Concrete Mailbox Post Calculator
Size the concrete footing for a mailbox post, sign post, or fence post.
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Short answer
A typical mailbox post footing — a 10-inch-diameter hole poured 24 inches deep — needs a little over 1 cubic foot of concrete, or about 3 bags of 60 lb fast-setting post mix. Heavier sign posts or larger fence posts need a wider, deeper hole and more bags.
Typical post footing sizes
| Post type | Hole diameter × depth | Bag size |
|---|---|---|
| 4×4 wood mailbox post | 10 × 24 in | 50–60 lb |
| 6×6 wood post | 12 × 30 in | 60–80 lb |
| Round metal mailbox post | 8 × 24 in | 50–60 lb |
| Heavy sign or fence post | 12 × 36 in | 80 lb |
How to set a mailbox or sign post
- Check local code for the required setback from the road and curb before digging.
- Dig or auger a hole at least a third of the post’s above-ground height, typically 24 to 36 inches deep.
- Set the post plumb in the hole, brace it temporarily, and check level on two sides.
- Enter your hole size above to get the concrete volume, then mix and pour around the post.
- Slope the top of the concrete slightly away from the post so water drains off instead of pooling.
- Let fast-setting post mix cure for the time on the bag (often 4 hours) before removing braces; standard mix needs 24–48 hours.
Worked example
- Hole size
- 10 in diameter × 24 in deep
- Volume
- π/4 × 10² × 24 = 1,885 in³ = 1.09 ft³, plus 10% waste = 1.20 ft³
- Bags (60 lb, 0.45 ft³ each)
- ceil(1.20 / 0.45) = 3 bags
Common mistakes
- Digging too shallow, leaving the post loose or prone to leaning after a season of frost heave.
- Not checking plumb before the concrete sets, resulting in a tilted post.
- Pouring the concrete top flat instead of sloped, which lets water pool and rot a wood post faster.
- Skipping the local setback check, which can mean redoing the post if it’s placed too close to the road.
Assumptions
- Standard premix or fast-setting post mix at roughly 150 lb per cubic foot.
- Bag yields follow manufacturer averages: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³.
- Hole is treated as a cylinder; the post cross-section can be subtracted using the checkbox above.
- Does not include the post itself, gravel base, or braces.
Frequently asked questions
How deep should a mailbox post footing be?
24 inches is standard for a typical 4×4 wood mailbox post. Taller posts, heavier sign posts, or areas with deep frost lines may need 30 to 36 inches for stability.
Do I need to subtract the post itself from the concrete volume?
For a rough estimate, most people don’t bother — the post displaces a small amount of volume. Check the box above if you want a more precise number, especially for larger posts.
Should I use fast-setting post mix or standard concrete?
Fast-setting post mix (often poured dry and watered in place) is convenient and sets in a few hours, which is handy for mailbox and fence posts. Standard premix is fine too and can be slightly cheaper for larger jobs.