How to Set Fence Posts in Concrete
A fence is only as straight as its posts. Bury a third of the post, dig the hole three times the post's width, put gravel in the bottom and stop the concrete just below the ground. Here are the steps, the numbers for common fence heights, and what to avoid.
The rules in one place
- Depth: bury one third of the post's overall length. That works out to half the height above ground: a 6 ft fence needs 3 ft in the ground. HGTV gives a range of one third to one half.
- Frost: in cold areas, the hole should reach at least 6 in below the frost line, or posts can heave.
- Width: make the hole about three times the post width.
- Drainage: put 6 in of gravel in the bottom of the hole.
- Top: stop the concrete 3–4 in below ground and finish with soil.
All five come from QUIKRETE's post-setting guide.
Depth and concrete for common fences
| Fence | Post | Hole depth | Hole width | Post length | 80 lb bags per post | 50 lb bags per post |
|---|---|---|---|---|---|---|
| 4 ft | 4×4 | 30 in | 10.5 in | 6 ft (buy 6 ft) | 1.5 | 2.4 |
| 6 ft | 4×4 | 42 in | 10.5 in | 9 ft (buy 10 ft) | 2.4 | 3.8 |
| 6 ft | 6×6 | 42 in | 16.5 in | 9 ft (buy 10 ft) | 5.8 | 9.3 |
| 8 ft | 6×6 | 54 in | 16.5 in | 12 ft (buy 12 ft) | 8.0 | 12.7 |
Lumber is nominal: a 4×4 is 3½ in square and a 6×6 is 5½ in. Bag figures subtract the post and are before any extra. For a run of twelve 6 ft posts on 4×4s with 10% extra you'd need about 32 bags of 80 lb mix.
Step by step
- Mark the line. Run a string between the end posts and mark each hole along it.
- Dig. Go to the hole depth from the table or calculator, about three times the post width across. Widen the bottom slightly rather than the top.
- Add gravel. Put in 6 in of gravel and tamp it, so water drains away from the end of the post.
- Set and brace. Stand the post on the gravel, check it is plumb on two faces with a level, and brace it with two stakes.
- Fill. With fast-setting mix, QUIKRETE's instructions are to pour the dry mix to 3–4 in below the top, then add about a gallon of water per 50 lb bag. Holes deeper than 2½ ft go in lifts. With standard mix, mix it first and pour.
- Check again for plumb before it sets, then leave the braces on until the concrete has cured as the bag directs.
- Backfill the last few inches with soil, sloped away from the post.
Mistakes that make posts lean or rot
- Holes too shallow. A 6 ft fence on 2 ft of post will move in the wind; follow the one-third rule.
- Holes too narrow. A thin collar of concrete cracks. Use about three times the post width.
- No gravel. Water sits at the bottom of the post and rots it from the end.
- Concrete up to grade. A flat concrete top holds water against the post. Stop below ground and finish with soil.
- Ignoring the frost line. In cold climates, shallow posts lift out of line over winters.
- Buying the exact post length. Buy the next stock length up and cut the top to height after setting.
Sources
- QUIKRETE: Setting Posts project guide
- QUIKRETE Fast-Setting Concrete Mix data sheet
- HGTV: how deep should I dig fence posts
- QUIKRETE Concrete Mix 1101 data sheet (bag yields)
Estimates for planning. Follow your plans, your supplier's instructions and your local building code.