The deck footing calculator answers the question the code's footing table starts from: how much deck does this post hold up? It works out that area, which the code calls the tributary area, from the joist length and the post spacing, then reads the footing size from Table R507.3.1 of the International Residential Code and the tallest post from Table R507.4, both at the 40 psf live load the model code sets for decks.
The area comes from the American Wood Council's prescriptive deck guide, DCA 6, whose Appendix B gives one formula for a post between two beam spans and another for the post at the end of a beam. The tables stop at 160 square feet, and the code does not allow reading past them; when a post carries more, the calculator says how far apart the posts can be instead.
How to use it
- 01
Pick the post
A post between two beam spans carries half of each span; the post at the end of a beam carries half of one span plus any beam that runs past it. Size the busiest post: on most decks that is an interior one.
- 02
Measure the joists
Joist length runs from the ledger face to the center of the beam. If the joists run past the beam, enter that overhang separately; DCA 6 limits it to a quarter of the joist span.
- 03
Enter the post spacing
The beam span, center of post to center of post. Wider spacing gives each post more deck to carry and a bigger footing.
- 04
Choose the soil, species and post size
Clay or silt is the code's 1,500 psf column, the lowest it has; use a firmer column only if the city accepts that value for your lot. Species and size set the post height limit.
How the math works
Tributary area
Center post: (joist length ÷ 2 + joist overhang) × post spacing. End post: (joist length ÷ 2 + joist overhang) × (post spacing ÷ 2 + beam overhang). A deck with 12 ft joists and posts 8 ft apart puts 48 sq ft on each interior post: (12 ÷ 2) × 8.
Reading the tables
The calculator reads the next row up: 48 sq ft is read at the 60 sq ft row, which on clay gives a footing 17 in square or 19 in round, 6 in thick. The code also allows interpolating between rows, which can give a slightly smaller footing; it never allows reading past the 160 sq ft row.
Post height
Table R507.4 gives the greatest height of a wood post under a single-level deck, from the underside of the beam to the top of the footing or pier, by species, post size and the same area. NP, not permitted, means the code allows no post of that size there.
Depth
Every deck footing goes at least 12 inches below undisturbed ground. Where the deck is attached to a house whose foundation is protected from frost, its footings go below the frost line too, or on solid rock or by another approved method.
Worked examples
| Layout | Deck area on the post | Footing on clay (1,500 psf) | Footing on sand (2,000 psf) | Tallest 4×4 |
|---|---|---|---|---|
| 10 ft joists, posts 6 ft apart | 30 sq ft | 14 in sq · 16 in round · 6 in | 12 in sq · 14 in round · 6 in | 13 feet 8 inches |
| 12 ft joists, posts 8 ft apart | 48 sq ft | 17 in sq · 19 in round · 6 in | 15 in sq · 17 in round · 6 in | 11 feet |
| 12 ft joists, posts 8 ft apart, end post | 24 sq ft | 14 in sq · 16 in round · 6 in | 12 in sq · 14 in round · 6 in | 13 feet 8 inches |
| 14 ft joists, 1.5 ft overhang, posts 8 ft apart | 68 sq ft | 20 in sq · 22 in round · 7 in | 17 in sq · 19 in round · 6 in | 9 feet 5 inches |
| 16 ft joists, posts 10 ft apart | 80 sq ft | 20 in sq · 22 in round · 7 in | 17 in sq · 19 in round · 6 in | 9 feet 5 inches |
Square side, round diameter and concrete thickness from IRC Table R507.3.1 at 40 psf, read at the next row up; post heights from Table R507.4. Interior posts unless the row says end post.
Where the number goes wrong
- Sizing the end post and using that footing everywhere. An interior post carries about twice the deck an end post does.
- Reading a firmer soil column without the city's say. The code assumes the clay value unless a soil evaluation or the building official sets another.
- Measuring the joist length to the outside of the rim when the joists overhang the beam. DCA 6 measures to the beam's center and counts the overhang separately.
- Using the 40 psf rows in California. California's residential code sets a deck's live load at 60 psf, and Los Angeles follows it.
- Treating the result as a design. Beams, joists, the ledger and lateral connections come from the rest of the code or a deck guide, and the city may ask for drawings.
Questions people ask
How big should a deck footing be?
It depends on the deck area the post carries and the soil. On clay, the code's 1,500 psf column, a post carrying 48 sq ft needs a footing 17 inches square or 19 inches round, 6 inches thick, read at the 60 sq ft row of IRC Table R507.3.1.
How do you calculate the tributary area of a deck post?
For a post between two beam spans, multiply half the joist length plus any joist overhang by the post spacing. For the post at the end of a beam, multiply the same joist share by half the post spacing plus any beam overhang. Those are equations B-1 and B-2 in the American Wood Council's DCA 6.
How deep do deck footings need to be?
At least 12 inches below undisturbed ground, and below the frost line where the deck is attached to a house whose own foundation is protected from frost. The frost line is in your city's design table, R301.2.
How far apart can deck posts be?
No farther than the beam's allowed span in the code's beam table, and no farther than keeps each post within the footing table's 160 square feet. With 12 ft joists and no overhang, the footing table alone allows interior posts up to 26 ft apart, so the beam span usually decides.