Post Hole Depth Calculator

Post hole depth is critical for structural stability. A post that is not deep enough will lean, sag, or fail over time.

This page starts from the post and works out the hole: tell it how tall you want the fence and where your frost line sits, and it returns the depth, the diameter and the concrete. If you already know the hole you are digging and only want the bag count, the post hole concrete calculator takes the depth and diameter directly. Both use the same volume formula and the same bag yields, so they agree on the concrete for any hole you can describe to both.

The general rule: Bury at least 1/3 of the total post length underground. That works out to half the height you want above ground, since burying h/2 out of a 1.5h post is exactly one third. For a 6-foot above-ground fence, that means a 9-foot post with 3 feet in the ground.

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) that is 10.5 inches, and for a 6×6 (5.5" actual) it is 16.5 inches. Augers do not come in those sizes, so in practice you round up: 12 inches for a 4×4, 18 for a 6×6, which is what this calculator assumes if you leave the diameter field blank. Rounding up is the right direction anyway, since a slightly wider collar of concrete is cheap and a hole too narrow to work the mix around the post is a real problem. Diameter matters more to the bag count than depth does, because volume goes with the square of the radius. Going from a 12-inch hole to an 18-inch one on the same 36-inch depth more than doubles the concrete.

Concrete calculation: The concrete fills the hole minus whatever the post itself occupies, so: Volume = π × (hole radius)² × depth − (post width)² × depth

The post term is squared, not circular, because fence and deck posts are square lumber.

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.