This tool does two jobs. For a continuous wall footing it works out concrete volume and checks the IRC minimum size. For a deck post footing it estimates the size of a round or square pad from the deck area the post carries and the soil type, using the method in the American Wood Council’s DCA 6 deck guide.
Planning estimate, not an engineering approval. Footing size and depth are set by your building department and, where needed, a design professional. Soil bearing must be confirmed on site, footings must go below the frost line, and decks with hot tubs, heavy snow or unusual loads need an engineer.
Footings calculator
How it works
Deck post footings. DCA 6 Appendix B works out the deck area each post carries (the tributary area):
- Center post: A = (joist length ÷ 2 + joist overhang) × beam span
- Corner post: A = (joist length ÷ 2 + joist overhang) × (beam span ÷ 2 + beam overhang)
- For a center post under a continuous (unspliced) beam, DCA 6 Table B3 multiplies the area by 1.25.
The load is that area × 50 psf (40 psf live load for decks plus 10 psf dead load). The footing must spread the load, plus the weight of its own concrete at 150 lb per cu ft, over enough soil that the soil’s allowable bearing value is not exceeded:
- Bearing area = load ÷ (soil bearing − 150 × thickness in feet)
- Thickness is at least 6 in, and at least as much as the footing projects beyond the post (IRC R403.1.1 for plain concrete footings)
Soil values are the IRC’s presumptive values (Table R401.4.1): 1,500 psf for clays and silts, 2,000 psf for sands, 3,000 psf for gravels. Sizes are rounded up to the next inch. We checked the method against DCA 6 Table B3: it matches the table or comes out up to 1 in larger.
Wall footings. Volume = length × width × thickness. The IRC minimum is 12 in wide and 6 in thick; required width depends on soil and the number of stories (Table R403.1(1)).
Worked example
A center post on a deck with 12 ft joists and posts 8 ft apart, on clay soil (1,500 psf), with a 6×6 post:
- What are you working out?: Deck post footing size (planning estimate)
- Joist length (ledger to beam): 12 ft
- Joist overhang past the beam: 0 ft
- Beam span between posts: 8 ft
- Beam overhang past the post: 0 ft
- Post position: Center post
- Soil bearing (IRC Table R401.4.1): 1,500 psf: clay, sandy clay, silty clay, silt (CL, ML, MH, CH)
- Post size: 6×6 (5.5 in)
- Number of footings like this: 3
- Extra for waste: 10%
The calculator gives:
- Round footing (estimate): 18 in diameter × 7 in thick
- Or square footing: 16 × 16 in × 7 in thick
- Load on this footing: 2,400 lb from 48.0 sq ft at 50 psf
- Bearing area needed: 1.68 sq ft
- Concrete, 3 round footings: 3.09 cu ft = 6 × 80 lb bags with 10% extra
The post carries 6 × 8 = 48 sq ft, or 2,400 lb. After allowing for the footing’s own weight, that needs about 1.7 sq ft of bearing, so an 18 in round or 16 in square footing, 7 in thick. DCA 6 Table B3 gives 18 in round for 50 sq ft on this soil, so the estimate is in the right range.
Material and waste notes
- Frost depth, minimum depth below grade (DCA 6 says at least 12 in below undisturbed ground) and any local amendments come from your building department.
- DCA 6 treats soils under 1,500 psf as needing a soils investigation. Fill, organic soils and wet ground are not covered.
- Deck footings closer than 5 ft to a house foundation must bear at the same elevation as the house footing (DCA 6).
- Snow loads above 40 psf, hot tubs, and roofs or second stories on the deck are outside this estimate.
- The extra percentage on concrete is your allowance, not a standard figure.
Frequently asked questions
How big should deck footings be?
It depends on how much deck each post carries and the soil. For a post carrying 40 sq ft on clay soil (1,500 psf), DCA 6 lists a 16 in round or 15 in square footing, 6 in thick. Larger areas and weaker soil need bigger footings.
How deep do footings need to be?
Below your local frost line and at least 12 in below undisturbed ground, whichever is deeper. Your building department sets the frost depth.
What is the minimum footing size in the IRC?
The IRC requires footings to be at least 12 in wide and 6 in thick (R403.1.1). Width for house walls then depends on soil bearing and the number of stories.
Is this an engineered footing size?
No. It is a planning estimate using a published prescriptive method. Your building department or an engineer decides what is acceptable for your deck.
Sources
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide (Appendix B tributary areas, Table B3 footing sizes; stair stringer rules)
- International Code Council, 2021 International Residential Code, Chapter 4: R401.3 drainage, Table R401.4.1 presumptive load-bearing values of soils, and Section R403.1 footings
- International Code Council, 2021 International Residential Code, Section R507 Exterior Decks (Tables R507.5 and R507.6)
- QUIKRETE Concrete Mix (No. 1101) data sheet: bag yields of about 0.60 cu ft (80 lb), 0.45 cu ft (60 lb) and 0.30 cu ft (40 lb)
Last reviewed: 25 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Results are planning estimates, not an engineering approval: confirm with your local building department or an engineer. Found an error? Please let us know.