Hole depth / post length
Fence Post Depth Calculator
Posts for a 6 ft fence go 3 ft into the ground by the bag makers' rule, a third of the post's length, so buy 9 ft posts; Master Halco's guide sets wood line posts 24 in deep.
Sources checked
Your estimate
3 ft in the ground
A third of the post’s length: 6 ft + 36 in = 9 ft, so buy 9 ft posts and dig 42 in with the gravel.
- A third of the post's length (half the height above ground)9 ft post36 in
- A third to a half of the height above ground9 ft post24 in to 36 in
- Master Halco, wood line posts8 ft post24 in
- Master Halco, wood corner and gate posts9 ft post30 in
How we calculate post depth
- A third of the whole post in the ground leaves two thirds above it, so the buried part is half the visible height: 6 ft / 2 = 36 in.
- Post length = height above ground + depth: 72 in + 36 in = 108 in, and the shortest 4x4 Master Halco sells that long is 9 ft.
- Dig 6 in deeper for the gravel QUIKRETE puts under each post: a 42 in hole for a 36 in setting. For the concrete in that hole see how many bags of concrete per fence post.
- A maker that prints a fixed depth for its own fence system gets its own row in the result; a rule written for another kind of fence stays off the list. Posts, rails and pickets for the whole fence are counted together by the fence calculator.
- Bag makers: buried part
- 1/3 of total post lengthQUIKRETE Concrete Mix 1101 data sheet (Oct 2022); Sakrete 5000 Plus Concrete Mix data sheet (2023)
- Wood line posts, Master Halco
- 24 inMaster Halco PostMaster Traditional installation guide
- Gravel below the post
- 6 inQUIKRETE how-to: Setting Posts without Mixing
Frost depth depends on the site and local rules. The optional frost field warns when the entered depth exceeds the listed setting depths; it does not silently change a maker’s rule. Barrette SlideLock vinyl fence instructions; Jerith aluminum fence installation instructions; QUIKRETE project guide: Setting Posts
Depth rules for a 6 ft fence
| Rule or maker | Fence | Depth | Source |
|---|---|---|---|
| A third of the post's length (half the height above ground) | wood, vinyl, aluminum, chain | 36 in | QUIKRETE Concrete Mix 1101 data sheet (Oct 2022); Sakrete 5000 Plus Concrete Mix data sheet (2023); QUIKRETE Concrete Calculator, help text |
| A third to a half of the height above ground | wood, vinyl, aluminum, chain | 24 in to 36 in | QUIKRETE how-to: Setting Posts without Mixing |
| Master Halco, wood line posts | wood | 24 in | Master Halco PostMaster Traditional installation guide |
| Master Halco, wood corner and gate posts | wood | 30 in | Master Halco PostMaster Traditional installation guide |
| Bufftech, vinyl | vinyl | 30 in | Bufftech molded and extruded fence installation guide (2201) |
| Jerith, aluminum, by fence height | aluminum | 24 in | Jerith aluminum fence installation instructions |
| Fortress A2, aluminum | aluminum | 24 in | Fortress A2 aluminum fence instructions |
| Merchants Metals, chain link: 24 in plus 3 in a foot over 4 ft | chain | 30 in | Merchants Metals chain link fence specification 32 31 13 (2014) |
Which wood for fence posts
Buried posts need wood suited to ground contact. Start with the purchase guidance below, then compare the species and treatment details before choosing a post.
- What to buy for buried posts
Choose a pressure-treated post labeled Ground Contact, including when setting it in concrete, or a naturally durable species with mostly heartwood. The Forest Products Laboratory advises pressure-treated lumber, or heartwood of a naturally durable species. A species name alone does not tell you how much durable heartwood is in a post; check the actual piece and its label.
USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021); Arxada, Wolmanized Outdoor: Above Ground vs Ground Contact applications sheet (2022)- Heartwood decay resistance
The Forest Products Laboratory rates heartwood this way: black locust is very resistant; western redcedar, eastern redcedar, white oak and old-growth redwood are resistant; Douglas-fir is moderately resistant; spruces, hemlocks and pines not named in its table are slightly or nonresistant. Sapwood is not covered by these ratings.
USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021)- Treated lumber
Arxada, a maker of treated lumber, lists posts holding up fences, in the ground or in concrete, under Ground Contact. Its guide prints Use Category 4A and 0.15 lb/ft³ of CA-C preservative for sawn fence posts; for pickets it prints 3A, 3B and 0.06. Buy posts rated for ground contact, not above-ground lumber.
Arxada, Wolmanized Outdoor: Above Ground vs Ground Contact applications sheet (2022); Arxada, Wolmanized Outdoor specification guide for treated wood end uses (WOW-5085-R11, 2022)- CCA
The EPA says makers of chromated arsenicals (CCA) voluntarily stopped making it for residential wood in December 2003.
U.S. EPA, Chromated Arsenicals (CCA), web page (January 2021 snapshot)- How long wood lasts in the ground
In Forest Products Laboratory stake tests, untreated southern pine stakes lasted 1.8 to 3.6 years on average in Mississippi, Florida and Louisiana, and a CCA-treated stake 28.7 years in Mississippi. Dipping or brushing added from a few months to 4 years. Stakes are small test pieces, not posts, so read them as a ranking, not a guarantee.
USDA Forest Products Laboratory, Comparison of Wood Preservatives in Stake Tests (FPL-RN-02, 2011 report)
Where depth rules disagree
Two rules called "one third", for a 6 ft fence
- 1/3 of the post's length: 36 inQUIKRETE Concrete Mix 1101 data sheet (Oct 2022)
- 1/3 to 1/2 of the height above ground: 24 in to 36 inQUIKRETE how-to: Setting Posts without Mixing
- At least 24 in for a 6 ft fenceQUIKRETE how-to: Setting Posts without Mixing
QUIKRETE's data sheets and Sakrete bury a third of the post; QUIKRETE's how-to page takes a third of the visible height as its shallow end, 12 in less. QUIKRETE's data sheets and calculator print the deeper rule, as Sakrete's sheet does, and the answer above leads with it.
Fence post depth questions
How deep should a fence post be for a 6 foot fence?
36 in by the rule QUIKRETE and Sakrete print, a third of the post's length. Master Halco sets its wood line posts 24 in deep and corner and gate posts 30 in.
How deep do posts go for a 4 foot fence?
24 in by the third-of-the-post rule, on 6 ft posts. Jerith's minimum for its 4 ft aluminum fence is 18 in.
How deep for an 8 foot fence post?
48 in, which calls for a 12 ft post. Jerith's table gives 36 in for its 8 ft aluminum fence.
How deep should gate and corner posts be?
Master Halco sets wood corner and gate posts 30 in deep against 24 in for line posts. A gate also needs its own opening worked out; the fence picket calculator takes each gate out of the run.
What length post do I buy for a 6 foot fence?
9 ft with 36 in in the ground. At Master Halco's 24 in depth an 8 ft post is enough.
How deep to set a vinyl fence post?
Bufftech sets vinyl posts 30 in deep, or to the frost line where that is deeper. Choose Vinyl above and enter your frost depth to see which one applies.
How deep to set a chain link fence post?
Merchants Metals asks 24 in plus 3 in for each foot over 4 ft: 24 in for a 4 ft fence and 30 in for a 6 ft one.
Do fence posts have to go below the frost line?
Some makers ask for it: Bufftech digs vinyl holes to the frost line when it is deeper than 30 in, and Barrette goes 6 in below it. QUIKRETE asks for 6 in below frost on structural posts such as a deck's.
A planning aid, not an engineering calculation: your local code and building inspector have the final word. Terms