4×4 fence posts
8" hole × 24" deep · 10 posts · 80 lb bags
- Bags per hole
- 1
- Total bags
- 10
- Weight
- 800 lbs
Typical 6 ft fence. Buy 1–2 extra bags for spillage.
How Many Bags of Concrete for Post Holes?
Enter posts, hole size, and bag size to get bags per hole and total bags to buy — for fence, deck, and mailbox posts.
Last updated: July 11, 2026
Post Hole Order Summary
10
bags to buy (80lb)
Bags per hole1
Concrete volume5.28 cu ft
Total weight800 lbs
10 posts · 8" × 24" holes · 3.5" post · 80lb bags. Volume subtracts the post. Add 1–2 bags for spillage. Over 500 lbs — consider store delivery.
Worked Examples
6×6 deck posts
10" hole × 36" deep · 8 posts · 80 lb bags
- Bags per hole
- 2
- Total bags
- 16
- Weight
- 1,280 lbs
Deeper holes for deck loads. Consider store delivery over ~500 lbs.
Mailbox post
8" hole × 18" deep · 1 post · 80 lb bags
- Bags per hole
- 1
- Total bags
- 1
- Weight
- 80 lbs
One 80 lb bag is usually enough for a single mailbox post.
Hole Size Guide
| Post | Hole Diameter | Typical Depth | Notes |
|---|---|---|---|
| 4×4 (3.5" actual) | 8–10 in | 24–36 in | 6 ft fence: start at 24 in; go deeper in frost areas. |
| 6×6 (5.5" actual) | 10–12 in | 30–42 in | Deck / pergola posts need more concrete around the post. |
| Round 2.5" | 6–8 in | 18–24 in | Light posts and small railings. |
| Round 3.5" | 8–10 in | 24–30 in | Same diameter rule as a 4×4. |
Rules of thumb: diameter ≈ 3× post width; depth ≈ ⅓ above-ground fence height.
Bag Yield Guide
| Bag | Yield | Notes |
|---|---|---|
| 40 lb | 0.30 cu ft | ~90 bags per cubic yard |
| 60 lb | 0.45 cu ft | ~60 bags per cubic yard |
| 80 lb | 0.60 cu ft | ~45 bags per cubic yard — most common for posts |
Frequently Asked Questions
For a 4×4 in an 8-inch hole that is 24 inches deep, you need about 1 bag of 80 lb concrete per hole after subtracting the post. Ten posts ≈ 10 bags — buy 1–2 extras for spillage.
It depends on diameter and how much of the post is buried. A common 8-inch hole about 24 inches deep for a 4×4 still lands near 1 × 80 lb bag per hole. Deeper or wider holes need more — use the calculator above.
Bury about ⅓ of the above-ground fence height (a 6-foot fence → at least 24 inches). Some manufacturer guides use up to ½ of post height. In cold climates, dig below the frost line so the post does not heave.
Hole diameter should be about 3× the post width. A 4×4 (3.5" actual) usually needs an 8- to 10-inch hole so you get roughly 2 inches of concrete around the post.
Yes. We calculate the round hole volume, subtract the post displacement, then convert to bags. That keeps the bag count closer to what you actually pour.