Decks, Fences & Siding
Fence Post Depth Calculator
Tell the tool how tall the fence stands above ground, which post size you use and how many posts you set. It returns how deep and wide to dig, how long a post to buy and how much bagged concrete to bring home.
Worked example
- Common rule: bury about one third of the post, which is half the height above ground: 72 / 2 = 36 in. Use 36 in.
- Hole about 3 times the post width: 3 x 3.5 = 10.5 in.
- Post length = 72 + 36 = 108 in; lumber comes in even feet, so 10 ft.
- Concrete over 6 in of gravel: 1.29 cu ft per hole; x 12 / 0.375 = 42 bags of 50 lb.
Common sizes at a glance
| Size | Hole depth | Hole diameter | Post length to buy | 50 lb concrete bags (all posts) |
|---|---|---|---|---|
| 3 ft fence, 4 x 4 | 18 in | 10 1/2 in | 6 ft | 2 |
| 4 ft fence, 4 x 4 | 24 in | 10 1/2 in | 6 ft | 3 |
| 5 ft fence, 4 x 4 | 30 in | 10 1/2 in | 8 ft | 3 |
| 6 ft fence, 4 x 4 | 36 in | 10 1/2 in | 10 ft | 4 |
| 6 ft fence, 6 x 6 | 36 in | 16 1/2 in | 10 ft | 9 |
| 8 ft fence, 6 x 6 | 48 in | 16 1/2 in | 12 ft | 12 |
How deep should a fence post be?
A common rule is to bury about one third of the post, which works out to half the height above ground. A 6 ft fence then needs holes about 36 in deep. That rule keeps posts from leaning in wind and gives the concrete enough grip.
In cold areas, the hole should also reach below the frost line so frozen soil does not push posts up. If you enter your local frost depth, the tool uses whichever number is deeper. Your local building office can tell you the frost depth for your area.
Hole width and gravel
The hole is about three times the post width. A 4 x 4 post, which is 3.5 in across, gets a hole about 10 1/2 in wide; a 6 x 6 needs about 16 1/2 in. A hole that is too narrow leaves a thin shell of concrete that cracks.
We assume 6 in of gravel at the bottom of each hole for drainage, with concrete above it. That keeps the end of the post out of standing water.
Buying posts in the right length
Add the buried depth to the height above ground, then round up to the next even foot, since lumber is sold that way. A 6 ft fence with a 36 in hole needs a 108 in post, so you buy 10 ft. Cut the tops level after the concrete sets rather than trying to set each post at exact height.
Counting concrete bags
We work out the volume of each hole, take away the post, and divide by what a 50 lb bag yields (about 0.375 cu ft). Twelve 4 x 4 posts at 36 in come to 42 bags. Bring a couple more than the count, since holes always end up a little wider than planned.
If you use 60 or 80 lb bags, the post hole concrete calculator and the concrete bags calculator handle other sizes.
- Call your state's dig line a few days before you dig to mark buried lines.
- Slope the concrete top away from the post so rain runs off.
- Brace each post plumb until the concrete sets.
Questions
How deep for an 8 ft fence post?
About 48 in, which is half of 96 in above ground, and deeper if your frost line is lower. A 6 x 6 post is common at that height.
Can I set posts in gravel instead of concrete?
Some builders do in well-drained soil. This tool assumes concrete, so the bag count would not apply.
How many posts does my fence need?
Get the post count from the fence calculator, then enter it here.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.