Deck Footing Size Calculator (IRC Table R507.3.1)

How big should the concrete footings under your deck posts be? This guide shows how to work out the tributary area each post carries, gives the IRC Table R507.3.1 footing sizes, covers footing depth and frost, and includes a free deck footing calculator.

Step 1: find the tributary area of each post

Each post holds up a rectangle of deck around it. For a deck attached to the house with one beam:

• Depth of the rectangle = half the joist span (the house ledger carries the other half) + any joist overhang past the beam.
• Width of the rectangle = the post spacing for an interior post, or half the spacing for an end post.
• Tributary area = depth × width.

Step 2: pick the footing size (IRC Table R507.3.1)

Softer soil needs bigger footings, firmer soil smaller ones. If you do not know your soil, the IRC lets you assume 1,500 psf for clays and silts and 2,000 psf for sands (Table R401.4.1); see footing size for a house for the full soil table.

📚 Recommended Download

Building Estimate & BOQ Excel Kit

Type your building dimensions — get quantities, materials, a priced BOQ and cost per sq ft. 6 linked sheets, IS 1200-style measurement, editable rates.

✔ Editable Excel  ·  ✔ Instant download  ·  ✔ Lifetime access

₹799₹299

Get the BOQ Kit →

Worked example

1. Deck 16 ft wide × 12 ft deep, attached to the house, one beam at 12 ft, posts at 0, 8 and 16 ft
2. Interior post: (12 ÷ 2) × 8 = 48 ft² → load 2,400 lb
3. On 2,000 psf soil, between the 40 and 60 ft² rows: 17 in round or 15 in square, 6 in thick
4. End posts: (12 ÷ 2) × 4 = 24 ft² → 12 in round or 11 in square
5. On 1,500 psf clay the interior footing grows to about 20 in round (estimate; check the table column for your soil)

How deep should deck footings be?

• Below the frost line for decks attached to a house, and at least 12 in below undisturbed ground. See frost line depth by state.
• Example: Anne Arundel County, MD requires attached-deck footings at least 30 in deep.
• On undisturbed soil, never on backfill next to the house foundation, which can settle for years.
• Some codes allow exceptions for certain free-standing decks; ask your building department.

Footing, pier and post tips

• Use a post base or anchor that holds the post off the concrete and resists uplift; follow the connector maker’s instructions.
• A tube form (Sonotube) on top of a wider footing pad is a common way to bring the pier above grade.
• Work out the concrete with our concrete calculator (round footing mode).
• Many building departments use the American Wood Council’s DCA 6 Prescriptive Residential Wood Deck Construction Guide; ask for your local deck handout.

FAQ

How big should deck footings be?

It depends on the area each post carries and the soil. On 2,000 psf soil, 40 ft² needs a 15 in round footing, 60 ft² needs 18 in, and 100 ft² needs 23 in (IRC Table R507.3.1, 50 psf).

How deep do deck footings need to be?

Below the local frost line and at least 12 in below undisturbed ground for attached decks. Anne Arundel County, MD, for example, requires 30 in.

How thick should a deck footing be?

At least 6 in for small footings; the IRC table calls for 7–10 in as footings get larger (100–160 ft² on 2,000 psf soil).

What is tributary area for a deck post?

The deck area the post supports: half the joist span (plus any overhang) times the post spacing, or half the spacing for an end post.

Can I use a 12 inch Sonotube as the footing?

Only if it meets the footing size required for the area it carries. Many posts need a wider pad at the bottom, with the tube as a pier above it.

References: International Residential Code (IRC) Table R507.3.1 and Table R401.4.1; Anne Arundel County (MD) 2021 IRC Deck Construction Guide; American Wood Council DCA 6. Local amendments can change these sizes; follow your building department’s requirements.