How many bags of concrete for a mailbox post?
Quick Answer
One 50-lb bag of fast-setting concrete is typically enough for a mailbox post set in a 6–8 inch diameter hole 24 inches deep. If you're using a 4×4 wood post with a wider or deeper hole, use 1–2 bags of 80-lb mix. Mailbox posts don't need large footings — USPS guidelines require the post to break away easily in a collision.
Quick Reference Table
| Hole size | Concrete bags for mailbox post |
|---|---|
| 6" dia × 18" deep | 0.5 bag (50-lb) |
| 6" dia × 24" deep | 1 bag (50-lb) |
| 8" dia × 24" deep | 1 bag (80-lb) |
| 8" dia × 30" deep | 1–2 bags (80-lb) |
How to Calculate It Yourself
- 1
Dig a hole 6" in diameter and 24" deep with a post hole digger or auger.
- 2
Place the post in the center; check for plumb with a level.
- 3
Pour 1 bag of Quikrete Fast-Setting concrete dry into the hole.
- 4
Add water (about 1 gallon) per the bag instructions — no mixing required.
- 5
Hold plumb for 4 minutes; support with stakes for 4 hours before mounting the mailbox.
Pro Tip
Use fast-setting concrete (Quikrete 5000 or Fast-Setting) — you pour it dry into the hole, add water on top, and it sets hard enough in 4 hours. No mixing, no tools.
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