A fence is only as strong as its posts. Most failures come from shallow holes, no drainage or posts that were moved while the concrete set.
Before you dig
In the US, call 811 a few days before digging so utility lines are marked for free. Then check your local rules for fence height and frost depth.
Hole size and depth
- Width: about three times the post width, so a 4 in post needs a hole near 10 to 12 inches wide.
- Depth: about one third of the post length, and below the frost line where required. A 6 ft fence on an 8 ft post needs a hole about 2.5 feet deep.
- Gravel: add 4 to 6 inches of gravel at the bottom for drainage.
How many bags
| Hole | Post | Concrete per hole | 80 lb bags |
|---|---|---|---|
| 10 in wide x 30 in deep | 4 in | 1.09 ft³ | 2 |
| 12 in wide x 36 in deep | 4 in | 2.02 ft³ | 4 |
An 80 lb bag yields about 0.60 cubic feet. For 10 posts in the first size you need about 20 bags, plus a spare. Use the fence post calculator for your sizes.
Step by step
- Dig the hole straight and a little wider at the bottom if you can.
- Add gravel and compact it.
- Place the post and brace it with stakes and scrap boards.
- Check that it is plumb (straight up) with a level on two adjacent sides.
- Mix concrete to the bag instructions, then pour in layers and poke out air bubbles.
- Shape the top so it slopes away from the post, so water runs off.
- Recheck plumb, then leave the braces on.
Common mistakes
- Skipping gravel, so water sits at the base and rots the post.
- Going too shallow. Depth matters more than extra concrete.
- Attaching rails and panels too soon. Fast-setting mixes set sooner, but check the bag. For standard mixes, a day or two is safer.
If you are also pouring a slab, see how much concrete you need for a patio.
Related
Slab calculator · Footing calculator · Fence post calculator · All guides
General information and estimates only. Follow your local building code and your supplier's instructions.