Fence Post Depth Calculator

How to use this calculator

1. Enter the fence height above ground. Use the height of the finished fence, not the length of the post, because every depth rule here is written from the exposed height.
2. Pick the post. Wood posts are entered at their actual size, so a 4x4 is 3.5 inches and a 6x6 is 5.5 inches under the American Softwood Lumber Standard, PS 20. For chain link or other steel pipe, choose round and type the outside diameter.
3. Say which post it is. Gate, corner and end posts carry more load than a line post and get a deeper minimum.
4. Add your frost depth if you live where the ground freezes. Your building department publishes it. There is no national figure.
5. Add a town minimum if your permit or fence ordinance names one. If it does, that is the number your inspector will measure.
6. Choose gravel if you are putting a layer under the post. It makes the hole deeper but does not change how much concrete goes in.

Leave the hole diameter blank and the calculator suggests a size you can buy an auger or digger for. Type one in to see the depth and bag count for the hole you actually plan to dig.

How the depth is worked out

No building code sets a depth for a fence post, so this is not a code calculation. It takes four published numbers and uses whichever one is deepest:

Post depth = the deepest of: ASTM F567 depth, gate or corner minimum, your frost depth, your local minimum

ASTM F567 is the chain link installation standard, and its footing rule is the only depth-by-height rule published by a standards body. The Chain Link Fence Manufacturers Institute reproduces it in its product manual as a "Minimum footing depth, 24 in. plus an additional 3 in. for each 1 ft. increase in the fence height over 4 ft." Written as a formula:

Depth in inches = 24 + 3 × (fence height in feet − 4), and never less than 24

Gate, corner and end posts get a deeper floor. Two USDA NRCS fence specifications publish it: the California 2019 specification sets wood line posts 24 inches deep and brace, corner, gate and end posts 3 feet deep, and the Vermont specification sets line posts at 30 inches and corner, gate and end posts at 36. The calculator applies 36 inches, or 6 inches more than the line post if that is deeper, since 6 inches is the smallest gap either specification leaves. Those are farm fence documents. Nobody publishes a gate post depth for a residential wood fence, so treat this as the nearest published figure rather than a rule written for your yard.

Frost depth is a local lookup. The model code prints the frost line column blank and tells each jurisdiction to fill it in, and the published values run from 0 inches in low-elevation Arizona to 5 feet in northern Minnesota under Minnesota Rules 1303.1600. Whether frost rules written for building footings strictly apply to a fence post is a question no authority has answered; the frost section of the depth guide sets out what is known. Getting below it is the safe call, so the calculator does.

A local minimum beats everything when you have one. The Village of Lombard, Illinois sets a 36 inch minimum and inspects the holes before you pour, and Cook County, Illinois requires 42 inch post holes on unincorporated land.

The result also shows the one-third rules printed on concrete bags, as a range rather than an answer. Quikrete and Sakrete publish four versions that disagree with each other, from a third of the exposed height to half of it plus 6 inches, which for a 6 foot fence is anywhere from 24 to 42 inches. Why the one-third rule disagrees with itself lays out all four.

Fence post depth chart

Run the method for the common fence heights and this is what comes out, for 4x4 posts with no frost or local minimum in play. The post length is the exposed height plus the depth, and the stock column rounds up to the next length a lumberyard sells.

The 6 foot row is the one that catches people. The standard advice is to buy an 8 foot post for a 6 foot fence, and Sakrete's own post setting guide says exactly that, because its rule is a third of the exposed height, or 24 inches. Set the post to ASTM F567 depth instead and you need 8 feet 6 inches of post, which means buying a 10 footer and cutting it. If you want the 8 foot post, you are choosing 24 inches in the ground, which is the shallowest of the published figures.

Wherever your frost line is deeper than the line post depth in this table, the frost line wins and the post gets longer with it. In Minnesota Zone I, a 6 foot fence set to 60 inches needs 11 feet of post and a 12 foot board.

How wide to dig the hole

Three published rules give three different widths, and the calculator reports all of the ones that apply to your post.

• The code minimum. IBC 1807.3.3, as adopted in Seattle, requires an embedded post's hole to be at least 4 inches larger than a round post, or 4 inches larger than the diagonal of a square one. For a 4x4 that is 8.95 inches.
• The bag rule. Both big bag makers say three times the post width, which is 10.5 inches for a 4x4.
• ASTM F567, for round pipe. Four times the post's outside diameter up to 4 inches, and three times above that. It is wider than the bag rule on purpose, because it was written for thin steel posts.

For a square wood post, the suggested hole is the auger size closest to the bag rule that still clears the code minimum. For a round post it is the first size at or above the F567 figure, because F567 is a minimum rather than a rule of thumb. Augers and diggers come in 6, 8, 10, 12, 14, 16, 18 and 24 inches.

Look at what the 6x6 does to the bag count. Going up one post size more than doubles the concrete, because the hole widens with the post and volume rises with the square of the width. If you are tempted to use 6x6 posts on a whole fence line for stiffness, price the concrete first. A deeper hole buys more holding power per bag than a wider one, which the depth guide explains with the code embedment equation.

A worked example

A 6 foot privacy fence in Columbus, Ohio, on 4x4 posts, with a walk gate hung on 6x6 posts. Columbus publishes a frost line depth of 32 inches, and the owner wants 6 inches of gravel under each post.

• ASTM F567 for a 6 foot fence: 24 + 3 × (6 − 4) = 30 inches
• Frost depth: 32 inches, which is deeper, so it sets the line posts
• Line post hole: 32 inches of concrete on 6 inches of gravel, so dig 38 inches deep and 10 inches across
• Line post length: 6 ft + 32 in = 8 ft 8 in, so buy 10 foot posts
• Concrete per line post: 1.23 cu ft, which is 4 bags of 50 lb fast-setting mix
• Gate posts: the 36 inch gate minimum beats the frost line, so 36 inches deep in a 16 inch hole for the 6x6, on a 10 foot post
• Concrete per gate post: 3.56 cu ft, which is 10 bags, more than twice a line post

For the whole fence, total the volume before you round. Fourteen line posts hold 17.19 cu ft, which is 46 bags, and the two gate posts 7.12 cu ft, which is 19 bags, so 65 bags before waste. Rounding up hole by hole would say 76, but fast-setting goes in dry, so the part bag left from one hole starts the next. Enter 14 and then 2 in the posts field to get those totals, or let the fence post concrete calculator count the posts from the length of the run.

What this calculator does not check

It does not design the post against wind. The building code does have a method for that, IBC 1807.3.2.1, but it needs the actual sideways force on the post, and that depends on how solid your fence is, how exposed the site is and how far apart the posts are. A solid privacy fence catches roughly six times the wind of the same area of chain link, by the industry's own wind load guide, which is why a depth borrowed from a chain link standard is a floor for a board fence rather than a ceiling.

It does not adjust for soil either, and soil matters. Clay resists a leaning post with about half the lateral pressure of sandy gravel under IBC Table 1806.2. If you are in heavy clay, on a windy lot, or building over 6 feet, go deeper than the number above. The soil section of the depth guide has the table.

And it is for fences. A post that holds up a deck, a pergola or a roof is a structural footing, sized from the load rather than the height, and the deck footing size calculator handles that case.