Calculator inputs
Example data table
These rows demonstrate how inputs affect depth. Your site conditions may differ.
| Soil | Frost | Spacing | Total load | Exposure | Depth result |
|---|---|---|---|---|---|
| Loam / Silt Loam | 30 in | 6×6 ft | 50 psf | Medium | ~42 in |
| Clay (plastic) | 24 in | 6×6 ft | 50 psf | High | ~48 in |
| Gravel / Sand | 12 in | 5×5 ft | 45 psf | Low | ~24 in |
Formula used
How to use this calculator
- Select units that match your measurements.
- Enter local frost depth and any code minimum depth.
- Choose the closest soil profile; use Unknown if unsure.
- Set post spacing to reflect your framing plan.
- Enter dead and live loads, then choose a safety factor.
- Pick footing type and uplift exposure for extra allowances.
- Press calculate, then review depth, hole depth, and notes.
- Download CSV or PDF to share with your crew.
Frost depth and local code minimum
Footing embedment is first governed by freezing conditions and local rules. The calculator starts with your frost depth and adds a 6 in buffer to reduce heave risk. It also compares that value against any code minimum you enter, then uses the larger depth as the base requirement for the site. On sloped ground, measure from the lowest finished grade near the post. Compact the base and keep post hardware above standing water during storms.
Soil profile and drainage behavior
Soils differ in bearing strength and how they hold water. Gravel and sand drain quickly, so the model applies a slightly lower drainage factor. Loams, clays, and unknown fill receive more conservative factors and higher minimum depths. If the soil is wet, an added allowance increases embedment for better stability. When you suspect organic topsoil, plan to excavate to firm subsoil first.
Load assumptions and tributary area
Each post supports a tributary area equal to spacing X multiplied by spacing Y. Surface loads are the sum of dead load and live load, entered as psf or kPa. The calculator multiplies total pressure by tributary area to estimate service load per post, then applies a safety factor to create a planning design load. Closer spacing usually reduces load per footing and helps limit deflection.
Bearing check and footing diameter
Depth is the focus, but footing size helps confirm the soil can support the post load. The calculator estimates required bearing area by dividing design load by assumed allowable soil bearing. It then converts area to an equivalent circular diameter and rounds up to practical increments for field layout. If you use square footings, match area rather than diameter for a similar check.
Uplift exposure and special conditions
Wind and uplift can pull on posts, especially for tall decks and open sites. Selecting medium or high exposure adds extra embedment as a simple resistance measure. Expansive soils add additional depth allowance to limit seasonal movement. The reported hole depth includes gravel base thickness and a concrete cover allowance for constructability. For planters or hot tubs, increase live load to reflect concentrated weight.
FAQs
1) Is the recommended depth a building permit value?
No. It is a planning estimate based on common footing principles. Always verify with your local authority, code tables, and any engineered requirements for your specific deck, soil, and exposure.
2) What should I enter for frost depth?
Use the published frost line for your area or the depth required by local code. If you are unsure, ask a local building office or use regional guidance from reputable construction references.
3) How do post spacing values affect the result?
Wider spacing increases tributary area per post, raising the load each footing must carry. This typically increases the suggested footing diameter and may influence depth when combined with soil and exposure allowances.
4) Why does wet soil increase depth?
Wet soil can lose strength and shift more easily. Adding embedment helps resist settlement and movement, especially during seasonal saturation. Improving drainage and using compacted base material can also help.
5) What does “hole depth” include?
Hole depth adds your gravel base thickness plus a concrete cover allowance below the post base. It helps you estimate digging depth even when the structural embedment depth remains the governing requirement.
6) Can I use this for helical piles?
Use it only as a baseline reference. Helical piles depend on torque, soil stratification, and manufacturer design tables. For piles, follow the supplier’s engineering documentation and any required inspections.