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Deck footing calculator: how big should a deck footing be?

19 in round, 6 in thick

or 17 in square · 48 sq ft per post (table row 60) · at least 12 in below grade · for 12 ft joists, posts 8 ft apart, 1500 psf soil, IRC Table R507.3.1

Run your own numbers

Enter it the way you measured it: 4.5, 4 1/2, 3/8 or 10' 6".

19 in round, 6 in thick

or 17 in square · 48 sq ft per post (table row 60) · at least 12 in below grade

    Source: IRC 2021 Table R507.3.1, Minimum Footing Size for Decks (International Code Council, 2021 International Residential Code), read October 18, 2026. Read the source

    A deck post carrying 48 square feet, with 12 ft joists and posts 8 ft apart, needs a footing 19 in round or 17 in square and 6 in thick on 1,500 psf clay soil at a 40 psf live load (IRC 2021 Table R507.3.1).

    A worked example

    A post 7 ft from its neighbors under 13 ft joists that overhang the beam 1.5 ft, on sand, with a 60 psf ground snow load needs 20 in round, 6 in thick. The post carries 7 × (6.5 + 1.5) = 56 square feet, so it takes the table's 60 square foot row: 17 in square or 20 in round on 2,000 psf soil.

    How deep should deck footings be?

    At least 12 inches below undisturbed ground, by IRC R507.3.2. When the deck is attached to a house whose foundation is protected from frost, R507.3.3 also takes the footings below the frost line in your local code's Table R301.2, onto solid rock, or uses another approved frost protection. The frost line is set county by county, so it is the one number on this page that you look up locally.

    How much deck does one post carry?

    Multiply the post spacing by half the joist span, plus any overhang past the beam. With 12 ft joists and posts 8 ft apart that is 8 × 6 = 48 square feet; an end post carries only half the spacing, 24 square feet, and can take the smaller footing of its own row. A post carrying more than 160 square feet is past the table, which allows no extrapolation.

    What soil bearing value should I use?

    Without a soil test, the IRC presumes 1,500 psf for clay and silt, 2,000 psf for sand and clayey or silty gravel, and 3,000 psf for gravel (Table R401.4.1). Clay is the weakest of the three and gives the largest footing: at 48 square feet per post and 40 psf it is 19 in round, against 17 in on sand and 14 in on gravel.

    What else it is called

    A deck footing calculator is also searched as deck footing size, deck pier size or concrete pier size for a deck. The round size is the diameter of a pad or of a form tube poured on its own footing; the thickness is the plain concrete under the post.

    The full IRC table

    IRC 2021 Table R507.3.1, minimum footing size for decks by load, area per post and soil
    Load · sq ft per post1,500 psf soil: squareroundthick2,000 psf soil: squareroundthick3,000 psf soil: squareroundthick
    40 psf live · 5 sq ft 7 in8 in6 in7 in8 in6 in7 in8 in6 in
    40 psf live · 20 sq ft 10 in12 in6 in9 in9 in6 in7 in8 in6 in
    40 psf live · 40 sq ft 14 in16 in6 in12 in14 in6 in10 in12 in6 in
    40 psf live · 60 sq ft 17 in19 in6 in15 in17 in6 in12 in14 in6 in
    40 psf live · 80 sq ft 20 in22 in7 in17 in19 in6 in14 in16 in6 in
    40 psf live · 100 sq ft 22 in25 in8 in19 in21 in6 in15 in17 in6 in
    40 psf live · 120 sq ft 24 in27 in9 in21 in23 in7 in17 in19 in6 in
    40 psf live · 140 sq ft 26 in29 in10 in22 in25 in8 in18 in21 in6 in
    40 psf live · 160 sq ft 28 in31 in11 in24 in27 in9 in20 in22 in7 in
    50 psf snow · 5 sq ft 7 in8 in6 in7 in8 in6 in7 in8 in6 in
    50 psf snow · 20 sq ft 11 in13 in6 in10 in11 in6 in8 in9 in6 in
    50 psf snow · 40 sq ft 15 in17 in6 in13 in15 in6 in11 in13 in6 in
    50 psf snow · 60 sq ft 19 in21 in6 in16 in18 in6 in13 in15 in6 in
    50 psf snow · 80 sq ft 21 in24 in8 in19 in21 in6 in15 in17 in6 in
    50 psf snow · 100 sq ft 24 in27 in9 in21 in23 in7 in17 in19 in6 in
    50 psf snow · 120 sq ft 26 in30 in10 in23 in26 in8 in19 in21 in6 in
    50 psf snow · 140 sq ft 28 in32 in11 in25 in28 in9 in20 in23 in7 in
    50 psf snow · 160 sq ft 30 in34 in12 in26 in30 in10 in21 in24 in8 in
    60 psf snow · 5 sq ft 7 in8 in6 in7 in8 in6 in7 in8 in6 in
    60 psf snow · 20 sq ft 12 in14 in6 in11 in12 in6 in9 in10 in6 in
    60 psf snow · 40 sq ft 16 in19 in6 in14 in16 in8 in12 in14 in6 in
    60 psf snow · 60 sq ft 20 in23 in7 in17 in20 in6 in14 in16 in6 in
    60 psf snow · 80 sq ft 23 in26 in9 in20 in23 in7 in16 in19 in6 in
    60 psf snow · 100 sq ft 26 in29 in10 in22 in25 in8 in18 in21 in6 in
    60 psf snow · 120 sq ft 28 in32 in11 in25 in28 in9 in20 in23 in7 in
    60 psf snow · 140 sq ft 31 in35 in12 in27 in30 in10 in22 in24 in8 in
    60 psf snow · 160 sq ft 33 in37 in13 in28 in32 in11 in23 in26 in9 in
    70 psf snow · 5 sq ft 7 in8 in6 in7 in8 in6 in7 in8 in6 in
    70 psf snow · 20 sq ft 12 in14 in6 in11 in13 in6 in9 in10 in6 in
    70 psf snow · 40 sq ft 18 in20 in6 in15 in17 in6 in12 in14 in6 in
    70 psf snow · 60 sq ft 21 in24 in8 in19 in21 in6 in15 in17 in6 in
    70 psf snow · 80 sq ft 25 in28 in9 in21 in24 in8 in18 in20 in6 in
    70 psf snow · 100 sq ft 28 in31 in11 in24 in27 in9 in20 in22 in7 in
    70 psf snow · 120 sq ft 30 in34 in12 in26 in30 in10 in21 in24 in8 in
    70 psf snow · 140 sq ft 33 in37 in13 in28 in32 in11 in23 in26 in9 in
    70 psf snow · 160 sq ft 35 in40 in15 in30 in34 in12 in25 in28 in9 in

    IRC 2021 Table R507.3.1, Minimum Footing Size for Decks, as printed; the third column is for soil rated 3,000 psf or more. Interpolation permitted, extrapolation not permitted. Based on highest load case: Dead + Live or Dead + Snow. Minimum thickness shall only apply to plain concrete footings.

    Where the numbers come from

    Your state or county adopts a version of the IRC and can amend these tables: New Jersey, for one, raises the presumptive soil values. The footing your inspector accepts is the one in your adopted code, and the frost depth is set locally in Table R301.2 of that code.

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