Deck footing calculator

With posts up to 8 ft apart and joists up to 10 ft long, each footing is 20 inches round and 8 inches thick, and a southern pine 6x6 post can stand up to 14 ft. Choose your spans to size the footings and count the concrete.

Footing under each post

20 in round

The footing under each post, 8 in thick, for a beam span up to 8 ft and joists up to 10 ft.

Corner posts may use 0.9 of that: 18 in round.
6x6 posts up to 14 ft tall, from the ground or the top of the footing to the underside of the beam.
Concrete for the pad alone: 1.5 cu ft a footing, about 3 bags of 80 lb; 18 bags for 6 footings.
Poured full width up to the ground instead, each foot above the pad adds 2.2 cu ft a footing; a 12 in round concrete stem on the pad adds 0.8 cu ft a foot.

Your local building code decides. These are the guide's prescriptive sizes for 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service.

The footing sits on undisturbed soil at least 12 in down, or below your frost line where that is deeper.

Deck footing size chart

Footing size and 6x6 post height, southern pine (DCA 6, Table 4)
Beam span	Joist span	6x6 post up to	Round	Square	Thick
6 ft	up to 10 ft	14 ft	18 in	16 × 16 in	7 in
6 ft	up to 14 ft	14 ft	21 in	18 × 18 in	8 in
6 ft	up to 18 ft	14 ft	24 in	21 × 21 in	10 in
8 ft	up to 10 ft	14 ft	20 in	18 × 18 in	8 in
8 ft	up to 14 ft	14 ft	24 in	21 × 21 in	10 in
8 ft	up to 18 ft	14 ft	27 in	24 × 24 in	11 in
10 ft	up to 10 ft	14 ft	23 in	20 × 20 in	9 in
10 ft	up to 14 ft	14 ft	27 in	24 × 24 in	11 in
10 ft	up to 18 ft	12 ft	31 in	27 × 27 in	13 in
12 ft	up to 10 ft	14 ft	25 in	22 × 22 in	10 in
12 ft	up to 14 ft	13 ft	30 in	26 × 26 in	13 in
12 ft	up to 18 ft	11 ft	34 in	30 × 30 in	15 in
14 ft	up to 10 ft	14 ft	27 in	24 × 24 in	11 in
14 ft	up to 14 ft	11 ft	32 in	29 × 29 in	14 in
14 ft	up to 18 ft	9 ft	37 in	33 × 33 in	16 in
16 ft	up to 10 ft	13 ft	29 in	26 × 26 in	12 in
16 ft	up to 14 ft	10 ft	35 in	31 × 31 in	15 in
16 ft	up to 18 ft	7 ft	40 in	35 × 35 in	18 in
18 ft	up to 10 ft	12 ft	31 in	27 × 27 in	13 in
18 ft	up to 14 ft	9 ft	37 in	33 × 33 in	16 in
18 ft	up to 18 ft	5 ft	42 in	37 × 37 in	19 in

From the American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6), based on the 2015 IRC, which assumes 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service. Footing sizes assume soil that bears 1,500 pounds per square foot and concrete of 2,500 psi.

How deep footings go

Every footing sits on undisturbed soil at least 12 inches below the ground, or below your frost line where that is deeper; your building department gives the frost depth. A footing closer than 5 ft to the house foundation must sit as deep as the house’s own footing.

Call 811 before you dig, and keep footings off septic tanks and drain fields, utility lines and meters.

Posts

Posts are 6x6 or larger and stand centered on the footing. An 8x8 post can replace a 6x6 anywhere in the table, up to 14 ft tall. Corner posts taller than 2 ft get a diagonal brace to the beam.

Concrete to order

The bag count is for the footing pad, the size and thickness in the table, at about 0.6 cubic feet an 80 lb bag. The guide also shows the footing poured full width up to the ground, or a 12 inch round concrete stem from the pad to the ground: both take more concrete than the pad, and the tool gives what each foot of height adds. For many footings, the concrete calculator says when a ready-mix truck makes more sense than bags.

What the table assumes

Joists and beams of No. 2 grade or better, treated for wet service, carrying 40 pounds per square foot of people and furniture plus 10 of the deck’s own weight. A hot tub, a roof over the deck or heavy snow needs an engineered design. Douglas fir-larch, hem-fir and SPF figures assume incised lumber, the small slits made for treatment.

Your building department decides what applies at your address, and most decks attached to a house need a permit.