Calculator inputs
Use feet for length and inches for widths in Imperial mode. Use metres for length and millimetres for widths in Metric mode.
Example data table
| Scenario | Above Grade | Frost Depth | Soil | Hole Diameter | Estimated Result Focus |
|---|---|---|---|---|---|
| Wood line fence post | 8.00 ft | 2.50 ft | Medium | 12 in | Balanced embedment with standard stock suggestion |
| Gate support post | 2.10 m | 0.90 m | Soft | 350 mm | Deeper embedment and higher concrete volume |
| Corner post on sloped ground | 7.00 ft | 3.00 ft | Dense | 12 in | Extra slope allowance with stronger footing check |
Formula used
1) Height-based embedment:
Height-based embedment = Above-grade height × Material ratio × Soil factor × Wind factor × Role factor
2) Design embedment:
Design embedment = max(Height-based embedment, Frost depth)
In manual mode, the entered embedment is still checked against frost depth.
3) Hole depth:
Hole depth = Design embedment + Footing thickness below the post
4) Cut and purchase length:
Cut length per post = Above-grade height + Design embedment + Slope allowance + Top trim allowance
Purchase length per post = Cut length × (1 + Waste % ÷ 100)
5) Concrete volume:
Hole volume = π × (Hole diameter ÷ 2)² × Hole depth
Concrete volume = Hole volume − (Post area × Design embedment)
This calculator is intended for planning and estimating. Final post sizing depends on code requirements, loading, post section properties, and product specifications.
How to use this calculator
- Select Imperial or Metric units before entering dimensions.
- Enter the visible height you want above finished grade.
- Add frost depth and footing thickness for below-grade design.
- Choose material, role, soil class, wind exposure, and post shape.
- Enter post width and hole diameter for volume estimates.
- Add slope, trim, and waste allowances if needed.
- Use auto mode for a planning estimate or manual mode for a fixed embedment.
- Submit the form to see results, graph output, and download options above the form.
Frequently asked questions
1) What does post length mean in this calculator?
It is the total cut length before installation. The value combines visible height, embedment below grade, slope allowance, trim allowance, and waste. Purchase length may be slightly larger when you add a waste percentage for estimating.
2) Why does frost depth matter?
Frost can move soil and disturb shallow foundations. This calculator checks embedment against frost depth so the planning depth is not shallower than the frost-related requirement entered by the user.
3) Should gate posts be longer than regular posts?
Usually yes. Gate posts resist higher lateral and repeated opening loads. That is why the calculator increases the embedment demand for gate and heavy-duty roles compared with standard line posts.
4) How large should the hole diameter be?
A common planning rule is roughly three times the post width, with a sensible minimum. The calculator compares your selected hole diameter with a recommended minimum and flags undersized input values.
5) What is the purpose of waste allowance?
Waste allowance covers trimming, cutting errors, damaged ends, and stock availability. It helps estimate a practical purchase length rather than only the ideal cut length needed in the ground.
6) Can I force a manual embedment depth?
Yes. Manual mode lets you enter a target embedment. The calculator still compares that value with frost depth and raises it when necessary, which avoids an obviously shallow planning result.
7) Can this work for wood, steel, and concrete posts?
Yes. The calculator includes different starting embedment ratios for wood, steel, concrete, and composite posts. Those ratios are estimating tools and should not replace project-specific structural design.
8) Why does slope allowance change the final post length?
Posts on sloped or stepped runs often need extra material to maintain visible height and achieve clean alignment. Adding slope allowance keeps the purchase estimate closer to field conditions.