Footing size depends on how much deck area each post carries (its tributary area) and the soil. This chart gives typical footing diameters for average soil at common deck loads.
| Footing diameter | Supports up to |
|---|---|
| 8 in | about 20 sq ft of deck |
| 10 in | about 30 sq ft |
| 12 in | about 45 sq ft |
| 16 in | about 80 sq ft |
Depth follows frost. Footings must extend below your local frost line so they do not heave. Weaker soil or larger tributary areas call for wider footings.
FAQ
Why does footing size depend on soil type, not just the load above it? Different soils bear weight differently — sandy or loose soil needs a larger footing to spread the same load over more area than dense clay or rock, which is why local code tables specify footing size based on both load and soil bearing capacity.
Size the concrete with the Concrete Footing Calculator and read Deck Footing Size Guide.