Post Depth & Materials Calculator
Enter your fence specs — get post depth, total post length, concrete bags needed, and a materials list
Your Results
Full Breakdown
How Post Depth Is Calculated
Fence post depth is determined by two independent factors — you must satisfy both, then use whichever result is deeper:
- The 1/3 rule: Posts should be buried at least one-third of their total length. A 9-foot post (for a 6-foot fence) should be buried at least 3 feet. This prevents the post from acting as a lever arm in high winds.
- The frost line: In cold climates, the bottom of the post footing must extend below the frost line — the depth at which soil freezes. Posts set above the frost line will heave as the ground freezes and thaws, destroying fence alignment within a few winters.
The frost line depth varies enormously by state: 0 inches in Hawaii and parts of Florida, 6 inches in the deep South, up to 60 inches in Minnesota, Vermont, and North Dakota. The calculator above uses USDA frost depth data by state.
Concrete: How Many Bags Per Post?
The standard rule of thumb is one 50-lb bag of concrete mix per post for a 4×4 post in an 8-inch diameter hole, two bags for a 6×6 post. For metal posts in a 10-inch diameter hole, use two 60-lb bags. Fast-setting concrete (Quikrete Fast-Setting or equivalent) is preferred for fence posts because it doesn't require mixing — pour the dry mix into the hole, add water, and it sets in 20–40 minutes.
Free: Fence Permit Application Checklist (PDF)
Includes a post depth worksheet and a pre-construction site layout checklist.
⬇ Download Free PDFFrequently Asked Questions — Post Depth
The 1/3 rule requires a 6-foot fence post to be buried at least 2 feet (using a 9-foot total post). However, if your frost line exceeds 24 inches, you must go deeper. In Minnesota or Vermont (60-inch frost line), posts for a 6-foot fence should be at least 66 inches (5.5 feet) deep — meaning total post length of at least 9.5 feet. Most building departments require posts to extend 6 inches below the frost line as a buffer.
Sometimes. Many jurisdictions include minimum post depth requirements as a permit condition, especially for fences over 6 feet or in high-wind areas. Check your permit approval documents — if a depth is specified, follow that rather than the calculator's output. In hurricane-risk counties (Florida Gulf Coast, Gulf Coast Texas), additional footing requirements often apply.
Yes, for wood posts — in fact, many fence installers prefer packed gravel (crushed stone) for wood posts because it drains better than concrete, reducing rot at the base. For metal and vinyl posts, concrete is generally preferred because the post can't swell to create friction grip. Never pack pure soil — it won't provide adequate lateral support. Some jurisdictions prohibit gravel footings for permitted fences; check your permit conditions.