How big does a deck footing need to be?

Divide the total load on the post by the allowable soil bearing pressure. A post carrying 100 sq ft of deck at 50 psf total load is 5,000 lb; on 1,500 psf soil that needs 3.33 sq ft of bearing, which is a 24.7 in. round footing or a 21.9 in. square footing.

What soil bearing capacity should I use for a deck footing?

IRC Table R401.4.1 gives presumptive load-bearing values; 1,500 psf is the common default for sand, silty gravel and clay mixtures. Your building department may require a different value or a soils report.

What loads apply to a residential deck?

The IRC uses a 40 psf live load for decks plus a dead load for the structure itself, commonly taken as 10 psf. Snow load can govern instead of live load in colder climates.