Question 1

How do I calculate the size of deck footings I need?

Accepted Answer

Multiply the tributary area (the deck area each footing supports) by the total load (50 psf is typical: 40 live plus 10 dead), then divide by your soil bearing capacity. For example, a 50 square foot tributary area with 50 psf load equals 2,500 pounds. In 1,500 psf silty clay soil, that requires 1.67 square feet of bearing, or a footing 17.5 inches in diameter. Always round up to the next standard size.

Question 2

How deep do deck footings need to be?

Accepted Answer

Deck footings must extend below the local frost line to prevent heaving. Typical depths are 12 inches in warm climates (Florida, Texas coast), 36 to 42 inches in northern US states, and up to 48 inches in Canada and Alaska. Check with your local building department for your exact frost depth. The footing must also bear on undisturbed native soil, not fill.

Question 3

What size concrete footing for a 12x12 deck?

Accepted Answer

A 12 foot by 12 foot deck (144 sq ft) typically uses 4 to 6 footings depending on beam layout. With 6 footings, each supports 24 square feet of tributary area at 50 psf, or 1,200 pounds. In 1,500 psf soil, a 12 inch diameter round footing (113 sq inch bearing) easily handles this load. Standard 10 or 12 inch Sonotubes are the most common choice.

Question 4

How many bags of concrete do I need per deck footing?

Accepted Answer

A 10 inch diameter by 48 inch deep Sonotube footing requires about 2.6 cubic feet of concrete, or 5 bags of 60 pound premix or 4 bags of 80 pound premix. A 12 inch by 48 inch footing needs 3.8 cubic feet, or 7 bags of 60 pound or 6 bags of 80 pound. For 6 or more footings, bulk ready-mix delivery is usually more economical.

Question 5

What is a Bigfoot or bell footing?

Accepted Answer

A Bigfoot is a plastic bell-shaped footing form that attaches to the bottom of a Sonotube to widen the bearing base. For example, a BF-20 Bigfoot creates a 20 inch diameter base under a 10 or 12 inch tube. This increases the bearing area dramatically (from 113 to 314 sq inches) without requiring a wider hole throughout. It is required in some jurisdictions for decks over 30 inches above grade.

Question 6

Do I need a footing for each deck post?

Accepted Answer

Yes. Every deck post requires its own footing bearing on undisturbed soil below the frost line. You cannot share a footing between two posts or cantilever posts off the beam. Footings under gates, stairs, or cantilevered sections may be smaller but are still required. Always verify post locations with your framing plan before digging.

Soil Type	Bearing (psf)
Sandy gravel / gravel	3,000
Sandy / clayey sand	2,000
Clay / silty clay	1,500
Soft clay / organic	1,000 (min)

Region	Typical Frost Depth	Example States
Deep South	0 to 12 in	FL, TX, LA, GA (coastal)
Mid-Atlantic / SW	12 to 24 in	VA, NC, TN, AZ, NM
Midwest / NE	36 to 48 in	IL, OH, PA, NY, MA
Northern border / Alaska	48 to 72+ in	MN, ND, ME, AK

Deck Footing Calculator

Deck Dimensions

Beam Layout

Footing Shape

Depth (below grade)

Soil Type

Design Load

Pricing

Footing Cross-section

Shopping List

How to Size Deck Footings

Load per Footing

Required Bearing Area

Soil Bearing Capacity (IRC R401.4.1)

Footing Depth by Frost Line

Common Footing Diameters

Frequently Asked Questions