Deck Footing Calculator

How big should a deck footing be?

ft
ft
ft
footings
Soil bearing
Load
in

Minimum footing diameter

16 inround footing

From IRC Table R507.3.1, rounded up to the next table row. Edition and local amendments vary, and footings must reach below your frost line. Confirm with your building department.

36 in16 in × 6 inEach post: 30 sq ft3 footings
Tributary area30 sq ft (table row 40)
Footing thickness6 in
Concrete for all footings2.3 cu ft
80 lb bags4

Footing size depends on how much deck each post carries and how strong your soil is. This looks up the IRC minimum for your numbers.

How it works

Tributary area is post spacing × half the joist span plus overhang. The next table row up in IRC Table R507.3.1 gives diameter and thickness for your soil and load.

Before you order

  • Use 1,500 psf unless you know your soil better, and ask your inspector what is assumed locally.
  • Footings must reach below your frost line regardless of the table size.
  • A heavier post load can often be reduced by adding a post, which shrinks the footing.

Common questions

What is tributary area?

The part of the deck a single post holds up, usually the post spacing × half the joist span plus any overhang.

How deep do deck footings need to be?

At least 12 inches under the common code and below frost in cold climates. Ask your building department.