Deck Footing Size and Depth, Explained

Footings do two jobs: spread the deck's weight over enough soil that it won't sink, and reach deep enough that frost can't lift them. Get either wrong and the deck racks, the ledger pulls, or doors stop closing after the first hard winter.

Footing size = load ÷ soil bearing

Each post carries a tributary slice of the deck — roughly the post spacing times half the joist span. Multiply that area by 50 psf (40 live + 10 dead) to get the load on the footing, then divide by your soil's bearing value to get the bearing area it needs. The IRC presumptive value for ordinary sand, silt or clay is 1,500 psf; gravel bears far more.

Depth follows the frost line

The bottom of the footing must sit below the local frost line so freezing soil can't heave it. That's about 12" in the frost-free South, 36–48" across the Midwest, and up to 60" in the upper Midwest and New England. Even where the ground never freezes, you still need roughly 12" to reach firm, undisturbed soil.

Get the footing inspected before you pour

Almost every jurisdiction requires an open-hole footing inspection before concrete goes in — it's the one part of the deck nobody can see afterward. Pick your state in the calculator to set the depth, and it sizes the footing diameter from your deck's actual loads.