Deckwright
Foundation · IRC R507.3

Deck Footing Calculator

Footing diameter or width and thickness from the area each post carries and your soil type, plus the concrete to fill them.

1 Load on one post
ft
Center to center along the beam
ft
Ledger to beam
in
Use the greater of 40 psf and your ground snow load
2 Soil & hole
Shape
in
Footing size
Ø18in
7″ thick minimum
Tributary area
48.0sq ft
2,400 lb at 50 psf
Concrete
7.32ft³
0.27 yd³ for 3 footings
80 lb bags
13
or 17 × 60 lb, 25 × 40 lb
Required bearing area: 2,400 lb ÷ 1,500 psf = 1.60 sq ft. A 18″ round footing gives 1.77 sq ft. The estimate assumes a 7″ pad, then a 10″ tube up to 2″ above grade.

Per footing: 2.44 ft³. No allowance for spillage or over-dug holes; most crews add 10%. Bag yields: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³.

How the size is set

IRC R507.3.1 basis
  1. Tributary area = post spacing × (joist span ÷ 2 + cantilever) = 8′ × 6′ = 48.0 sq ft.
  2. Load = area × (40 psf live + 10 psf dead) = 2,400 lb. The 2021 IRC table adds a 50 psf row for snow; we compute any load the same way.
  3. Bearing area = load ÷ soil bearing. Rounded up to the next inch, with a minimum of 12″ square or 14″ round.
  4. Thickness ≥ the footing’s projection beyond the 6x6 post, and never less than 6″.

These steps reproduce every row of IRC Table R507.3.1 to within an inch, always rounding up. End posts carry less than interior posts, but sizing all footings for the largest load keeps one hole size.

Tributary area, explained

Each post carries the deck area halfway to its neighbors. For a beam supporting joists that come from a ledger, an interior post carries post spacing × (half the joist span + any cantilever). End posts carry less, but most builders size every hole for the biggest load so there is only one size to dig.

Multiply that area by 50 psf (the IRC’s 40 psf live load plus 10 psf for the deck itself), and you have the load the soil must hold up. Divide by the soil’s bearing value and you have the bottom area the footing needs.

Depth: frost line and undisturbed soil

The IRC requires footings to bear at least 12 inches below undisturbed ground and below the frost line. Frost depth is set locally, so call your building department or check its deck handout rather than relying on a map. Freestanding decks that are not attached to the house may be allowed to use shallower footings in some jurisdictions. Check before you rely on it.

Pad and pier, or one big tube?

A common method is a wide, shallow pad at the bottom of the hole with a narrower tube up to grade. It uses far less concrete than a full-width tube. Bell-shaped plastic footing forms do the same job in one piece. Either way, the bottom must have the required area and the pad must be at least as thick as it projects beyond the post. See deck footing size and depth for the full method.

Questions people ask

How big should deck footings be?
Size each footing for the deck area it supports: tributary area × 50 psf (40 live + 10 dead), divided by your soil’s bearing capacity. In 1,500 psf clay, a post carrying 40 sq ft needs about a 14″ square or 16″ round footing; 100 sq ft needs about 22″ square or 25″ round (IRC Table R507.3.1).
How deep do deck footings need to be?
At least 12″ below undisturbed ground, and below the frost depth set by your local building department where the ground freezes. Frost depths range from zero in the far South to 48″ or more in northern states.
How many bags of concrete per deck footing?
A 10″ tube 36″ deep holds about 1.7 ft³: three 80 lb bags (0.60 ft³ each). Belled or wider pad footings take more. Enter your sizes above for an exact count.
What soil bearing should I use?
Unless you have a soil report, the IRC presumes 1,500 psf for clays and silts, 2,000 psf for sands and silty or clayey gravels, and 3,000 psf for clean sandy gravel. When you are unsure, use 1,500 psf.

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