How to Size Deck Footings

What determines deck footing size

Two separate questions decide a footing: how wide and how deep. Diameter handles the weight, so a heavier load or weaker soil means a wider footing. Depth handles frost, so a colder climate means a deeper hole. Get one right and the other wrong and the footing still fails.

The governing code is IRC R507.3.1, which sizes deck footings from the tributary area on each post and the soil bearing value. The footing size does not come from the post size. A 6 by 6 post does not need a 14 inch footing just because the post is large; the diameter follows the load.

Step 1: find the tributary area per footing

The tributary area is the slice of deck whose weight lands on one post. For a deck attached to the house with a ledger, the house carries half the load and the beam posts carry the other half, so each post takes the post spacing along the beam times half the joist span.

Take a 12 by 16 foot deck with a beam 6 feet out from the house and posts 8 feet apart. Each interior post carries half the joist span, 3 feet, times the beam spacing on each side, 8 feet, which is a 24 square foot tributary area. Corner posts carry less, so size for the worst loaded post.

If your layout is not a clean rectangle, split it into sections and add the areas that land on each post. To size the footing once you have the area, enter it in the concrete footing calculator and skip the lookup table.

Step 2: apply the deck load

The IRC sets the design load for you. A residential deck is built for 40 pounds per square foot live load plus 10 pounds dead load, a total of 50 psf. Live load is people and furniture; dead load is the deck structure itself.

In snow country you use the greater of the live load or the ground snow load, not both stacked. So a 40 psf deck in a 40 psf snow zone still designs at 40 psf live, but a heavier 50 or 70 psf snow load takes over where it is larger.

Multiply the tributary area by the load to get the pounds on each post. The 24 square foot example at 50 psf is 1,200 pounds per post without snow, or about 2,160 pounds if a 40 psf snow load is added on top of the dead load.

Step 3: divide by soil bearing to get footing area

Soil bearing value is the load the ground can hold per square foot. The IRC default, used when you have no soil test, is 1,500 psf, which assumes clay or silt and gives the largest, safest footing. Sandy or gravelly soil can reach 2,000 to 3,000 psf and allows a smaller footing.

Required footing area is the total post load divided by the soil bearing value. The 1,200 pound post on 1,500 psf soil needs 0.80 square feet, about 115 square inches. A 12 inch round footing is 113 square inches, borderline, so go up to a 14 or 16 inch tube.

Deck footing diameter chart by tributary area

The table below shows typical round footing diameters from IRC R507.3.1 for a 40 psf deck on 1,500 psf soil, the common default. Interpolate between rows if your tributary area falls in the middle, and round up. Extrapolating past the table is not allowed.

A 12 inch round footing is the practical minimum in most jurisdictions, with 18 inches a common safe default for standard decks. When in doubt, go bigger, since a slightly oversized footing costs a few extra dollars of concrete but adds real margin.

Footing depth and the frost line

Footing depth is a separate calculation from diameter, driven by frost. Code requires footings to extend at least 12 inches below grade and below the local frost line, whichever is deeper. In warm climates that is often 12 to 24 inches; in cold regions it runs 36 to 48 inches.

Measure depth from undisturbed soil, not loose backfill, and many builders add 6 inches below the frost line as a margin. Where bedrock is near the surface, footings can bear directly on rock, since frost does not heave solid stone.

Always confirm the frost depth and footing inspection rules with your local building department before digging, because jurisdictions amend the IRC for local snow, soil and frost. Most areas require an inspection of the open holes before any concrete is poured.

How much concrete each footing needs and what it costs

Once the diameter and depth are set, the concrete is a simple tube volume: pi times the radius squared times the depth. A 12 inch round footing 4 feet deep holds about 3.1 cubic feet, roughly 5 to 6 bags of 80 lb mix once you add waste. A 16 inch footing the same depth nearly doubles that.

In 2026 an 80 lb bag of concrete runs about 3.50 to 5 dollars, so a single small footing is often under 30 dollars in material. An installed footing, with excavation and labor, typically costs 200 to 400 dollars each. For the exact bag count per footing, run the diameter and depth through the footing calculator.

Above roughly one cubic yard total, ready-mix delivered by the truck usually beats hauling bags. If you are also pouring a slab or pad near the deck, the concrete slab calculator handles that flat work separately.

Frequently asked questions

References

• IRC 2021 R507.3.1 Minimum Footing Size (ICC Digital Codes)
• AWC DCA-6 Prescriptive Residential Deck Construction Guide
• Shallow Foundation (Wikipedia)
• QUIKRETE Concrete Mix yield (spec sheet)

Related