By Zencalcy Editorial TeamLast reviewed About us
A post resists wind load through the ground it is buried in. The rule of thumb is one third of the post above grade and two thirds below, but in cold country the hole must reach below the frost line first, or the post will heave out with the winter.
How deep to set a post, and how much concrete it takes.
depth = max(frost depth, height ÷ 2) total post = height + depth concrete = π ÷ 4 × (hole² − post²) × depth
The frost depth varies from about 4 inches in the deep south to 48 inches or more along the northern border. Drainage matters as much as depth: 6 inches of gravel at the bottom of the hole lets water escape instead of freezing under the post.
| Symbol | Meaning |
|---|---|
| h | Post height above grade. |
| d | Embedment below grade. |
| f | Local frost depth. |
The hole goes 36 in down — the frost depth — giving a 108 in 4×4 with about 1.44 cu ft of concrete, or four 60 lb bags.
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Below the frost line where one exists, otherwise at least half the above-ground height. A 6 foot privacy post typically sets 30 to 36 inches deep in temperate climates.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026