Assuming a minimum soil bearing capacity of 1,500 psf, 8-inch-diameter concrete piers bearing on square footings measuring 2 feet on a side and 9 to 11 inches thick are adequate for most single-story decks where beams are spaced 14 feet apart or less and joist spans are 14 feet or less (see table below).

Beam
Span
Joist
Span
Round
(diameter)
Square Footing
Thickness
8' <10' 14" 18"x18" 8"
<14' 11" 21"x21" 10"
<18' 8" 24"x24" 11"
10' <10' 23" 20"x20" 9"
<14' 27" 24"x24" 11"
<18' 31" 27"x27" 13"
12' <10' 25" 22"x22" 10"
<14' 30" 26"x26" 13"
<18' 34" 30"x30" 15"
14' <10' 27" 24"x24" 11"
<14' 32" 29"x29" 14"
<18' 37" 33"x33" 16"
16" <10' 29" 26"x26" 12"
<14' 35" 31"x31" 15"
<18' 40" 35"x35" 18"


Adapted from "Prescriptive Residential Wood Deck Construction Guide" (DCA6)
For more about designing and building decks, visit the JLC Field Guide.