Fence Post Depth and Hole Diameter: A Field-Tested Guide for 4×4 and 6×6

The first time we built a fence we used the 1/3 rule we read on a Home Depot bag of concrete. 8-foot posts, 32 inches in the ground, 64 inches above. That fence is still standing 11 years later. The second fence we built we tried to cheap out, 24-inch holes for 8-foot posts. That fence started leaning in year 3 and was visibly off-plumb in every section by year 5.

The math is not complicated and the consequences of getting it wrong are slow but expensive.

The 1/3 rule

The starting point for fence post depth is: one-third of the total post length goes in the ground.

• 6-foot fence with 8-foot posts: 32 inches in the ground, 64 inches (5'4") above. The extra 8 inches gets you to a full 6 feet above grade because you'll set the post about 4 inches up from the bottom of the hole on a gravel pad.
• 8-foot fence with 10-foot posts: 40 inches in the ground.
• 4-foot fence with 6-foot posts: 24 inches in the ground.

The 1/3 rule assumes:

• Normal soil (not pure sand, not pure clay, not loose fill)
• No frost line concerns (more on this below)
• Standard fence height with normal wind exposure
• Concrete or properly compacted gravel setting

Where the rule breaks

The 1/3 rule needs adjustment in three situations.

Freeze zones. Anywhere the ground freezes in winter, your hole has to go below the frost line. Frost line depth varies from 0 inches in south Florida to 60+ inches in northern Minnesota. The IRC publishes a frost depth map and most county building departments have local-specific numbers.

If your frost line is 36 inches and the 1/3 rule says 32 inches, you need to dig to 36 (or actually deeper, see the gravel pad section below). If the 1/3 rule says 40 inches and frost line is 36, the 1/3 rule wins and you dig 40.

Loose or sandy soil. Sand doesn't grip a concrete-set post as well as loam or clay. In sandy soil (common in coastal areas, parts of Florida, parts of Texas), go 1.5× the 1/3 depth. A 6-foot fence in sandy soil wants 48-inch holes, not 32.

Tall fences and gate posts. Anything over 6 feet of fence, or any post that holds a gate, carries more leverage and load. Go to 1/2 of post length in the ground, not 1/3. Gate posts especially, the gate's swinging weight tries to tip the post backward every time it opens. We've seen 4-foot gate posts pulled out of the ground by 5-year-old kids hanging on them.

Hole diameter

The hole has to be wider than the post for the concrete to grip it.

Rule of thumb: hole diameter = 3 × post width.

• 4×4 post: nominal 4 inches, actual 3.5 inches. Hole = 10 to 12 inches.
• 6×6 post: nominal 6 inches, actual 5.5 inches. Hole = 16 to 18 inches.
• 8×8 post: nominal 8 inches, actual 7.5 inches. Hole = 22 to 24 inches.

Going wider gives the concrete more grip but uses more concrete (and more digging). Going narrower than 3× saves time but the concrete plug is thinner around the post and breaks under load.

A 10-inch auger bit works fine for most 4×4 fences. A 12-inch auger is better for corner posts and gate posts. For 6×6 posts you'll want a 16-inch hole, which usually requires a one-person gas auger or a two-person manual auger.

The gravel pad

A 4-6 inch layer of gravel at the bottom of every fence post hole is non-negotiable. We see fence posts rot out at the bottom every year in reader emails, and almost always the failure is "I didn't put gravel in."

Why gravel matters: water flows down through the soil around the post. When it hits the bottom of the hole, it has nowhere to go if you've packed dirt or concrete down there. The water sits at the post bottom, soaks into the wood, and rots it from the underside up. Within 7-10 years the post bottom is mush and the fence starts to settle and lean.

A 4-inch layer of #57 gravel at the bottom of the hole creates a drainage layer. Water that gets down there can move sideways through the gravel into the surrounding soil. The post bottom stays dry, the wood doesn't rot.

This also means your hole has to be 4-6 inches deeper than your target post depth, because the gravel takes up space. A 32-inch target depth becomes a 36-38 inch dig.

How much concrete per post

For a 4×4 post in a 10-inch hole at 32 inches deep with a 4-inch gravel pad (so 28 inches of concrete around the post):

• Hole volume: π × 5² × 28 = 2,199 cubic inches
• Post volume in the hole: 3.5 × 3.5 × 28 = 343 cubic inches
• Concrete needed: 2,199 - 343 = 1,856 cubic inches = 1.07 cubic feet

That's roughly 2 bags of 60 lb concrete or 1.5 bags of 80 lb concrete per post.

For a 6×6 post in a 16-inch hole at 36 inches deep with a 4-inch gravel pad (32 inches of concrete):

• Hole volume: π × 8² × 32 = 6,434 cubic inches
• Post volume: 5.5 × 5.5 × 32 = 968 cubic inches
• Concrete needed: 6,434 - 968 = 5,466 cubic inches = 3.16 cubic feet

That's roughly 6 bags of 60 lb concrete or 4-5 bags of 80 lb concrete per post.

For a 50-post fence that's a lot of concrete. The fence calculator on this site does this math for you across the entire perimeter, with frost-line adjustment.

Should you use concrete at all?

Two camps on this.

Concrete camp. Sets fast, holds firm, you can release the post brace in 4-24 hours.

Gravel-only camp. Tamped gravel around the post drains better than concrete and lets the post breathe. Many old farm fences use no concrete at all, just tamped gravel and clay.

We use concrete. The reason: concrete doesn't move once it sets. Gravel-set posts can slowly shift over decades, especially in clay soil that expands and contracts with moisture. For a fence you want to look straight in 15 years, concrete wins.

The one exception: in areas with no frost (south Florida, deep south Texas) and well-draining sandy soil, a tamped gravel set works fine. Saves money, drains naturally, and the soil doesn't shift seasonally enough to move the post.

Common mistakes

Setting posts without bracing them plumb. Concrete sets in 30-60 minutes, you don't get a second chance. Brace every post with two 1×4 stakes before pouring.

Pouring concrete on a hot dry day without misting. Cures too fast, develops a chalky weak surface around the post.

Filling the hole with concrete all the way to grade. Water pools on top of the concrete plug and stays against the post. Bring the concrete to about 2 inches below grade, then mound dirt and grass on top so water sheds away.

Setting the post directly on the dirt at the bottom of the hole. No gravel pad means the post bottom sits in standing water for the life of the fence. Use gravel.

Using fast-setting concrete for tall fence posts. Fast-setting is fine for short fence posts and mailboxes, but for anything over 6 feet, standard-cure concrete bonds better and stays stronger long-term.

Skipping the permit. Most municipalities require a permit for any fence over 6 feet. The inspector checks post depth.

Field-tested rules from 20 years of fence projects

These are the rules we actually use, summarized:

• 6-foot fence, normal soil: 8-foot 4×4 posts, 10-inch holes, 32 inches deep + 4-inch gravel pad. 36-38 inch dig.
• 6-foot fence, sandy soil: 8-foot 4×4 posts, 12-inch holes, 40 inches deep + 4-inch gravel pad. 44-46 inch dig.
• 8-foot fence: 10-foot 6×6 posts, 16-inch holes, 40 inches deep + 6-inch gravel pad. 46-48 inch dig.
• Gate posts (any fence): Always one size up. If the fence uses 4×4, use 6×6 for gate posts. Same on diameter and depth. Brace gate posts to adjacent line posts with horizontal beams.
• Corner posts: Same as gate posts. The fence pulls at corners.

This guide isn't a substitute for your local code. The frost line, wind zone, and soil type rules in your county can override anything we've said here. Check with your building department before you dig.

Last updated May 2026