Calculator Inputs
Use deck mode for plan-based estimating, or direct mode when you already know the load on each footing.
Example Data Table
These examples are illustrative. Final footing design should always follow local code, site conditions, and professional review.
| Deck Size | Footings | Total Load (psf) | Soil Capacity (psf) | Required Area per Footing | Approx. Square Size | Approx. Round Diameter |
|---|---|---|---|---|---|---|
| 10 ft × 12 ft | 6 | 50 | 1,500 | 0.83 sq ft | 11 in × 11 in | 12.4 in dia |
| 12 ft × 16 ft | 8 | 60 | 2,000 | 0.90 sq ft | 11.4 in × 11.4 in | 12.8 in dia |
| 14 ft × 20 ft | 10 | 60 | 2,500 | 0.87 sq ft | 11.2 in × 11.2 in | 12.7 in dia |
Formula Used
Deck Area (sq ft) = Length × Width
Total Uniform Load (lbs) = Deck Area × (Dead Load + Live Load + Snow Load)
Base Load per Footing = (Total Uniform Load + Additional Point Load) ÷ Number of FootingsIn direct mode, the base load is your entered load per footing.
Design Load per Footing = Base Load per Footing × Safety Factor
Required Area (sq ft) = Design Load per Footing ÷ Allowable Soil Bearing Capacity
Square Side (ft) = √Required Area
Round Diameter (ft) = √(4 × Required Area ÷ π)
Volume (cu ft) = Actual Footing Area × Thickness
How to Use This Calculator
- Choose the calculation mode that matches your project information.
- Enter deck dimensions and load values, or enter a direct footing load.
- Input the number of footings planned under the deck.
- Enter the allowable soil bearing capacity for your site.
- Add a safety factor and a footing thickness you want to review.
- Pick a rounding increment to size footings to practical dimensions.
- Click the calculate button to see the results above the form.
- Review the table, the Plotly chart, and download CSV or PDF if needed.
Frequently Asked Questions
1) What does this calculator estimate?
It estimates required bearing area, square footing size, round footing diameter, approximate bearing pressure, and concrete volume. It is useful for early planning and comparison, not final stamped design.
2) Which soil bearing value should I enter?
Use the allowable soil bearing capacity from your geotechnical report, building department, or approved local reference. Avoid guessing because footing size is highly sensitive to soil strength.
3) Why is a safety factor included?
The safety factor increases the footing design load above the raw calculated load. It gives added conservatism for uncertainty, uneven loading, and practical field conditions.
4) Should I choose square or round footings?
Square footings are often easier to form with lumber. Round footings are common with augered holes and tube forms. This calculator shows both so you can compare practical size and concrete volume.
5) Does this replace local code checks?
No. Local code may require larger minimum dimensions, frost-depth embedment, reinforcement, uplift checks, lateral resistance, and inspection rules. Always verify the final design locally.
6) What if I already know the load on each footing?
Use direct mode. It skips deck area loading and sizes the footing from your entered footing load, soil capacity, safety factor, and thickness assumptions.
7) Why are actual pressures lower than soil capacity?
The calculator rounds footing dimensions upward to your selected increment. That larger practical size creates extra bearing area, which lowers the actual bearing pressure.
8) How accurate is the concrete volume result?
It is a planning estimate based on the actual rounded footing area and the entered thickness. Excavation shape, pier extensions, waste, and field tolerances can increase actual concrete use.