Get the right post hole depth for your frost zone — the single most important number in a fence that stays straight. Enter your post height and local frost line depth to get hole depth and concrete per hole.
How to use this calculator
- Enter post height above ground in feet.
- Enter your frost line depth in inches — check your local building code or ask your building department.
- Enter the number of posts for a total concrete estimate.
The math behind it
Hole depth = your frost line depth, with a 24-inch minimum. Concrete per hole assumes a 12-inch diameter hole: volume = π × (6 in)² × depth, converted to 60 lb bags at 0.45 cu ft per bag, rounded up.
FAQs
How deep should a 4 ft fence post be?
At least 24 inches, and deeper than your frost line — 30–36 inches in northern states. The old rule of thumb (one-third of the post underground) is a minimum, not a target, in freeze zones.
Should I use concrete or gravel for fence posts?
Concrete for gate posts and high-wind areas; compacted gravel works for line posts and drains better, which extends post life in wet soil. Either way, crown the top so water sheds away from the post.