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Fence post depth calculator

Enter the fence height and your state. We apply the 1/3 rule, check it against the frost depth where you live, and give the hole depth and width and the post length to buy.

Set the post 36 in deep. Dig the hole 42 in deep and 10.5 in wide, with 6 in of gravel in the bottom. The post is 9 ft long, so buy a 4x4 x 10 ft post.
By the depth rule
36 in (half the height above ground)
Frost depth
0 in
Concrete
3.8 bags of 50 lb fast-setting per post, filled to 3 in below grade
Fence post depth by fence height, 4x4 posts
Fence height1/3 of the postShallowestPost to buy
3 ft18 in12 in8 ft
4 ft24 in16 in8 ft
5 ft30 in20 in8 ft
6 ft36 in24 in10 ft (8 ft)
8 ft48 in32 in12 ft
Count the bags: concrete for fence posts

Bags of fast-setting or regular mix per post and for the whole fence.

Planning figures for a wood fence. The frost depths are typical state values; your building department sets the one that applies to your lot. Call 811 before you dig.

How the fence post depth calculator works

Enter the height of the fence above ground. The calculator buries half that height, which puts a third of the whole post in the ground, or a third of the height if you pick the shallow end. If you set the posts below the frost line and your frost depth is deeper, the frost depth wins. It then adds 6 in of gravel for the hole depth and rounds the post up to a length the store stocks.

Why a third of the post

The part in the ground is what keeps the fence standing against wind on the boards. A privacy fence catches more wind than a picket fence, so it is worth staying at the deep end of the range, and corner, end and gate posts carry the most.

The gravel and the concrete

The 6 in of gravel under the post lets water drain away from the end grain. The concrete stops 3 in below grade so soil and sod can cover it, and its top should slope away from the post.

Frost line depth by state

The frost line is how deep the ground freezes in a cold winter. Below it, the soil does not freeze and lift. These are typical values; many states set the depth county by county, so treat them as a starting point and ask your building department.

StateTypical frost depthRangeWho sets it
Alabama12 in6 to 18 invaries by county
Alaska42 in32 to 120 inset locally
Arizona12 in6 to 30 inset locally
Arkansas12 in12 to 18 invaries by county
California12 in0 to 36 invaries by county
Colorado36 in30 to 48 inset locally
Connecticut42 insame statewidestatewide code
Delaware24 in24 to 32 invaries by county
Floridanonesame statewidestatewide code
Georgia12 insame statewidestatewide code
Hawaiinonesame statewidestatewide code
Idaho24 in24 to 48 invaries by county
Illinois42 in30 to 42 inset locally
Indiana30 in24 to 36 invaries by county
Iowa42 in42 to 48 invaries by county
Kansas36 in24 to 36 inset locally
Kentucky24 in24 to 33 instatewide code
Louisiananonesame statewidevaries by county
Maine48 in36 to 60 invaries by county
Maryland30 in24 to 42 invaries by county
Massachusetts48 insame statewidestatewide code
Michigan42 insame statewidestatewide code
Minnesota60 in42 to 60 instatewide code
Mississippi12 insame statewidevaries by county
Missouri36 in12 to 36 inset locally
Montana48 in36 to 60 invaries by county
Nebraska42 in36 to 42 invaries by county
Nevada24 in12 to 30 inset locally
New Hampshire48 in48 to 60 invaries by county
New Jersey36 in30 to 36 instatewide code
New Mexico18 in12 to 30 instatewide code
New York48 in36 to 60 invaries by county
North Carolina12 in12 to 24 instatewide code
North Dakota48 in48 to 60 invaries by county
Ohio32 in30 to 42 invaries by county
Oklahoma18 in12 to 18 invaries by county
Oregon12 in12 to 24 instatewide code
Pennsylvania36 in30 to 42 invaries by county
Rhode Island40 in30 to 40 instatewide code
South Carolina12 insame statewidestatewide code
South Dakota48 in42 to 60 inset locally
Tennessee12 in6 to 18 invaries by county
Texas6 insame statewideset locally
Utah30 in12 to 42 invaries by county
Vermont60 in48 to 72 instatewide code
Virginia18 in12 to 24 invaries by county
Washington12 in12 to 24 invaries by county
West Virginia30 in24 to 36 invaries by county
Wisconsin48 in48 to 60 instatewide code
Wyoming36 in36 to 48 inset locally

Where the table says none, the ground does not freeze deep enough to matter, and the post depth follows the 1/3 rule alone.

Next steps

To count the bags for the holes, use the concrete for fence posts calculator. For the posts, rails and pickets of a whole fence, use the fence calculator. Setting posts for a deck instead? The deck footing calculator sizes footings from DCA 6.

Where the numbers come from

The depth rule, the hole width of 3 times the post, the 6 in of gravel and the fill line 3 in below grade come from Quikrete, Setting Posts in Concrete, which gives a depth of 1/3 to 1/2 of the above-ground length of the post, plus 6 in.

The typical frost depth and county range for each state are from footingdepth.com, Frost Line Depth by State or ZIP Code, full US chart, read Nov 6, 2026. We checked Minnesota against Minnesota Rules 1303.1600, which sets 5 ft in the northern zone and 3 1/2 ft in the southern zone by county. Setting fence posts below the frost line is our editorial default, not a code requirement for fences.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

Bury one third of the whole post, which is the same as burying half the height that stands above ground. A 6 ft fence on a 9 ft post has 36 in in the ground. Quikrete's instructions give a range of 1/3 to 1/2 of the above-ground height, plus 6 in for gravel, so the shallowest you should go for a 6 ft fence is 24 in.

36 in by the 1/3 rule, and never less than 24 in. Add 6 in of gravel under the post, so the hole is 42 in deep, and buy a 10 ft post. Where the frost line is deeper, go to the frost line: in northern Minnesota, at 60 in, a 6 ft fence needs a 12 ft post.

24 in by the 1/3 rule and 16 in at the shallow end, with 6 in of gravel under it. An 8 ft post covers both. An 8 ft privacy fence needs 48 in and a 12 ft post.

Where the ground freezes, setting the bottom of the concrete below the frost line keeps frost heave from pushing the posts up over the winters. Building codes usually set frost depth for house and deck footings, not fences, so ask your building department what it expects for a fence. The calculator lets you switch the frost check off.

3 times the width of the post, by Quikrete's instructions. A 4x4 is 3 1/2 in wide, so the hole is 10.5 in; a 6x6 is 5 1/2 in, so the hole is 16.5 in. A 10 or 12 in auger is the usual tool for 4x4 posts.