Fence Post Depth Calculator — Hole Depth & Frost Line
Calculate the minimum fence post hole depth based on post height, soil type, and local frost line. Follows the standard one-third-above-grade burial rule.
Enter Dimensions
Typically 1/3 of post height above grade
Results
Bags per Post
5bags
Total Bags
50bags
Volume per Hole
1.909cu ft
Concrete per post hole
Total Volume
19.09cu ft
All post holes combined
Editorial Standards
Each calculator is reviewed for formula accuracy, unit consistency, and alignment with current U.S. building practices before publication. We verify outputs against published engineering references and real-world project data. Learn more about our methodology.
Fence-post depth is the single biggest predictor of how long your fence will last. A post set too shallow will lean within a year, heave in the first winter, or rot at grade. The two governing rules are simple but non-negotiable: at least 1/3 of the post must be below grade, and the bottom of the hole must be below the local frost line.
Use this calculator to size the hole — then it returns the concrete bag count needed to fill it. The result accounts for the post displacing concrete inside the hole.
How to Determine Fence Post Hole Depth
Hole depth = max(above-grade height ÷ 3, local frost line) + 4″ gravel pad. Concrete volume = hole volume − post volume.
Frost Line Quick Reference (USA)
- Northern tier (MN, ND, MT, ME): 48–60 inches. Set posts 60 inches deep for guarantee.
- Midwest (IA, IL, OH, PA): 36–48 inches.
- Mid-Atlantic / Mountain West: 24–36 inches.
- Southern states (TX, GA, AL): 6–24 inches.
- Coastal South / FL: 0–6 inches — the 1/3 rule governs, not frost.
Frequently Asked Questions
How deep should I set a 6 ft fence post?
Apply the 1/3 rule: at least 1/3 of the post should be below grade. A 6 ft fence on an 8 ft post needs ≈ 2 ft 8 in of hole depth. If your frost line is deeper, set below frost — frost heave will lift shallow posts.
What is the frost line and why does it matter?
The frost line is the maximum depth seasonal freezing reaches in your area. Soil expands when it freezes and can push shallow posts upward — heaving them out of plumb. Local building codes publish the frost line depth for your jurisdiction; common values: Maine 60″, Iowa 42″, Texas Panhandle 24″, Gulf Coast 0″.
How wide should a fence post hole be?
A fence post hole should be approximately 3× the width of the post. A 4×4 post (3.5″ actual) needs a hole ≈ 10–12″ in diameter; a 6×6 post needs a 16–18″ hole.
Should I use gravel under fence posts?
Yes — add 3–4 inches of compacted gravel at the bottom of each hole for drainage. This prevents water from collecting around the post base and dramatically extends post life, especially for wood posts.
How much concrete per fence post?
For a 4×4 post in a 10″ hole 2'8" deep: hole volume ≈ 1.45 cu ft − post volume ≈ 0.25 cu ft = 1.2 cu ft concrete. That's ≈ 2 bags of 80 lb fast-set mix per post.
Looking for the general calculator?
Calculate how many bags of concrete you need per fence post hole. Supports 4×4 and 6×6 wood posts plus metal pipe posts in standard hole sizes.
Open the Concrete Fence Post Calculator: Bag Count →