Deck Footing Size Calculator

A deck footing size calculator returns the footing size for each post. It reads AWC DCA 6 Table 4 by beam span and joist span, then gives the round or square size, thickness, and maximum post height. A 12 ft beam with 14 ft joists needs a 26 in. square footing, 13 in. thick.

Last reviewed September 2026Method & sources Report an error

Square Footing (per post)

26 in. × 26 in.

Breakdown
Square Footing (per post)	26 in. × 26 in.
Round Footing Diameter	30 in.
Footing Thickness	13 in.
Maximum Post Height (6x6 Southern Pine)	13 ft

How to Calculate Deck Footing Size

AWC DCA 6 Table 4 sets the footing size for each post from the beam span and joist span it carries. This calculator finds the row for your beam span and the joist band that covers your joist span, then reads the Southern Pine columns for the maximum post height and the round or square footing size. The table assumes 1,500 psf soil bearing capacity and 150 pcf concrete. The guide allows a corner post footing to be 0.9 times the size listed.

Example: Post under a 12 ft beam span, with joists up to 14 ft, Southern Pine

A post under a 12 ft beam span, with joists up to 14 ft, Southern Pine: 1. Table row: beam span 12 ft. 2. Joist band: joists up to 14 ft is the ≤14 ft band. 3. Table value: 13 ft maximum post height, 30 in. round footing, 26 in. × 26 in. square footing, 13 in. thick. Use a 26 in. × 26 in. square footing, 13 in. thick, and keep the post no taller than 13 ft.

Deck Footing Size: Compare Scenarios

Scenario	Square Footing (per post)	Round Footing Diameter	Footing Thickness	Maximum Post Height (6x6 Southern Pine)
Short span (8 ft beam, joists up to 14 ft)	21 in. × 21 in.	24 in.	10 in.	14 ft
Medium span (12 ft beam, joists up to 14 ft)	26 in. × 26 in.	30 in.	13 in.	13 ft
Long span (16 ft beam, joists up to 18 ft)	35 in. × 35 in.	40 in.	18 in.	7 ft

What Is Deck Footing Size?

A deck footing size calculator tells you how big each concrete footing under a deck post needs to be. The footing spreads the post load into the soil. A footing that is too small can settle or shift, which moves the post and the deck above it. The size depends on how much load the post carries, which the guide ties to the beam span and joist span.

Deck Footing Size: Pros and Cons

Pros

Gives the round and square footing sizes and thickness from the same table row, so you can compare the two shapes directly
Shows the maximum post height next to the footing size, so a tall post and a large footing are checked together
Uses a free AWC guide table with its assumptions printed on the page

Cons

Covers Southern Pine 6x6 posts only in this version. Other species have different post heights and footing sizes in the same table, and are not included yet
Assumes 1,500 psf soil bearing capacity. Weaker soil needs a larger footing, and the guide says the soil must be checked on site by the building official
Does not account for frost depth. Footings must bear at least 12 in. below undisturbed ground or below the frost line, whichever is deeper, and the building official confirms the depth on site

Deck Footing Size: What a Pro Checks

Measure post height from grade or the top of the foundation to the underside of the beam, whichever is higher. The table limit applies to that measurement. (AWC DCA 6 Table 4 notes and Post Requirements)
Confirm the soil bearing capacity with your building official, since the table assumes 1,500 psf. Weaker soil needs a larger footing. (AWC DCA 6 Table 4 notes and Figure 12)
Keep footings at least 12 in. below undisturbed ground and below the frost line, whichever is deeper. (AWC DCA 6 Figure 12 and footings section)

Frequently Asked Questions

How big should a deck footing be?

It depends on the beam span and joist span. AWC DCA 6 Table 4 gives a 16 in. square footing for a 6 ft beam span with joists up to 10 ft, and a 37 in. square footing for an 18 ft beam span with joists up to 18 ft, in Southern Pine.

How deep should a deck footing be?

The guide says footings must bear on undisturbed soil at least 12 in. below the undisturbed ground surface or below the frost line, whichever is deeper. Confirm the local frost depth with your building department.

Can I use a square footing instead of a round one?

Yes. The table gives both shapes. A round footing needs a larger diameter than a square one for the same load, so pick the shape that fits your post base and digging method.

Why does the footing get bigger when the beam span gets longer?

A longer beam span carries more load onto each post, so the footing has to spread that load over more soil. The table reflects that with larger footings and shorter maximum post heights as the beam span increases.

Sources

AWC Prescriptive Residential Wood Deck Construction Guide (DCA 6, 2015 edition), Table 4: Post Height for 6x6 and Footing Sizes for all Posts (opens in a new tab)

Concrete & Masonry