Calcoid

Deck Footing Size Calculator

Size a deck footing from joist span, beam span, soil bearing, and load. Get the minimum diameter, bearing area, and concrete needed per footing.

Deck footing size

Ledger to beam. The beam carries half of this span.

Distance between footings along the beam.

IRC presumptive values. Use 1,500 psf when the soil is unknown.

Recommended footing diameter

18 in

Minimum required: 17.13 in (round footing). Next standard tube form rounds up to 18 in.

Bearing area and footing options

Required bearing area
230.4 sq in
Bearing area
1.6 sq ft
Round footing (min diameter)
17.13 in
Square footing (min side)
15.18 in

Load on the footing

Tributary area
48 sq ft
Beam tributary width
6 ft
Design load
50 psf
Total load on footing
2,400 lb

Concrete per footing (recommended diameter)

Volume
1.18 cu ft
Volume
0.0436 cu yd
60 lb bags
3
80 lb bags
2

Sizes assume an interior footing, the governing case. Corner and end footings carry less. Confirm footing depth, frost line, and post connections with your local building department.

Frequently Asked Questions about the Deck Footing Size Calculator

How does this calculator size a deck footing?
It carries the deck load down to the soil. First it finds the tributary area each post supports, multiplies that by your live plus dead load to get the load in pounds, then divides by the soil bearing capacity to get the required footing area. The minimum round diameter comes from the circle area formula, area equals pi times the diameter squared divided by 4. The method follows the IRC and the American Wood Council DCA 6 deck guide.
How is the tributary area figured?
The beam carries half of the joist span, because the ledger at the house carries the other half. An interior post then carries the full beam span between the posts on either side. So the tributary area is half the joist span times the beam span. This calculator sizes the interior footing, which is the worst case, so the same size also covers the end and corner footings that carry less.
What live and dead load should I enter?
The IRC sets a deck live load of 40 psf and a typical wood-frame dead load of about 10 psf, which is 50 psf total. The defaults match those values. Raise the live load for a hot tub, heavy planters, or deep snow, and raise the dead load if you use heavier decking or framing.
What soil bearing capacity should I pick?
Use the IRC presumptive value for your soil from Table R401.4.1: 1,500 psf for clay or silt, 2,000 psf for sand, 3,000 psf for sandy gravel, 4,000 psf for sedimentary rock, and 12,000 psf for bedrock. When you are not sure, pick 1,500 psf, the most conservative choice. A soil report or your building inspector can confirm the right value for your site.
How much concrete does each footing need?
The calculator pours a cylinder of the recommended diameter to the footing depth you enter, so the volume is pi times the radius squared times the depth. It reports cubic feet, cubic yards, and the number of 60 lb and 80 lb pre-mix bags, rounding bags up. A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.6 cubic feet.
Is this a substitute for a permit or an engineer?
No. This is a prescriptive estimate based on the IRC and DCA 6, and it does not check frost depth, post and beam sizing, hardware, or local code amendments. Footing depth in particular has to reach below your frost line. Confirm the design with your local building department before you dig or pour.

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