it.
Does the type of post change the hole diameter rule?
The 3 times rule applies to the post diameter or width for any material. Round posts use the diameter, square posts the width. A 4 inch round post gets a 12 inch hole, and a 75 mm post gets a 225 mm hole. For vinyl or metal posts follow the instructions that come with them, since they may specify a different size.
References
- [1] The QUIKRETE Companies. “QUIKRETE Fast-Setting Concrete #1004-50 (SPEC-DATA): hole diameter 3 times the post, depth one half of above-ground height.” QUIKRETE. Accessed 2026-10-07.
- [2] Penn State Extension. “Fence Planning for Horses (line post 36 inches, corner and gate posts 48 inches, frost heave, wet soils).” Penn State Extension. Accessed 2026-10-07.
- [3] Wikipedia contributors. “Frost line.” Wikipedia. Accessed 2026-10-07.
- [4] Wikipedia contributors. “Frost heaving.” Wikipedia. Accessed 2026-10-07.
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