Deck Footing Size Calculator (Tributary Load)
Size deck post footings from the tributary area each post supports, the design live and dead load, soil bearing capacity and footing shape (round or square), with the IRC R507.3 minimum footing sizes, concrete volume per footing and frost-depth reminder.
How it works
Tributary area = (joist span ÷ 2) × post spacing (× ½ for end posts); load = area × (live + dead); bearing area = load ÷ soil capacity; diameter = √(4A ÷ π)
Each deck post carries the weight of the deck within its tributary area — half the joist span (the other half goes to the ledger or the opposite beam) times the post spacing — at the design load of 50 psf for a normal residential deck. Dividing that by the soil's safe bearing pressure gives the bearing area the footing must spread it over: a few hundred square inches on gravel, far more on soft clay, which is why the same deck needs 12-inch footings in one town and 20-inch in another. The footing must also be thick enough that the concrete does not snap at the edge (projection no more than thickness) and must bear below frost depth so heave does not lift the deck; sonotube piers achieve that with a bell or pad at the bottom.
Worked example
An interior post under 12 ft joists at 7 ft post spacing carries 42 ft² × 50 psf = 2,100 lb; on 1,500 psf clay that needs 1.4 ft² (202 in²) — a 16″ round or 15″ square footing. Each footing with an 8″ pad and 10″ pier to 36″ frost depth takes about 2.2 ft³ of concrete, four 80 lb bags.
Frequently asked questions
How do I know my soil bearing capacity?
Building departments publish a presumptive value (often 1,500 psf unless a soil test shows better); IRC Table R401.4.1 lists 1,500 for clay, 2,000 for sandy clay/silt, 3,000 for sand and gravel. If water stands in the hole or the soil is fill, get it tested.
Does a hot tub change this?
Dramatically — a filled tub with people is 100–150 psf over its footprint. Posts under a tub need their own calculation and usually a separate, closer set of footings.
Related calculators
Span values follow the IRC 2021 deck provisions (R507) and AWC DCA 6 for #2 grade lumber at 40 psf live + 10 psf dead load, wet-service, simplified to the common cases. Local amendments, snow loads, hot tubs, cantilevers and other species/grades change the numbers — confirm with the local building department or an engineer.