How Much Concrete for a Mailbox Post? (2026 Guide)
About 1.1 cu ft of concrete — a 10-in diameter hole 24 in deep needs roughly 2 bags of 80-lb mix for a mailbox post. Estimate any hole size with our concrete calculator.
Figuring the concrete for one post hole
A round post hole is just a cylinder, so cu ft = π × r² (ft) × depth (ft). Then divide by what one bag makes: an 80-lb bag yields about 0.60 cu ft. We'll run a typical mailbox hole — 10 in diameter, 24 in deep.
1. Convert the diameter to a radius in feet
A 10-in hole has a 5-in radius, and 5 ÷ 12 = 0.4167 ft.
2. Find the area of the hole
r² = 0.4167 × 0.4167 ≈ 0.174, so the area is π × 0.174 ≈ 0.545 sq ft.
3. Multiply by the depth
24 in = 2 ft, so 0.545 × 2 = 1.09 cu ft of concrete in the hole.
4. Convert to bags
Divide by the 80-lb yield: 1.09 ÷ 0.60 ≈ 1.8 bags, so buy 2 bags of 80-lb mix — or about 3 bags of 60-lb mix, since a 60-lb bag yields about 0.45 cu ft and 1.09 ÷ 0.45 ≈ 2.4. Yields vary a little by brand, so check the bag, and let the concrete calculator do the counting for your actual hole.
Mailbox post concrete by hole depth (10-in hole)
Keep the 10-in diameter and change the depth — this is what the bag math looks like at each step. The buy column rounds the 80-lb count up to a whole bag:
| Hole depth (in) | Concrete needed (cu ft) | 80-lb bags | Buy |
|---|---|---|---|
| 18 | 0.82 | 1.4 | 2 |
| 24 | 1.09 | 1.8 | 2 |
| 30 | 1.36 | 2.3 | 3 |
| 36 | 1.64 | 2.7 | 3 |
Spot-check the math: 0.545 sq ft × 1.5, 2, 2.5 and 3 ft of depth gives 0.82, 1.09, 1.36 and 1.64 cu ft, and each divided by 0.60 is 1.4, 1.8, 2.3 and 2.7 bags. In frost-line regions, dig below your local frost depth — often 30 in or more — so winter heave can't lift the post; the deeper rows are the ones to plan around.
Tips for a straight, long-lasting mailbox post
- Match the post to the soil — a 4×4 or 5×5 pressure-treated wood post handles most residential mailboxes; a steel post is a good option where you want the permanent look the manufacturer designs for.
- Set the post before you pour — drop the post in, level and plumb it on both faces, and brace it; concrete locks in whatever angle it cures at, so fix the lean before the bag is opened.
- Add 4–6 in of gravel — tamp a few inches of gravel in the bottom of the hole to drain water and keep the post base out of standing moisture.
Pouring concrete for a whole run of supports? The same bag math carries over to concrete for fence posts, and when you're covering an area rather than filling a hole, see how many bags of concrete per square foot.
FAQ
How many bags of concrete do I need for one mailbox post?
About 2 bags of 80-lb mix. A standard 10-in diameter hole 24 in deep holds about 1.09 cu ft, and an 80-lb bag yields about 0.60 cu ft: 1.09 ÷ 0.60 ≈ 1.8, so round up to 2 bags. With 60-lb bags (about 0.45 cu ft each), buy about 3.
How deep should I dig the hole for a mailbox post?
About 24 in for a standard residential mailbox post. In frost-line regions, dig deep enough that the base of the post sits below your local frost depth — shallower in mild climates, 30 in or more where the ground freezes hard — so winter frost heave can't tilt the post.
Do steel mailbox posts need concrete too?
Most do — follow the instructions that come with your post. Many steel posts call for a concrete footing or an anchor sleeve set in concrete, while a few use a ground spike instead. Set whatever your post specifies so the mailbox stays level and secure.
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