Deck Footing Calculator: Size, Depth & Concrete Bags

Type your deck's size, where the beam sits and how far apart the posts are. You get the footing each post needs from the building code's own table, how deep it goes, the post size that reaches your height, and how many bags of concrete to buy. Your city has the last word on snow load and frost depth.

Width, along the house ft

Depth, out from the house ft

Is it attached to the house?

Joists past the beam ft

Posts at most every ft

Snow where you live (the deck is built for the greater of 40 psf or your ground snow load)

The soil under the deck

Footing shape

Post height, beam to footing ft

Frost line (optional) in

The posts' wood

A form tube on the footing

Concrete bags

Your deck's footings

18″ round footings, 6″ thick, below your frost line3 posts · the middle post carries 52 sq ft · 40 psf · soil 1,500 psf · IRC 2021 Table R507.3.1

Your deck from above: the posts, and the deck the busiest one carries

Does it need a permit? →

Quick answer: a deck footing is sized by the deck area one post carries, the snow load and the soil, from the building code’s Table R507.3.1. A middle post holding 50 square feet of a 40 psf deck on clay needs an 18-inch round (or 16-inch square) footing, 6 inches thick. Every footing goes at least 12 inches below the ground, and below the frost line when the deck is attached to the house.

How to use the deck footing calculator

Type the deck’s width along the house and its depth out from it (feet, or meters with the toggle), and whether it hangs on a ledger board on the house or stands on its own.
Where the beam sits: how far the joists run past it (0 when the beam is at the edge), and the most the posts may be apart. The calculator spaces the posts evenly.

Your snow load and soil. The deck is built for 40 pounds per square foot or your area’s ground snow load, whichever is greater. Not sure of the soil? Clay is the code’s lowest value, so it is the safe guess.
The post’s height and wood, and your frost line if you know it: you get the post size the code allows and the concrete for the whole hole. Pick a form tube and a bag size to count the bags. Copy link keeps your deck.

The code’s rule: one post’s share of the deck

The IRC 2021 says “The minimum size of concrete footings shall be in accordance with Table R507.3.1, based on the tributary area and allowable soil-bearing pressure in accordance with Table R401.4.1”. The tributary area is the “Area, in square feet, of deck surface supported by post and footings”: the joists put half their span on each end, so a beam carries half the joist span plus any overhang past it, and a post in the middle of that beam carries one post spacing of that strip. A corner post carries half as much.

The soil’s value comes from the code too: “In lieu of a complete geotechnical evaluation, the load-bearing values in Table R401.4.1 shall be assumed.” Clay and silt are 1,500 psf, sand 2,000, gravel 3,000 and sedimentary rock 4,000.

Minimum deck footing size at 40 psf (IRC 2021 Table R507.3.1): square side / round diameter / thickness, in inches
Area per post	Clay, silt (1,500 psf)	Sand (2,000 psf)	Gravel, rock (3,000+ psf)
5 sq ft	7 / 8 / 6	7 / 8 / 6	7 / 8 / 6
20 sq ft	10 / 12 / 6	9 / 9 / 6	7 / 8 / 6
40 sq ft	14 / 16 / 6	12 / 14 / 6	10 / 12 / 6
60 sq ft	17 / 19 / 6	15 / 17 / 6	12 / 14 / 6
80 sq ft	20 / 22 / 7	17 / 19 / 6	14 / 16 / 6
100 sq ft	22 / 25 / 8	19 / 21 / 6	15 / 17 / 6
120 sq ft	24 / 27 / 9	21 / 23 / 7	17 / 19 / 6
140 sq ft	26 / 29 / 10	22 / 25 / 8	18 / 21 / 6
160 sq ft	28 / 31 / 11	24 / 27 / 9	20 / 22 / 7

The table’s own notes: “Interpolation permitted, extrapolation not permitted.” Between two rows the calculator interpolates and rounds up to the inch; past 160 square feet a post is off the table, so the posts go closer or an engineer sizes it. “Minimum thickness shall only apply to plain concrete footings.” And “Footing dimensions shall allow complete bearing of the post”, so the calculator widens a footing that would be narrower than its post. The snow columns (50, 60 and 70 psf) are in the calculator.

How deep the footings go

“Deck footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface.” For a deck on the house, the code adds: “Where decks are attached to a frost-protected structure, deck footings shall be protected from frost by one or more of the following methods: Extending below the frost line specified in Table R301.2.” Your building department publishes that frost line, and it differs from town to town. A free-standing deck may stop at 12 inches, but frost can lift it.

Worked examples

A 12 × 10 ft free-standing deck, beams 1 ft in from each edge, posts every 6 ft, clay: 6 posts, a middle post carries 30 sq ft → 14″ round footings, 6″ thick, 12″ deep, 2 bags of 80 lb a hole.
A 16 × 12 ft deck on the house, the beam 1 ft in, posts every 8 ft, clay, a 36″ frost line: the middle post carries 52 sq ft → 18″ round footings, 6″ thick, 36″ deep, 9 bags of 80 lb a hole if you fill it all the way up.
A 20 × 14 ft deck on the house in 50 psf snow country, the joists 2 ft past the beam, sandy soil, a 42″ frost line: 4 posts 6.7 ft apart, 53.3 sq ft on a middle post → 17″ round footings, 6″ thick, 42″ deep; with a 10″ tube on the footing, 5 bags of 80 lb a hole.

A 24 × 16 ft deck on the house, the beam at the edge, posts 12 ft apart, clay: 96 sq ft on the middle post → 25″ round footings, 8″ thick, 36″ deep. Posts 8 ft apart would bring it down to 64 sq ft.

Concrete: how many bags a hole

The calculator fills the hole at the footing’s width from the bottom to the ground, or pours the footing and a form tube on top of it. It counts whole bags of QUIKRETE Concrete Mix by its data sheet’s yields: “An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L)”, “A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L)”, “A 50 lb (22.6 kg) bag yields approximately 0.375 ft3 (10.6 L)” and “A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 (8.5 L)”. Mix each hole’s bags on their own, and in cold weather “Protect concrete from freezing during the first 48 hours.”

Common mistakes

Sizing every footing by the whole deck. A footing carries its own post’s share: divide the deck into the strips each post holds.

Forgetting the snow. “Decks shall be designed for the live load required in Section R301.5 or the ground snow load indicated in Table R301.2, whichever is greater.” In 60 psf country the same deck needs bigger footings.
Stopping at 12 inches on an attached deck. The house sits below the frost line, and a deck footing that heaves pulls the ledger with it.
Setting a post on a footing narrower than it. The whole post must bear on the concrete, and a brick or block pier needs 2 inches of footing all round it: “If the support is a brick or CMU pier, the footing shall have a minimum 2-inch projection on all sides.”

Leaving the post loose on the footing. “Where posts bear on concrete footings in accordance with Section R403 and Figure R507.3, lateral restraint shall be provided by manufactured connectors or a minimum post embedment of 12 inches (305 mm) in surrounding soils or concrete piers.”

Deck footings: questions people ask

How big should deck footings be?

As big as Table R507.3.1 says for the deck area each post carries, your snow load and your soil: from 8 inches round for a small share on any soil, to 31 inches round for 160 square feet on clay at 40 psf. The calculator works out the share and reads the table.

How deep should deck footings be?

At least 12 inches below the undisturbed ground (R507.3.2), and below your frost line when the deck is attached to the house (R507.3.3). Your building department publishes the frost line.

How much concrete do I need for deck footings?

The hole’s volume: an 18-inch round hole 36 inches deep holds about 5.3 cubic feet, 9 bags of 80 lb concrete mix at 0.60 cubic feet a bag. A form tube on a footing pad uses less. The calculator counts it per hole and for the whole deck.

What size concrete footing for a 6×6 post?

The post size does not set the footing; the deck area it carries does. The footing only has to be wide enough for the whole post to bear on it: at least a 6-inch square or an 8-inch round for a 6×6, and the table is bigger than that for any real deck.

What size post do I need for a deck?

Table R507.4 gives the tallest each post may be by its wood, its load and the area it carries: a 4×4 of southern pine is allowed up to 14 feet at 20 square feet and 40 psf, less as the area grows; a 6×6 reaches 14 feet in most cases.

Can I use a sonotube for deck footings?

Yes, as the pier on top of a footing of the table’s size, or a tube as wide as the footing all the way down. Pick the tube size in the calculator and it counts the concrete for the footing plus the tube.

Deck Footing Calculator