How big should deck footings be?
Enter your deck layout and soil, get the footing size for each post and the concrete.
Joists run from the house ledger to the center line of the beam; with no overhang, to the outside edge of the deck.
The part of the deck surface that rests on that post (tributary area).
IRC Table R401.4.1 values, for use without a soil report. Not sure? Use the 1,500 psf clay row.
From your local code. The deck is designed for the greater of this and the 40 psf live load.
Depth and extra are examples: your frost depth sets the depth, and the footing must be at least 12 in below undisturbed ground.
How big should a deck footing be?
Find the load on the post, then divide by what the soil can carry. A center post under a 16 ft beam on three posts, with 12 ft joists, carries 6 ft × 8 ft = 48 ft² (60 ft² with the 1.25 factor for a continuous beam). The design load is (40 + 10 psf) × 60 ft² = 3,000 lb. On 1,500 psf soil that needs 3,000 ÷ 1,500 = 2 ft² = 288 in²: a footing 17 inches square or 20 inches round (19.15, rounded up). IRC Table R507.3.1 prints 17 inches square, 19 round and 6 thick for the same 60 ft² on 1,500 psf soil; the table rounds to the nearest inch and the calculator rounds up. The footing must also let the whole post bear on it, and a brick or block pier needs the footing to project at least 2 inches on every side (table notes c and d).
What load does a deck footing carry?
The IRC designs a deck for the greater of the 40 psf live load and the ground snow load (R507.1), plus a dead load of 10 psf, over the tributary area: the part of the deck surface that rests on that post. A 50 psf snow load governs: (50 + 10) × 60 ft² = 3,600 lb, which on 1,500 psf soil needs a 19 inch square or 21 inch round footing. For comparison, the American Wood Council’s DCA 6 guide also counts the weight of the concrete itself and lists a 26 inch round footing for 100 ft² on 1,500 psf soil, where the IRC table says 25.
What is the soil bearing capacity?
Without a soil report the IRC lets you assume the values of Table R401.4.1: 1,500 psf for clay and silt soils (CL, ML, MH, CH); 2,000 psf for sand and for silty or clayey sand and gravel; 3,000 psf for gravel and sandy gravel (GW, GP); 4,000 psf for sedimentary and foliated rock; and 12,000 psf for crystalline bedrock. Where the building official expects soil weaker than 1,500 psf, a soils investigation must set the value. The footing table has columns for 1,500, 2,000 and 3,000 psf or more.
How deep should deck footings go?
The IRC asks for footings at least 12 inches below the undisturbed ground (R507.3.2). Where a deck is attached to a frost-protected structure, the footings must also be protected from frost: below the frost line in the code’s climate table, on solid rock, or by another approved method (R507.3.3). The frost line is local, so ask your building department what depth it requires.
How much concrete do I need for deck footings?
Multiply the footing area by its depth. Six round footings 20 inches across and 36 inches deep take 6 × π ÷ 4 × 20² × 36 ÷ 1,728 = 39.3 ft³ — 1.45 cubic yards; with 10% extra that is 43.2 ft³, or 72 bags of 80 lb at 0.6 ft³ each. For tube forms and post holes, see the sonotube calculator.
Sources: International Residential Code 2021 and 2024 (ICC, codes.iccsafe.org, free view): R507.1, Table R301.5, R507.3, R507.3.1 with Table R507.3.1, R507.3.2, R507.3.3 and Table R401.4.1 (R401.4.1(1) in 2024); American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6, 2015), Appendix B, for the tributary area equations, the 1.25 factor at continuous beams and the 10 psf dead load; bag yields from QUIKRETE. Check your joists with the deck joist calculator and the beam with the deck beam span calculator; boards and screws: deck calculator; tube forms and post holes: sonotube calculator; more concrete: concrete calculator. How we check our calculators.