Deck Footing Size and Load Calculator
Calculate the required footing diameter/size and total load for deck posts based on tributary area, dead load, live load, and soil bearing capacity.
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Formulas Used
Tributary Area (Atrib):
Atrib = (Joist Span / 2) × (Beam Span / 2)
Design Load per Area (wdesign):
wdesign = Dead Load + max(Live Load, Snow Load) [per IBC/IRC load combination]
Total Footing Load (P):
P = wdesign × Atrib + Post & Beam Weight
Required Footing Area (Areq):
Areq = P / qallow where qallow = allowable soil bearing capacity
Round Footing Diameter (d):
d = √(4 × Areq / π) → round up to nearest inch
Square Footing Side (s):
s = √(Areq) → round up to nearest inch
Assumptions & References
- Live load default of 40 psf per IRC R507 for residential decks.
- Dead load default of 15 psf covers decking, framing, railings, and finishes.
- Load combination follows IBC/IRC: use the greater of live load or snow load combined with dead load.
- Default soil bearing capacity of 1,500 psf is conservative for undisturbed native soil; sandy/gravelly soils may support 2,000–3,000 psf.
- Tributary area assumes a simple rectangular deck with uniformly spaced posts.
- Footing depth must extend below the local frost depth — not calculated here.
- This calculator does not account for seismic, wind uplift, or lateral loads.
- Results are for preliminary sizing only. A licensed structural engineer should review final designs.
- References: IRC 2021 Section R507, IBC 2021 Chapter 18, AWC DCA6 Prescriptive Residential Wood Deck Construction Guide.