How Much Concrete Do You Need for Fence Posts?

Setting fence posts in concrete is the gold standard for stability, but how much do you actually need? This guide breaks down the math so you buy exactly what you need.

The Quick Answer

For a standard 4×4 fence post set in a 10-inch diameter hole that’s 24 inches deep:

• 1 bag (50lb) of fast-setting concrete per post
• Or about 0.4 cubic feet of standard concrete mix

For 6×6 posts or deeper holes, plan for 1.5–2 bags per post.

The Formula

Fence post holes are cylinders, so we use:

Volume = π × r² × depth

Where the variables are:

• r = hole radius (half the diameter)
• depth = hole depth minus the post depth

Example Calculation

For a 10” diameter hole, 24” deep, with a 4×4 post:

1. Hole radius = 10” ÷ 2 = 5” = 0.417 feet
2. Post takes up about 16 sq inches of the hole
3. Remaining area = π × (5²) - 16 = 62.5 sq inches
4. Volume = 62.5 × 24 = 1,500 cubic inches = 0.43 cubic feet

One 50lb bag of Quikrete yields 0.375 cubic feet, so one bag per post with a little extra.

Post Size Reference Chart

Post Size	Hole Diameter	Hole Depth	Bags per Post
4×4	10”	24”	1 bag
4×4	12”	30”	1.5 bags
6×6	12”	30”	1.5 bags
6×6	14”	36”	2 bags

Pro Tips

1. Don’t Fill to the Top

Leave 2–3 inches at the top for soil. This hides the concrete and allows water to drain away from the post.

2. Crown the Concrete

Slope the concrete away from the post so water runs off instead of pooling. Water pooling leads to rot.

3. Use Fast-Setting for DIY

Fast-setting concrete (like Quikrete Fast-Setting) lets you pour dry mix directly into the hole and add water. No mixing required. The post is set in 20–40 minutes.

4. The One-Third Rule

Your hole depth should be one-third of the total post length. For example, a 6-foot fence with posts that extend 6 inches above the fence needs 8-foot posts buried 32 inches.

5. The Water Test

Before pouring concrete, fill the hole with water. If it drains within 10 minutes, you have good drainage. If water pools and sits, add more gravel to the base. Poor drainage leads to water accumulation around the post, which accelerates rot and can cause concrete to crack in freeze-thaw cycles.

6. Set Posts High, Cut Later

For level fence lines on uneven ground, set all posts 6 inches higher than your final needed height. After the concrete cures, use a laser level or water level to mark the exact height across all posts, then cut them to create a perfectly level fence line. This prevents the nightmare scenario of ending up with posts that are too short on high spots.

Total Project Calculation

For a 100-foot fence with posts every 8 feet:

• Posts needed: 100 ÷ 8 + 1 = 14 posts (don’t forget the end post)
• Bags needed: 14 × 1 = 14 bags minimum
• With 10% buffer: 16 bags

At roughly $5 per 50lb bag, that’s $80 in concrete for the entire fence.

Frost Line Considerations by Region

One critical factor many DIYers overlook: your hole depth must extend below the frost line to prevent frost heaving. When water in the soil freezes, it expands and pushes posts upward, cracking concrete and tilting your fence. The frost line varies dramatically by region.

Region	Frost Depth	Minimum Hole Depth
Southern US (FL, TX, AZ)	0-6”	24”
Mid-Atlantic (VA, NC, TN)	12-18”	30”
Northern US (OH, PA, NY)	24-36”	36-42”
Upper Midwest (MN, WI, MI)	42-60”	48-60”
Alaska	60-100”	60”+ (consult local codes)

Important: These are general guidelines. Always check your local building codes for exact frost line depth requirements in your area. Some municipalities require inspections before concrete is poured, especially for fence lines near property boundaries.

When to Skip Concrete

Not every post needs concrete. Here are some exceptions:

• Dry, stable soil: Tamped gravel can work for interior fence posts
• Temporary fencing: Just tamp soil firmly
• Extremely rocky ground: Rocks can provide natural stability

But for gate posts, corner posts, and end posts, always use concrete because these carry the most stress.

Common Mistakes

1. Holes too shallow: Posts will lean within a year
2. No gravel base: Add 3” of gravel before concrete for drainage
3. Touching the concrete: Wood-to-concrete contact accelerates rot, so use a post base bracket for longevity
4. Mixing too wet: Soupy concrete is weak concrete, so follow package directions

Frequently Asked Questions

How long should I wait before attaching fence panels to concrete posts?

For fast-setting concrete (like Quikrete Fast-Setting), wait at least 4 hours for light work such as attaching fence panels, but give it a full 24 hours before hanging gates or applying heavy stress. For standard concrete mix, wait a minimum of 48 hours before attaching anything, and ideally 7 days for full strength. Gate posts especially need the full cure time because they carry the most stress and movement.

Can I reuse the hole if I need to replace a fence post?

Yes, if the concrete is still intact and the old post simply broke at ground level. Use a sledgehammer to break out the old concrete sleeve from inside the hole. Clean out all debris, add a fresh 3-inch gravel base, and reset with new concrete. However, if the concrete has crumbled or cracked badly, dig a new hole about 6 inches to the side where you’ll hit undisturbed soil. Compromised concrete won’t provide adequate support.

What’s the best concrete mix for fence posts?

Fast-setting concrete (Quikrete Fast-Setting or similar) is best for DIY projects. You can pour the dry mix directly into the hole, add water, and it sets in 20-40 minutes with no mixing required. Standard concrete mix is cheaper for large projects but requires mixing and has a 24-48 hour cure time. Avoid mortar mix entirely because it’s designed for laying blocks, not structural support. When fully cured, both fast-setting and standard mixes provide similar strength, so choose based on convenience and budget.

How do I remove an old concrete fence post?

Method 1 (Easiest): Dig around the concrete footer until you expose the sides, then rock the post back and forth while pulling upward. Use a 2x4 as a lever against the ground for extra lift.

Method 2 (For stubborn posts): Place a car jack on a piece of plywood for stability, position a 2x4 across the post as a crossbar, and jack it straight up.

Method 3 (Leave it): If the post is truly stuck and not in your way, cut it off at ground level with a reciprocating saw and leave the concrete in place. Cover with soil.

For extremely stubborn posts: Drill several holes into the concrete with a masonry bit, then use a sledgehammer and cold chisel to crack it apart in sections.

Use our concrete calculator to dial in the exact amount for your specific hole dimensions.