Mailbox Post Concrete Calculator — Bags Per Mailbox
Estimate concrete bags for a typical residential mailbox post. Includes USPS-compliant setback height and break-away mounting guidance for standard post sizes.
Enter Dimensions
Typically 1/3 of post height above grade
Results
Bags per Post
5bags
Total Bags
50bags
Volume per Hole
1.909cu ft
Concrete per post hole
Total Volume
19.09cu ft
All post holes combined
Editorial Standards
Each calculator is reviewed for formula accuracy, unit consistency, and alignment with current U.S. building practices before publication. We verify outputs against published engineering references and real-world project data. Learn more about our methodology.
A mailbox post is one of the smaller post-setting projects in residential work — a single 4×4 wood post or metal stand cemented in a single hole. The math is identical to fence-post installation but at the simplest end of the scale.
USPS specifies the mailbox-to-curb distance (6-8 inches) and mailbox height (41-45 inches to bottom of box). Local jurisdictions may add additional requirements; check before installing.
How to Concrete a Mailbox Post
Concrete per post (cu ft) = π × (Hole D ÷ 24)² × Depth × 12 − (3.5 ÷ 12)² × Buried Depth × 12. Typically 0.6-0.8 cu ft = 1 bag 80 lb.
Mailbox Post Reference
- 4x4 post, 8″ × 24″ hole: 0.62 cu ft = 1 bag 80 lb.
- 4x4 post, 10″ × 30″ hole (frost zone): 1.10 cu ft = 2 bags 80 lb.
- Decorative metal post, 6″ × 24″ hole: 0.39 cu ft = 1 bag 60 lb.
- Heavy decorative post (8×8), 12″ × 30″ hole: 1.40 cu ft = 3 bags 80 lb.
- Above-grade height: 41-45 inch to bottom of box (USPS spec).
Frequently Asked Questions
How much concrete for a mailbox post?
For a 4×4 wood mailbox post in an 8 inch × 24 inch hole: 0.62 cu ft = 1 bag of 80 lb fast-set concrete. Smaller decorative posts: 1 bag of 60 lb. Always plumb the post before pour and let set before installing the box.
How tall should a mailbox post be?
USPS requires the bottom of the mailbox door to be 41-45 inches above ground. With a 4-inch tall mailbox base, the post needs to extend 37-41 inches above grade. Common: 42 inch above-grade height.
What is a break-away mailbox post?
USPS and many municipalities require mailbox posts to yield in a vehicle impact rather than become a hazard. A break-away post uses a hinge or score-line that fails under impact. Standard residential 4×4 posts qualify if installed without reinforcement.
Should the post hole reach below frost line?
Recommended but not always required for mailboxes. Below frost (24-36 inches in northern climates) prevents frost heave that can tilt the post. Decorative mailbox installations often skip this; functional ones should follow the same depth rules as fence posts.
Can I use Postmaster foam for a mailbox?
Yes — 1 cartridge of expanding foam replaces 1 bag of concrete and sets the post in minutes. Especially convenient for replacing a single damaged mailbox post.
Looking for the general calculator?
Calculate how many bags of concrete you need per fence post hole. Supports 4×4 and 6×6 wood posts plus metal pipe posts in standard hole sizes.
Open the Concrete Fence Post Calculator: Bag Count →