Table of Contents
Post Hole Concrete Basics
Setting posts in concrete provides a stable, long-lasting foundation for fences, decks, mailboxes, and signs. The concrete fills the annular space between the post and the hole walls, locking the post firmly in place.
The hole should typically be three times the post width in diameter and one-third of the total post length in depth. For a 6-foot fence with a 4x4 post, dig the hole at least 24 inches deep (with the post being 8 feet total: 6 feet above ground + 2 feet below).
Calculation Formula
Post Hole Size Guide
| Post Size | Hole Diameter | Hole Depth | 80lb Bags per Post |
|---|---|---|---|
| 4x4 (fence) | 10" | 24" | 1-2 |
| 4x4 (fence) | 10" | 36" | 2-3 |
| 6x6 (deck) | 12" | 42" | 3-4 |
| 4" round (mailbox) | 10" | 24" | 1-2 |
| 6x6 (pergola) | 14" | 48" | 5-6 |
Setting Posts Tips
- Add 6 inches of gravel at the bottom of each hole for drainage before placing the post.
- Use a level to ensure the post is plumb before the concrete sets.
- Crown the concrete slightly above ground level so water drains away from the post.
- Fast-setting concrete sets in 20-40 minutes but allow 4 hours before applying weight or attaching rails.
- For standard concrete mix, wait at least 24-48 hours before loading the posts.
Frequently Asked Questions
How many bags of concrete do I need per fence post?
For a standard 4x4 fence post set in a 10-inch diameter hole that is 36 inches deep, you need approximately 2 bags of 80-lb fast-setting concrete. The exact amount depends on hole size and whether you include gravel at the bottom.
Should I use fast-setting or regular concrete?
Fast-setting concrete (like Quikrete Fast-Setting) is ideal for posts because you can pour it dry into the hole and add water on top. It sets in 20-40 minutes. Regular concrete provides a stronger bond but requires mixing and 24-48 hours to cure.
How deep should fence post holes be?
The general rule is to bury one-third of the total post length. For a 6-foot privacy fence, use 8-foot posts buried 24-30 inches deep. In areas with frost, holes should extend below the frost line, which can be 36-48 inches in northern climates.