How to use
- Enter how much post must remain above the ground line.
- Use actual post width, rather than a nominal lumber label.
- Obtain local frost depth and enter it if applicable; leaving it blank means frost is not checked.
- Review embedment, gravel depth, and stock length as separate quantities.
How the estimate is calculated
Planning embedment in inches is the greater of half the above-ground height, expressed in inches, and the entered frost depth. Hole depth adds the gravel beneath the post. Hole diameter is three times actual post width. Required post length adds above-ground height and embedment, excluding the gravel.
Example: a 6 ft exposed post, 3.5 in wide, with 6 in of gravel and no frost input gives 36.0 in embedment, a 42.0 in deep × 10.5 in diameter hole, and a 9.0 ft post. The stock comparison selects 10.0 ft.
Frost depth and the bottom of the hole
| Planning item | Value used | Basis |
|---|---|---|
| Embedment rule of thumb | Half the above-ground height | Quikrete guidance summary; not a code requirement |
| Hole diameter rule of thumb | 3 × actual post width | Same manufacturer summary |
| Gravel allowance | 6 in, editable | Same planning summary |
| Stock comparison list | 6, 8, 10, 12, 14, 16 ft | Tool comparison sizes; verify availability |
A gravel layer adds excavation below the post tip; it does not add post embedment. Ask the building department how local frost conditions, soil, fence height, and wind exposure affect the detail. Do not treat the stock suggestion as approval of the post or footing.
Common mistakes
- Including the gravel layer in the post’s cut length.
- Entering a nominal 4 in width when the actual post measures 3.5 in.
- Taking a blank frost field as evidence that frost does not matter.
Sources
Quikrete Fast-Setting Concrete Mix guidance is represented by a search-only excerpt in the project snapshot. Its dimensions are used here as commonly used planning figures, not verified code limits. Local requirements remain for the user to check.
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Frequently asked questions
Is the result a building-code minimum?
No. It combines a planning rule of thumb with the frost depth you supply. Obtain the applicable local requirements before choosing a footing.
What does leaving frost depth blank do?
The calculation uses the height-based rule alone and warns that local frost depth has not been checked.
Why is a 4×4 preset 3.5 inches wide?
The example represents a post with that actual measured width. Enter the dimensions of your own post, which can differ.
Does a 9 ft post mean I should buy 9 ft stock?
The comparison chooses the next size in its stated stock list. A 9 ft requirement selects 10 ft from that list; check what your supplier carries.
What if the required post exceeds 16 ft?
The tool reports no matching stock size in its comparison list. Discuss stock and the design with a supplier or qualified designer.