Three things set the size of a deck footing: how much deck area each post carries (its tributary area), the design load, and how much weight your soil can bear. The code tables turn those three into a minimum diameter and thickness. Depth is a separate question. Footings go at least a code minimum below undisturbed ground, and in cold climates below the local frost line, which your building department sets.
This guide shows how to read the footing tables, how to work out tributary area, and how to estimate concrete. It’s planning information, not an engineered design. Soil, frost depth and local amendments vary, so your permit drawings and inspector have the final word.
Why footings matter more than they look
Footings are the part of a deck you never see again once it’s built, and the hardest part to fix. An undersized footing can settle and tilt a post. A footing above the frost line can heave as the ground freezes and lift one side of the deck. If the deck is attached to the house, that movement pulls on the ledger connection. Most building departments inspect footing holes before you pour for this reason. Anne Arundel County, for example, requires separate inspections for footings, framing and final (Anne Arundel County 2021 IRC deck guide).
Step 1: work out tributary area
Tributary area is the patch of deck surface whose weight ends up on one post. For a typical deck, with joists running from a house ledger to a beam on posts, each beam post carries:
- Half the joist span (the other half goes to the ledger), times
- Half the distance to the post on each side.
Anne Arundel’s guide works this example for an interior post on a 20 × 12 ft deck. The length is 20 ft × 1/2 = 10 ft, the width is 12 ft × 1/2 = 6 ft, and the tributary area is 60 sq ft (Anne Arundel County deck guide).
Picture that deck with three posts along the outer beam, one at each corner and one in the middle, 10 ft apart:
| Post | Width share | Depth share | Tributary area |
|---|---|---|---|
| Middle post | 5 ft + 5 ft = 10 ft | 6 ft | 60 sq ft |
| Corner post | 5 ft | 6 ft | 30 sq ft |
Joist or beam cantilevers add area to the posts nearest them. Count the overhang in your shares.
Post spacing isn’t free either. The distance between posts is limited by the beam span table (IRC Table R507.5), which depends on beam size and joist span. The deck framing calculator checks post spacing and footing loads together.
Step 2: read the footing size table
IRC Table R507.3.1 gives minimum footing sizes by tributary area. The version in Anne Arundel County’s guide is for a 50 psf load on soil with a 2,000 psf bearing capacity (Anne Arundel County deck guide):
| Tributary area | Square footing (side) | Round footing (diameter) | Thickness |
|---|---|---|---|
| 20 sq ft | 10” | 11” | 6” |
| 40 sq ft | 13” | 15” | 6” |
| 60 sq ft | 16” | 18” | 6” |
| 80 sq ft | 19” | 21” | 6” |
| 100 sq ft | 21” | 23” | 7” |
| 120 sq ft | 23” | 26” | 8” |
| 140 sq ft | 25” | 28” | 9” |
| 160 sq ft | 26” | 30” | 10” |
For the 20 × 12 example, the 60 sq ft middle post needs at least an 18” round footing, 6” thick. The 30 sq ft corner posts fall between the 20 and 40 sq ft rows. The 2021 table notes say interpolation is permitted but extrapolation is not (2021 IRC Table R507.3.1 via Island County, WA). Many builders just use the next row up for simplicity.
Soil and snow change the answer
The table above covers only one load and one soil type. The full 2021 table has columns for 1,500, 2,000 and 3,000+ psf soils, and separate rows for higher live or snow loads. At 60 sq ft under a 60-70 psf load, for example, the 2021 table calls for a 24” round footing 8” thick on 1,500 psf soil, but a 17” round footing 6” thick on soil rated 3,000 psf or more (Island County, WA 2021 IRC deck handout).
The same 60 sq ft post can need anything from a 17” to a 24” footing depending on soil and load. That’s why no single footing size is right for every deck. Your building department will tell you which soil value and design load to use. If the soil is fill, clay that swells, or anything questionable, ask for an engineer.
Step 3: set the depth
The minimum depth
The 2021 IRC says deck footings must be placed at least 12” below the undisturbed ground surface (R507.3.2) (2021 IRC text via Island County, WA). “Undisturbed” matters. Anne Arundel’s guide also states that footings must bear on undisturbed soil (Anne Arundel County deck guide). Near a house, the soil against the foundation is often backfill that was loosened during construction. You may need to dig deeper there to reach firm ground.
Frost depth
Where a deck is attached to a frost-protected structure (in practice, most houses), the 2021 IRC requires the footings to be protected from frost by one of three methods: extending below the frost line listed in Table R301.2, bearing on solid rock, or another approved method (R507.3.3) (2021 IRC text via Island County, WA).
The code doesn’t print a national frost depth. Each jurisdiction fills in its own value in Table R301.2 when it adopts the code. Anne Arundel County, Maryland, for example, requires footings at least 30” deep for attached decks (Anne Arundel County deck guide). Colder regions require far more, and some warm regions have little or no frost requirement. Call your building department or check its deck handout before you dig.
Freestanding decks
Freestanding decks get some relief under the code, but local rules often add their own requirements. The 2021 IRC lists exceptions where footings aren’t required at all for freestanding decks. One covers decks with joists supported directly on grade over their entire length. The other covers decks that meet all of these conditions: the joists bear directly on precast concrete pier blocks at grade without beams or posts, the deck is no larger than 200 sq ft, and the walking surface is no more than 20” above grade at any point within 36” of the edge (2021 IRC R507.3 via Island County, WA).
Many jurisdictions still require frost-depth footings for any deck on posts. The ledger vs freestanding guide covers the trade-offs in more detail.
Step 4: estimate the concrete
Once you know the footing shape, concrete is simple geometry:
- Round: volume (cu ft) = 3.1416 × (diameter in ft ÷ 2)² × height in ft
- Square: volume (cu ft) = side in ft × side in ft × height in ft
Quikrete’s data sheet lists an 80 lb bag of its standard concrete mix as yielding about 0.60 cu ft, and a 60 lb bag about 0.45 cu ft (Quikrete Concrete Mix data sheet). Check the bag you buy, since mixes differ.
Worked example: the 18” footing
Using the middle post from the 20 × 12 deck, with a 30” depth requirement:
Option A: 18” pad, 6” thick, with a narrower pier to grade. The pad is 3.1416 × 0.75² × 0.5 = 0.88 cu ft. A 12” diameter pier rising the remaining 24” to grade adds 3.1416 × 0.5² × 2 = 1.57 cu ft. The total is 2.45 cu ft, or about 4.1 bags at 80 lb each, so buy 5.
Option B: 18” diameter poured the full 30”. 3.1416 × 0.75² × 2.5 = 4.42 cu ft, or 7.4 bags, so buy 8.
The difference adds up quickly across a deck. Which shapes are acceptable depends on the footing details your code and plans allow. The IRC shows several footing and post-connection configurations, so confirm yours before you buy concrete. Add a bag or two for waste, holes that come out oversized, and any bell at the bottom. The deck footing calculator does this math per post.
Connecting the post
The footing has to hold the post sideways as well as up. Anne Arundel’s guide states that where posts bear on concrete footings, lateral restraint must come from manufactured connectors or at least 12” of post embedment in concrete piers (Anne Arundel County deck guide). The 2021 code text adds that where soils are expansive, compressible or shifting, the surrounding soil can’t be relied on for lateral support (Island County, WA handout).
Most builders use a galvanized post base set in the wet concrete, which keeps the post end off the concrete and makes alignment easier. Post size and height have their own table (IRC R507.4). Wood in ground contact must be labeled for that use. The pressure-treated lumber guide explains the ratings.
Practical digging tips
- Call 811 before you dig so buried utilities get marked. Anne Arundel’s guide lists the local one-call number and notice period (Anne Arundel County deck guide).
- Lay out with batter boards and string, not by eye. Footings that are off by a few inches throw off beam lengths.
- Keep the bottom flat and clean. Loose dirt at the bottom of a hole is disturbed soil.
- Schedule the inspection before the pour, with the holes open.
- Rent a power auger for more than a few holes, and expect rocks and roots.
Footings are usually a modest share of total deck cost, but they take a lot of labor. The deck cost breakdown shows how they fit into the full budget.
Frequently asked questions
How deep do deck footings need to be?
At least the code minimum below undisturbed soil (12” in the 2021 IRC) and, for most attached decks, below your local frost line. The frost depth is set by your jurisdiction, not by the national code. One Maryland county requires 30”, and colder areas require more. Ask your building department for the local number.
What size footing does a deck post need?
It depends on the post’s tributary area, the design load and the soil bearing capacity. In one common version of the IRC table, a post carrying 60 sq ft on 2,000 psf soil needs an 18” round footing, 6” thick. Weaker soil or heavier snow loads increase that. Use the table your building department enforces.
How many bags of concrete per deck footing?
Calculate the footing volume in cubic feet and divide by the bag yield. Quikrete lists about 0.60 cu ft for an 80 lb bag. An 18” × 6” pad with a 12” pier to 30” depth comes to about 2.45 cu ft, or 5 bags. A full 18” column 30” deep takes about 8.
Can I use deck blocks instead of poured footings?
Only in limited cases. The 2021 IRC exempts some small, low freestanding decks with joists bearing directly on precast pier blocks. Many local codes are stricter. Any deck attached to the house, raised on posts, or over the size and height limits needs proper footings.