Fence Post Depth Calculator
How deep should your fence posts go? Get the exact number.
Enter fence height, frost line and soil type — the calculator applies the 1/3 rule, the frost-depth rule and a soil adjustment, then returns hole depth, hole diameter, post length and concrete needed per post.
Height of the finished fence, not the post
Your county building department publishes the official depth
4 for a 4x4, 6 for a 6x6
4–6 in recommended for drainage
Calculation Results
Planning estimate — actual quotes vary
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Smart Fencing Calculator keeps your fence projects on your phone and works offline on the job site. On iPhone? It's inside Roofing Calculator & Estimator.
📌 Set posts at least 30" deep — that clears both the 1/3 rule and a 24" frost line.
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🛠️ How This Calculator Works
Takes the deepest of three requirements — one third of the above-ground fence height, your frost line plus 6 inches, and a 24 inch minimum — then adds a soil adjustment for loose or sandy ground. Hole diameter is three times the post width, and concrete volume is the hole minus the post.
📐 Formula
Depth = max(height×12 ÷ 3, frost line + 6 in, 24 in) + soil adjustment📝 Example Calculation
6 ft fence, 24" frost line, loam soil, 4x4 post: 1/3 rule = 72 ÷ 3 = 24" Frost rule = 24 + 6 = 30" Depth = 30" · Hole = 12" wide · Post = 9 ft
💡 Pro Tips from Fence Builders
- Gate and corner posts carry the most load — go 6 inches deeper than line posts and use a 6x6 where you can.
- Always put 4–6 inches of gravel in the bottom of the hole so water drains away from the end grain.
- Crown the concrete above grade and slope it away from the post — a flat collar holds water and rots posts from the top down.
- Call 811 before you dig. It's free and it's the law in every US state.
- In sandy soil, widen the hole rather than deepening it — the concrete bell is what resists tipping.
❓ Frequently Asked Questions
How deep should a 6 ft fence post be?
At least 24 inches for the 1/3 rule, but in frost country the frost line usually rules: 30 inches for a 24 inch frost line, up to 54 inches in the far north. Use the calculator with your local frost depth.
How deep for an 8 ft fence?
The 1/3 rule gives 32 inches. With any frost line deeper than 26 inches, go with frost depth plus 6 inches instead. An 8 ft fence catches serious wind, so a 12 inch wide hole is the minimum.
Is the 1/3 rule enough?
Only where the ground doesn't freeze. Frost heave lifts a shallow post out of the ground a little each winter — that's the number one cause of leaning fences in northern states.
Do I need concrete for every post?
Line posts can be set in compacted gravel in well-drained soil, which actually helps wood last longer. Corner, end and gate posts should always be concreted.
How wide should the post hole be?
Three times the post width. A 4x4 wants a 12 inch hole, a 6x6 wants 18 inches. Narrow holes are the most common DIY shortcut and the reason posts wobble.
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