Concrete for Fence Posts: Post Hole Concrete Done Right

For a standard wood privacy fence (4x4 posts, 8 feet apart), you're looking at roughly 1-2 bags of concrete per hole depending on hole size and depth. Multiply that by 15-25 posts for a typical yard and you're buying 20-40 bags. Use our post hole calculator to get the exact count for your project dimensions.

How Much Concrete Per Hole

The amount depends on three variables: hole diameter, hole depth, and post size (which displaces volume in the center).

Standard Residential Fence Posts

Post Size	Hole Diameter	Hole Depth	Concrete per Hole	50-lb Bags
4x4 (3.5" actual)	10 inches	24 inches	~0.6 cu ft	1-1.5
4x4 (3.5" actual)	10 inches	36 inches	~0.9 cu ft	1.5-2
6x6 (5.5" actual)	12 inches	30 inches	~1.1 cu ft	2-2.5
6x6 (5.5" actual)	12 inches	42 inches	~1.7 cu ft	3-3.5

The hole diameter should be 3 times the post width. A 4x4 post (3.5 inches actual) needs a 10-12 inch hole. A 6x6 post (5.5 inches actual) needs a 14-16 inch hole. Undersized holes don't allow enough concrete around the post for strength.

For exact calculations based on your specific hole dimensions, use our concrete calculator with the post hole mode to account for the post displacement automatically.

Hole Depth: Frost Line Matters

The Two Rules

1. 1/3 rule: Bury 1/3 of the total post length. An 8-foot post gets 2.5-3 feet underground.
2. Frost line rule: The bottom of the concrete must be below your local frost line.

Use whichever is deeper. In southern states with shallow frost lines, the 1/3 rule governs. In northern states where frost lines reach 36-48 inches, the frost line governs.

Region	Approximate Frost Line	Minimum Hole Depth (4x4, 8 ft post)
Deep South (FL, LA, TX coast)	0-6 inches	24-30 inches (1/3 rule)
Mid-Atlantic (VA, NC, TN)	12-18 inches	24-30 inches (1/3 rule)
Midwest (OH, IN, IL)	30-36 inches	36 inches (frost line)
Northern (MN, WI, MI)	42-60 inches	42-48 inches (frost line)
Mountain West (CO, MT)	36-48 inches	40-48 inches (frost line)

Check your exact local frost line with your building department or county extension office. Getting this wrong leads to fence posts that heave out of the ground over the first winter.

Setting Posts: Step by Step

Preparation

1. Mark post locations. Set string line between corner points. Mark each post location (typically 6-8 feet apart for privacy fence, 8-10 feet for rail fence).
2. Call 811 at least 2 business days before digging. This is free and legally required.
3. Dig holes. A power auger (rental $50-75/day) saves hours compared to a manual post hole digger. Make holes 2-3 inches wider than your target diameter--precision isn't critical.

Gravel Base

Place 3-4 inches of gravel at the bottom of each hole before anything else. This serves two purposes:

• Drainage: Water runs past the post bottom instead of pooling. Standing water at the base is the primary cause of premature post rot.
• Stable base: The gravel gives the post a solid surface to sit on while you plumb and brace it.

Read our gravel guide for more on why proper base material matters for any concrete project.

Wet Mix Method (Recommended for Gate and Corner Posts)

1. Pre-mix the concrete in a wheelbarrow following proper water ratios. Aim for a thick, workable consistency--not soupy.
2. Set the post on the gravel base and plumb it in both directions using a level
3. Brace the post with temporary 2x4 supports so it stays plumb
4. Shovel concrete into the hole around the post
5. Tamp with a stick or piece of rebar to eliminate air pockets
6. Fill to within 2-3 inches of ground level
7. Crown the concrete slightly so water sheds away from the post
8. Keep braces in place for 24-48 hours

Dry-Set Method (Fine for Standard Line Posts)

1. Set the post on the gravel base and plumb it
2. Brace with temporary supports
3. Pour dry concrete mix straight from the bag into the hole around the post
4. Add water on top according to the bag instructions (typically 1 gallon per 50-lb bag)
5. The water seeps down through the mix. No manual mixing required.
6. Crown the top and keep braces for 24-48 hours

When to use wet vs. dry:

Situation	Method
Standard 4x4 line posts (basic fence)	Dry-set is fine
Gate posts (higher stress)	Wet mix for better strength
Corner and end posts	Wet mix
6x6 posts or larger	Wet mix
Fast-setting concrete products	Follow product directions (usually dry-set)

Common Mistakes

Hole Too Shallow

The most consequential error. Posts in shallow holes wobble in wind and heave in frost. If you're in a frost-heave zone, this isn't fixable without pulling the post and re-setting it. Proper subgrade preparation principles apply to post holes too--solid base material prevents settling.

Concrete Below Ground Level (Good) vs. Above (Bad)

Keep concrete at or slightly below final grade. Concrete that sticks up above ground level creates a basin that traps water against the post, accelerating rot. Crown the concrete surface so it slopes away from the post on all sides.

Too Much Water in the Mix

Soupy concrete is weak concrete. The finished post will wobble. For fence post work, the mix should be thick enough to hold its shape on a shovel--like thick oatmeal, not pancake batter.

Skipping the Gravel Base

Without gravel, water pools at the bottom of the post. For treated lumber posts, this accelerates decay at the most critical point--where the post exits the concrete. For steel posts, it causes corrosion.

Not Plumbing Before Concrete

Once concrete is in the hole, you have minutes to adjust. Set and brace posts perfectly plumb before adding any concrete. Check plumb on two adjacent faces.

Backfilling Around Still-Soft Concrete

Wait at least 24 hours before backfilling soil around the post. Pushing soil against uncured concrete can shift the post.

Material Costs

For a typical 150-foot fence with 19 posts (8-foot spacing):

Item	Quantity	Unit Cost	Total
50-lb bags concrete mix	25-40 bags	$4-6 each	$100-240
Gravel (for hole bottoms)	0.25-0.5 cu yd	$30-50/cu yd	$15-25
Post hole digger or auger rental	1 day	$50-75	$50-75
Total materials			$165-340

Concrete for fence posts is one of the most affordable concrete projects. The posts themselves (at $8-25 each) typically cost more than the concrete to set them.

Key Takeaways

• Standard 4x4 fence posts need 1-1.5 bags of 50-lb concrete per hole (10-inch diameter, 24-inch depth)
• Hole depth must exceed both the 1/3 rule and your local frost line--use whichever is deeper
• Always place 3-4 inches of gravel at the hole bottom for drainage
• Wet mix is stronger and better for gate, corner, and end posts; dry-set is faster for line posts
• Crown concrete below grade and slope away from the post to prevent water pooling
• Budget $165-340 in concrete and gravel for a typical residential fence

For more project guidance, browse our complete library of concrete guides and tutorials.