How to Set Fence Posts in Concrete: Hole Size and Bags per Post
How wide and deep to dig, why gravel goes in the bottom, and how many bags of concrete each post takes.
A fence is only as straight as its posts. Setting them in concrete at the right depth keeps them upright through wind, wet soil and frost. Here’s how big to dig and how much concrete each post takes.
How big to dig the hole
Quikrete’s post-setting instructions give a simple rule:
- Width: three times the width of the post. A 4×4 (actually 3½” square) needs a hole about 10” across; a 6×6 needs about 16”.
- Depth: one third of the post’s total length, plus 6” for gravel in the bottom. An 8 ft post goes about 32” into the ground, so dig 38”.
- Frost: for posts that carry a structure, such as a deck, the hole must go at least 6” below your area’s frost line. Your building department can tell you how deep that is.
The 6” of gravel at the bottom lets water drain away from the end of the post instead of sitting against it.
Bags of concrete per post
With 6” of gravel in the bottom, the post’s volume taken out and 10% extra:
| Post | Post length | Hole | 60 lb bags | 80 lb bags | 80 lb bags for 10 posts |
|---|---|---|---|---|---|
| 4×4 | 6 ft | 10" × 30" | 2.25 | 1.69 | 17 |
| 4×4 | 8 ft | 10" × 38" | 3 | 2.25 | 23 |
| 4×4 | 10 ft | 10" × 46" | 3.75 | 2.81 | 29 |
| 6×6 | 8 ft | 16" × 38" | 7.73 | 5.8 | 58 |
| 6×6 | 10 ft | 16" × 46" | 9.67 | 7.25 | 73 |
| 6×6 | 12 ft | 16" × 54" | 11.6 | 8.7 | 87 |
If you mix one hole at a time, round up for each hole. When buying for a whole fence, multiply first and round once at the end.
Setting the posts
- Mark the line. Run a string between the end posts and mark each hole. Call 811 first in the US to get buried lines marked.
- Dig to the full depth with a post-hole digger or a rented auger. Keep the sides straight; a hole wider at the bottom than the top holds the post best.
- Add 6” of gravel and tamp it down.
- Set the post on the gravel, check it’s plumb on two sides with a level, and brace it with scrap boards and stakes.
- Fill with concrete to a few inches below ground. With fast-setting mix you can pour the dry mix in and add water as the bag directs; with standard mix, mix it first.
- Slope the top away from the post so rain runs off, then cover it with soil later if you like.
- Wait before hanging rails or gates. Check the bag; fast-setting mix is usually ready for rails much sooner than standard mix.
Common mistakes
- Holes too shallow, especially for gate posts, which take far more force than line posts. Many builders dig gate posts deeper and wider.
- No gravel in the bottom, so water collects against the end grain and rots the post.
- Concrete domed below ground level, which traps water against the post.
- Not checking plumb before the concrete starts to set.
Enter your number of posts, hole size and post type in the Concrete Slab & Footing Planner (choose “Post holes”). For slab and footing bag tables, see How Many Bags of Concrete Do You Need?