Post Hole Depth Calculator
Calculate the correct post hole depth and concrete needed for fence posts, deck posts, and mailboxes by post height.
Post hole depth is critical for structural stability. A post that is not deep enough will lean, sag, or fail over time.
The general rule: Bury at least 1/3 of the total post length underground. For a 6-foot above-ground fence, use an 8-foot post and bury 2 feet.
Minimum depth requirements:
- Fence posts (under 6 ft above ground): 24 inches deep minimum
- Fence posts (6 ft and above): 30-36 inches deep
- Deck posts: Must reach below the frost line (varies by location)
- Mailbox posts: 18-24 inches deep
- Gate posts: Add 6 inches deeper than adjacent fence posts
Frost line consideration: In cold climates, posts must extend below the frost line to prevent heaving. Frost lines range from 12 inches in the southern US to 60+ inches in northern states and Canada.
Post hole diameter should be 3 times the post width. For a 4×4 post (3.5" actual), the hole should be about 10-12 inches in diameter. For a 6×6 post (5.5" actual), the hole should be about 16-18 inches in diameter.
Concrete calculation: The volume of concrete needed per hole is calculated as: Volume = π × (hole radius)² × hole depth
A standard 50-lb bag of premixed concrete yields about 0.375 cubic feet. An 80-lb bag yields about 0.6 cubic feet.
Best practices:
- Crown the concrete above ground level to shed water away from the post.
- In wet soil, add 2-3 inches of gravel at the bottom for drainage.
- Allow concrete to cure for 24-48 hours before attaching panels or rails.