Bury at least one third of the post, which works out to 24 inches minimum for a 4 to 6 foot fence and about 36 inches for an 8-foot fence. If you live where the ground freezes, the frost line overrides that rule: the bottom of the footing should sit at least 6 inches below your local frost depth. Gate and corner posts go about 6 inches deeper than line posts.
The one-third rule and the frost line override
The one-third rule is the usual starting point. It comes in two versions, and they give different answers:
- One third of the overall post length. Quikrete's Fast-Setting Concrete Mix data sheet says hole depth should be 1/3 of the overall post height. For a 6-foot fence that means a 9-foot post with 3 feet in the ground.
- One third to one half of the height above ground, plus gravel. Quikrete's post-setting how-to sets the hole at 1/3 to 1/2 of the above-ground height, plus about 6 inches for a compacted gravel base. For a 6-foot fence that is 24 to 36 inches of post in the ground, in a hole 30 to 42 inches deep.
An 8-foot post on a 6-foot fence leaves about 24 inches in the ground, which meets the second version and not the first. That is fine in firm soil in a mild climate. It is the bare minimum, with nothing in reserve for frost or wind.
The rule that beats both is the frost line. Water in the soil freezes, expands and lifts anything that is not anchored below it. A post or footing that sits in the frozen layer gets jacked upward a little every winter, and the fence line leans and waves as posts move by different amounts. The common practice is to put the bottom of the footing at least 6 inches below the local frost depth.
So your working depth is the deeper of two numbers: the one-third depth for your fence height, or frost depth plus 6 inches.
CalculatorFind your post depth with the post depth calculatorFence post depth by fence height
These are embed depths, meaning how much post is below grade. Add the gravel base to get the hole depth.
| Fence height | Minimum embed | Typical post | Notes |
|---|---|---|---|
| 3 to 4 ft | 24" | 4x4x6 or 4x4x8 | 24" is also the ASTM F567 minimum for chain link |
| 5 ft | 24" to 30" | 4x4x8 | 30" if the site is windy or the soil is loose |
| 6 ft | 24" to 36" | 4x4x8 (about 24" to 27" embed) | Use 4x4x10 in frost or high-wind areas |
| 8 ft | 36" | 6x6x12 | Taller fences need a bigger post, not just a deeper one |
The 6-foot numbers come from the way wood fences are actually built. America's Fence Store lists 69 inches above grade for a 6-foot board fence, which leaves 27 inches of an 8-foot post in the ground.
For chain link, ASTM F567 section 5.1 sets a minimum of 24 inches plus 3 inches for each foot of fence height over 4 feet. That is 24 inches for a 4-foot fence, 30 inches for a 6-foot fence, and 36 inches for an 8-foot fence.
Frost depth by region
The table below gives typical code frost depths as broad ranges. It is only a starting point. Frost depth is set locally, often county by county, and it changes with elevation.
| Region | Typical code frost depth | Example |
|---|---|---|
| South Florida, Gulf Coast, low deserts of Arizona and southern California | 0" to 12" | Frost rarely controls; use the one-third rule |
| Southeast and southern Plains (Georgia, the Carolinas, Oklahoma, north Texas) | 6" to 18" | One-third rule usually controls |
| Mid-Atlantic and lower Midwest (Virginia, Maryland, Kentucky, Missouri) | 18" to 36" | Frost and one-third depths are close |
| Northeast and Great Lakes (Pennsylvania, New York, Ohio, Illinois, Michigan) | 36" to 48" | Michigan requires 42" (Mich. Admin. Code R 408.30449) |
| Upper Midwest, northern New England, northern Plains | 42" to 60" or more | Minnesota requires 42" in the south and 60" in the north (Minn. R. 1303.1600) |
| Mountain West | Varies widely with elevation | Always check locally |
To get the real number, call your local building department or look up the residential code your city or county has adopted. The frost line depth is listed in the climate and geographic design criteria table, usually Table R301.2(1). Many jurisdictions do not require a permit for a fence, but the frost number still applies to the ground.
The one-third rule says 24 to 36 inches. Frost depth plus 6 inches says 48 inches. The frost number wins, so you need 48 inches of embed. With about 69 inches above grade, that is a 117-inch post, so you buy 4x4x10s.
Gate posts and corner posts go deeper
Line posts mostly resist wind. Terminal posts do more work. A corner post takes rail and board load from two directions, an end post has the whole run pulling on one side, and a gate post carries a swinging, cantilevered load every time the gate opens.
Set corner, end and gate posts about 6 inches deeper than your line posts, in a wider hole. For gates wider than 4 feet, or any gate on a 6-foot privacy fence, consider stepping up to a 6x6 or a steel post.
Hole diameter
A post hole should be about three times the post width. Quikrete's data sheets say to dig "about 3 times the diameter of the post," and America's Fence Store specifies a 10-inch hole for 4-inch square posts.
| Post | Line post hole | Corner, end and gate hole |
|---|---|---|
| 4x4 wood (3½" actual) | 10" | 12" |
| 6x6 wood (5½" actual) | 12" to 14" | 14" to 16" |
| 5x5 vinyl | 10" to 12" | 12" (Bufftech manual) |
| Chain link, 1⅝" line post | About 6" to 7" | Not applicable |
| Chain link, 2⅜" terminal post | Not applicable | About 9" to 10" |
| Chain link gate post | Not applicable | 8" to 10" for residential gates; 12" for larger gates |
Chain link follows a different rule. ASTM F567 section 5.1 sets the hole diameter at four times the largest cross section of the post for posts up to 4 inches outside diameter, and three times for larger posts. A 1⅝-inch line post gets a 6½-inch hole by that math. For residential gate posts, the standard's Table 1 calls for 8-inch holes for gate leaves up to 4 feet wide and 10-inch holes for wider leaves, both 30 inches deep in dirt.
Wider is not always better. Every extra inch of diameter adds concrete fast, because volume grows with the square of the diameter. A 12-inch hole holds about 50 percent more concrete than a 10-inch hole at the same depth.
Gravel base
Put about 6 inches of compacted gravel in the bottom of the hole before you set the post. It lets water drain away from the end grain of a wood post, and it gives you a firm, level base to set the post on. Quikrete's post-setting guide recommends a 6-inch gravel base under each post. Tamp it firm with the end of a 2x4 before the post goes in.
Remember to dig for it. If you want 30 inches of embed, dig a 36-inch hole. The post sits on the gravel, so the gravel counts toward how much post is below grade, but not toward the concrete volume.
Soil adjustments
The same depth behaves differently in different ground.
| Soil | Adjustment |
|---|---|
| Sand or loose fill | Dig about 6" deeper, or 25% deeper, and widen the hole 2" |
| Clay | Standard depth, gravel base for drainage, slope the concrete top away from the post |
| Firm loam | Standard depth |
| Solid rock at the surface | ASTM F567 section 5.5 allows a hole 3 times the post's cross section deep and ½" wider than the post, for chain link set in rock |
| Rock partway down | Go as deep as you can, widen the hole above the rock, and consider a rock bar or a rented hydraulic auger |
In rocky ground, a hole that is shallower but anchored into solid rock can be stronger than a deeper hole in loose soil. In sand, the opposite is true: the soil does not grip, so you need more depth and a wider footing to spread the load.
How to choose post length
Post length is the height above grade plus the embed depth, rounded up to the next stock length. Wood posts are sold in 8, 10 and 12-foot lengths, and some yards also stock 6-foot 4x4s.
| Fence | Above grade | Embed | Total | Buy |
|---|---|---|---|---|
| 4 ft wood picket | 45" | 24" | 69" | 4x4x6 or 4x4x8 |
| 4 ft wood, 42" frost | 45" | 48" | 93" | 4x4x8 |
| 5 ft wood | 60" | 30" | 90" | 4x4x8 |
| 6 ft wood privacy, mild climate | 69" to 72" | 24" to 27" | 96" | 4x4x8 |
| 6 ft wood privacy, frost or wind | 69" to 72" | 36" to 48" | 105" to 120" | 4x4x10 |
| 8 ft wood privacy | 96" | 36" or more | 132" or more | 6x6x12 |
A few other fence types come in lengths sized for the job:
- Chain link. The usual rule of thumb is line posts at fence height plus 2 feet and terminal posts at fence height plus 3 feet. For a 6-foot fence that is 8-foot line posts and 9-foot terminals. Check the result against ASTM F567's 30-inch minimum for a 6-foot fence, and go up a size if your line posts will not reach it.
- Vinyl. Posts are usually sold for the fence height. A 5x5x9 post is common for 6-foot privacy panels, and 84-inch and 96-inch posts are typical for 4-foot and 5-foot fences. Bufftech's manual calls for 30-inch holes or the frost line, whichever is deeper.
- Split rail. Posts are mortised for the rails, so the length is fixed. The Split Rail Fence Store sets 7-foot 3-rail posts 30 inches deep.
Cut posts to final height after they are set and the concrete has cured. It is far easier to trim a row of posts to a string line than to set each one at an exact height.
CalculatorWork out the concrete for your holesQuestions people ask
How deep should a 6 foot fence post be?
At least 24 inches, and 30 to 36 inches is better. In a cold climate the bottom of the footing should sit at least 6 inches below your local frost line, which often means 42 to 54 inches and a 10-foot post.
Is 2 feet deep enough for a fence post?
Two feet is the minimum for a 4 to 6 foot fence in firm soil where the ground does not freeze deeply. Go deeper for gate and corner posts, sandy soil, windy sites, and anywhere the frost line is below 18 inches.
What is the one-third rule for fence posts?
Bury about one third of the post. Quikrete's post-setting guide phrases it as one third to one half of the height above ground, plus 6 inches for a gravel base. The frost line overrides the rule when it calls for a deeper hole.
How deep should gate posts be?
Set gate posts and corner posts about 6 inches deeper than line posts, in a wider hole. They carry the gate's weight or the pull from two directions, and they carry the gate's weight through every swing.
How wide should a fence post hole be?
About three times the width of the post. That is 10 inches for a 4x4 line post, 12 inches for 4x4 corner, end and gate posts, and 12 to 16 inches for a 6x6. Chain link holes are about four times the pipe diameter under ASTM F567.
How do I find my frost line depth?
Call or check the website of your local building department. The frost depth adopted for your area is listed in the local residential code, usually in Table R301.2(1). Regional maps are only a starting point.
What length post do I need for a 6 foot fence?
An 8-foot 4x4 gives about 24 to 27 inches in the ground for a 6-foot privacy fence, which works in mild climates. In frost or high-wind areas buy 10-foot posts so you can bury 36 to 48 inches.
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Written by the My Fence Calculator editorial team. Quantities are estimates; verify with your supplier and local code.