The rule of thumb
Quikrete, one of the big makers of post concrete, puts it simply: dig to a third or a half of the post's height above the ground, then add 6 inches for gravel. Worked out for the common fence heights:
| Fence height | Post in the ground | Hole, with 6 in of gravel | Post length to buy |
|---|---|---|---|
| 4 ft | 16–24 in | 22–30 in | 8 ft |
| 5 ft | 20–30 in | 26–36 in | 8 ft |
| 6 ft | 24–36 in | 30–42 in | 8 ft or 10 ft |
| 8 ft | 32–48 in | 38–54 in | 12 ft |
Go toward the deeper end for gate posts, corner posts, tall privacy fences that catch the wind, and soft or sandy soil. The shallow end is fine for a short fence in firm ground.
How wide to dig
About three times the width of the post. That's roughly 10 to 12 inches across for a 4×4 and 16 to 18 inches for a 6×6. A narrower hole leaves too little concrete around the post to hold it. A much wider one only uses more concrete.
In loose or sandy soil where the sides keep caving in, a cardboard tube form keeps the hole round and the concrete where it belongs.
Why gravel goes in the bottom
Six inches of gravel, tamped down, gives rainwater somewhere to drain. Without it the end of a wood post sits in a cup of concrete that holds water, and that's where wood posts rot first. Set the post on the gravel, not in it.
The frost line
In places where the ground freezes in winter, the soil swells as it freezes and can push a post up a little every year. That's called frost heave, and it's why fence posts in cold places lean and rise after a few winters.
The fix is to set the bottom of the post below the frost line, the depth the ground freezes to in your area. It ranges from a few inches in the South to several feet in the northern states. Your building department knows the number for your town, and for deck posts and other structural footings it's usually required, not just recommended.
How much concrete each post takes
More than most people guess. For a 4×4 post in a hole three times its width, with no gravel:
| Hole depth | Concrete, cu ft | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|
| 24 in | 1.0 | 2.8 | 2.3 | 1.7 |
| 30 in | 1.3 | 3.4 | 2.9 | 2.2 |
| 36 in | 1.5 | 4.1 | 3.4 | 2.6 |
| 42 in | 1.8 | 4.8 | 4.0 | 3.0 |
Add the gravel and the number goes down a little, a bigger post or a wider hole and it goes up. The post hole concrete calculator works it out for your post, hole and bag size.
Setting the post, step by step
- Call 811 a few days before you dig. It's free, and buried lines get marked.
- Dig the hole to depth and width, and tamp 6 inches of gravel into the bottom.
- Stand the post on the gravel and brace it plumb on two sides.
- Fill the hole with concrete to a few inches below the surface, and slope the top away from the post so water runs off.
- Leave it alone until it has set. Fast-setting post mixes harden in well under an hour, but give the post a few hours before hanging anything heavy on it, and longer in cold weather.
Fair questions
Can I set posts without concrete?
Yes. Posts set in tamped gravel or soil are common, especially for wire and split rail fences, and they drain better. Concrete gives the firmest hold, which is why it's the norm for gate posts and privacy fences that catch the wind.
Do vinyl and metal posts go as deep?
The same depth rule applies, since it's about leverage, not material. Vinyl posts are often set over a wood or metal insert, and the manufacturer's instructions give the depth for their system.
What length of post should I buy?
Fence height plus the depth in the ground. The table above gives the stock length for each height. For a 6-foot fence, an 8-foot post leaves 2 feet in the ground and a 10-foot post leaves room to go deeper.
Sources
- Quikrete, Setting Posts in Concrete: hole width, hole depth and how much concrete a post takes.
- Call 811: the national call-before-you-dig service, for when and how to have buried utilities marked.
- How we calculate: every suggested price behind this calculator, with the date it was last reviewed.
