How the footing size is worked out
The IRC’s deck footing table (R507.3.1 in the 2021 and 2024 editions) lists footing sizes by the deck area a post carries, called its tributary area, and the soil’s bearing pressure. The numbers come from simple arithmetic: the area times the design load gives the weight on the footing, and that weight divided by what the soil can bear gives the footing’s area.
The beam carries the joists’ outer half plus their overhang: that strip of deck is shared among its posts. A middle post takes one post spacing of the strip, an end post half a spacing plus the beam’s overhang. The thickness rule, at least the distance the footing reaches past the post on each side, reproduces the table’s thicknesses.
Worked example: a 16 ft deck on three posts
- Joists span 10 ft from the ledger to the beam and overhang it 2 ft. The beam is 16 ft, with posts at most 8 ft apart and 1 ft in from each end: three posts, 7 ft apart.
- Strip: 10 ÷ 2 + 2 = 7 ft. Middle post: 7 × 7 = 49 sq ft. End posts: (3.5 + 1) × 7 = 31.5 sq ft.
- Middle footing: 49 × (40 + 10) = 2,450 lb. On 1,500 psf clay: 1.63 sq ft, which is 15.3 in square or 17.3 in round. Rounded up: 16 in square or 18 in round, 6 in thick.
- End footings: 31.5 × 50 = 1,575 lb, 1.05 sq ft: 13 in square or 14 in round.
- Concrete for round pads with 10 in tube piers, 36 in deep and 2 in above the ground: 2.34 ft³ for the middle footing and 1.99 ft³ for each end, 6.32 ft³ in all; 6.95 ft³ with 10% extra, or 12 bags of 80 lb mix.
Penn State’s housing research center works one with 14 ft joists, a 2 ft overhang and posts 8 ft apart: (14 ÷ 2 + 2) × 8 = 72 sq ft per post, and on 2,000 psf soil a footing 16¼ in square. Worked exactly it’s 16.1 in, so in whole inches the calculator says 17.
How this compares with the IRC table
The table rounds its sizes to the nearest inch; the calculator rounds up. We checked the method against fifteen cells of the table’s 40 psf rows, from 20 to 160 sq ft on all three soils: every size the calculator gives is the table’s or one inch bigger. The table allows interpolation between its rows but not extrapolation, and it stops at 160 sq ft per footing. Past that, the calculator still shows a size and warns you to have it designed or checked.
The table’s loads are 40 psf live load plus 10 psf for the deck’s own weight, with more rows for ground snow loads of 50, 60 and 70 psf. It uses whichever is bigger, live load or snow, so choose the snow row only where your ground snow load is over 40 psf.
Soil bearing
Without a soil test, the IRC lets you presume what common soils bear (Table R401.4.1(1) in the 2024 edition):
| Soil | Presumed bearing |
|---|---|
| Clay, sandy clay, silty clay, clayey silt, silt and sandy silt | 1,500 psf |
| Sand, silty sand, clayey sand, silty gravel and clayey gravel | 2,000 psf |
| Sandy gravel and gravel | 3,000 psf |
Where the building official thinks the soil may bear less than 1,500 psf, a soils investigation has to find out. If you don’t know your soil, 1,500 psf is the cautious choice: it gives the biggest footings.
Depth, frost and posts
The bottom of a deck footing goes at least 12 in below undisturbed ground. Where the deck is attached to a house protected from frost, the 2024 IRC (R507.3.3) asks for the deck’s footings to be protected from frost too, which usually means taking them below the local frost depth (R403.1.4). Freestanding decks may be treated differently; ask your building department, which also sets the frost depth.
Near the house
AWC’s DCA 6 requires footings closer than 5 ft to the house’s foundation wall to bear at the same depth as the house’s own footing. The ground beside a basement or crawlspace wall is often backfill rather than undisturbed soil.
The footing must allow the post to bear completely on it, so a round footing is never smaller than the post’s diagonal: 7.8 in for a 6×6. Posts are held to the footing by an approved connector or by being embedded at least 12 in in the soil or a concrete pier (R507.4.1). AWC’s DCA 6 calls for 6×6 posts; check post sizes and heights against IRC Table R507.4.
Pads, piers and tubes
The calculator offers three kinds of footing. A pad with a tube pier is a spread footing at the bottom of the hole with a round form from its top to above the ground; the post base goes on the pier. A tube poured to the bottom is a footing only as wide as the tube, so it has to be as wide as the footing needs to be; tubes on the list go up to 24 in, the sizes the Sonotube calculator uses. A pad alone suits a post base set into the footing itself.
The concrete is the pads, at the rounded-up size and the thickness above, plus the piers or tubes up to the height you enter above the ground, with your extra on top. Bag counts use the makers’ yields: an 80 lb bag makes 0.6 ft³. For holes dug wider than planned, the post hole calculator works from the hole’s size instead.
Questions people ask
How big should deck footings be?
Big enough to spread the load on each post over the soil. A post that carries 49 sq ft of deck at the IRC’s 40 psf live load plus 10 psf dead load puts 2,450 lb on its footing; clay or silt is presumed to bear 1,500 psf, so the footing needs 1.63 sq ft: a 16 in square or an 18 in round, at least 6 in thick. The same post on gravel (3,000 psf) needs half the area, an 11 in square or a 13 in round.
How deep do deck footings have to be?
At least 12 in below undisturbed ground (IRC R507.3.2). Under a deck attached to a house, the footings must also be protected from frost, usually by going below the frost line (R507.3.3 in the 2024 IRC). Your building department sets the frost depth. AWC’s DCA 6 adds that footings within 5 ft of the house’s foundation must bear as deep as the house’s footing, since the soil beside a foundation is often backfill.
How far apart can deck posts be?
As far as the beam can span. The IRC’s beam tables (R507.5) and AWC’s DCA 6 give the span for each beam size, species and joist span. Wider spacing means fewer footings, but each carries more deck and needs to be bigger. The calculator spreads the posts evenly at no more than the spacing you enter.
Can a tube form be the whole footing?
Only if its bottom is as big as the footing has to be. A 12 in tube bears on 0.79 sq ft, which on 1,500 psf soil carries about 1,180 lb, the load from about 24 sq ft of deck. Most posts carry more, so a narrow tube usually stands on a wider pad. Choose “Tube poured to the bottom” and the calculator picks the smallest tube that’s wide enough, or says none is.
Does a floating deck need footings?
The 2024 IRC lets two kinds of freestanding deck skip dug footings (R507.3): one whose joists lie on the ground along their whole length, and one of no more than 200 sq ft, with its surface no more than 20 in above the ground, whose joists bear directly on precast concrete blocks with no beams or posts. Anything else stands on footings.
Round or square footings?
Either; what matters is the bearing area. A round footing of the same area is about 1.13 times as wide as a square one, which is why the IRC table lists both: 16 in square and 18 in round are about the same. The footing must also take the whole post, so a round one is never smaller than the post’s diagonal.
Sources for the numbers on this page
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R507: exterior decks (2024 IRC text; footings, posts, beams) · checked October 4, 2026
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R401.4: presumptive soil-bearing values (2024 IRC text) · checked October 4, 2026
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R403.1.4: footing depth and frost protection (2024 IRC text) · checked October 4, 2026
- Colorado Chapter of the International Code Council, via Adams County — Single family residential uncovered decks and porches, 2024 IRC (ledger fastener spacing and loads, PDF, 03/20/26) · checked October 4, 2026
- American Wood Council — DCA 6: Prescriptive residential wood deck construction guide (2015 IRC edition) · checked October 3, 2026
- Pennsylvania Housing Research Center, Penn State — Decked out: deck requirements of the 2021 IRC, webinar handout (January 2025, PDF) · checked October 4, 2026
- Sonoco — Sonotube Builder’s Tube concrete forms — specification sheet · checked October 1, 2026
- Quikrete — Concrete Mix No. 1101 — technical data sheet (bag yields, strength gain, curing) · checked October 1, 2026
Corrections and updates are logged on the sources page.