Deck Footing Size Calculator
The minimum concrete footing for a deck post, straight from IRC 2021 Table R507.3.1 — by tributary area, soil bearing, and live or ground-snow load. Works for square and round footings, and estimates the concrete for the pad.
Concrete — footing pad only
Volume covers the footing pad itself, not the above-grade pier/tube that carries the post — add that separately. IRC 2021 Table R507.3.1. Verify local amendments before you pour.
What a footing actually does
A deck post collects the decking, joists, beam, and everyone standing on the area it serves, then drops all of it onto one small patch of ground. The footing spreads that concentrated load over enough soil to carry it without settling. Every size in Table R507.3.1 is the smallest pad that keeps the bearing pressure under a post at or below what the soil can hold. The post itself is a separate question, sized from IRC Table R507.4 — see the deck post spacing guide for the post side, including how its spacing follows the beam span — and this calculator covers the concrete pad only, not the above-grade pier.
Finding the tributary area for one post
Tributary area is the deck surface one post carries. It is not the deck's total square footage, and it is not post spacing times the full deck depth. For the ordinary attached deck — joists from the ledger to one beam, beam on posts — AWC DCA 6 Appendix B gives:
- Interior post: A = (½ × joist span + joist overhang) × post spacing
- End or corner post: A = (½ × joist span + joist overhang) × (½ × post spacing + beam overhang)
Measure the joist span from the ledger face to the beam center, halve it — the beam carries half the span, the ledger the other half — add any joist overhang past the beam, then multiply by the post spacing along the beam, or by half of it plus the beam overhang for an end post. Interior posts usually govern, but check the end posts rather than assuming.
Soil bearing and the table columns
The three soil columns — 1,500, 2,000, and 3,000 psf or greater — come from IRC Table R401.4.1: 1,500 psf for clay and silty clay (CL, ML, MH, CH), 2,000 psf for sand and silty or clayey gravel (SW, SP, SM, SC, GM, GC), and 3,000 psf and up for clean gravel (GW, GP), with rock about 4,000 to 12,000 psf. Weaker soil means a bigger pad: at 40 ft² per post and 40 psf live load the table gives 14″ square at 1,500 psf, 12″ square at 2,000 psf, and 10″ square at 3,000 psf. When you cannot tell, use 1,500 psf.
Frost depth: attached vs. free-standing
Depth is a separate question from size. R507.3.2 puts the bottom of the footing at least 12″ below undisturbed ground. An attached deck also needs frost protection (R507.3.3): the bottom goes below the site frost line in Table R301.2 unless it bears on rock or another approved method. Water in the soil expands as it freezes, the ground lifts, and a footing in that layer rides up with it. A free-standing deck shares no load path with the house, which is why R507.3 exempts one of 200 ft² or less, walking surface 20″ or less above grade, joists on precast pier blocks at grade.
Worked example — 16 ft deck, 8 ft post spacing
Joists span 10 ft from the ledger face to the beam center, posts sit 8 ft apart along the beam, no joist overhang, soil 2,000 psf, 40 psf live load.
- Interior tributary area: (½ × 10 + 0) × 8 = 5 × 8 = 40 ft²
- 40 psf load, 2,000 psf soil, 40 ft² row: 12″ × 12″ square, 14″ round, 6″ thick
- Pad volume: 12 × 12 × 6 ÷ 1728 = 0.50 ft³ per footing
- 80-lb bags: 0.50 ÷ 0.60 = 0.83 → 1 bag per footing; six footings is 3.0 ft³, or 5 bags
An end post with a 1-ft beam overhang: (5 + 0) × (4 + 1) = 25 ft², between the 20 and 40 ft² rows. Footnote a allows interpolation: the square side is 9 + 3 × 0.25 = 9.75″, rounded up to 10″, the round diameter 10.25″, rounded up to 11″, thickness stays 6″. Extrapolation is not allowed — past 160 ft² the answer is another post or an engineer.
Common mistakes
- Using the deck's total area instead of tributary area. 300 ft² of deck is not 300 ft² per post.
- Assuming one soil across the site. Rock under one corner and clay under the other changes the footing size; go to 1,500 psf when unsure.
- Skipping frost depth. Size and depth are separate checks — a correctly sized 12″ pad above the frost line heaves.
- Forgetting the 2″ projection on a brick or CMU pier. Footnote d requires it on every side, so a 12″ pier needs a 16″ footing.
- Extrapolating past the table. Rows stop at 160 ft²; a bigger area needs another post.
- Reading the thickness as reinforced. Footnote f: the tabulated thickness is for plain concrete only.
Deck footing FAQ
How do I figure the tributary area for a deck post?
Interior post: (half the joist span + joist overhang) × post spacing. End post: that width × (half the post spacing + beam overhang). The beam carries half the joist span, not the whole deck depth. A 10-ft joist span with 8-ft post spacing and no overhang is 5 × 8 = 40 ft².
How deep does a deck footing need to be?
At least 12″ below undisturbed ground (R507.3.2), and on an attached deck below the site frost line in Table R301.2 (R507.3.3). Table R507.3.1 sizes the pad; it does not set the depth.
Which soil bearing capacity should I use?
1,500 psf for clay and silty clay, 2,000 psf for sand and silty or clayey gravel, 3,000 psf or greater for clean gravel. Use 1,500 psf when you cannot tell.
Can I size a footing past 160 ft² of tributary area?
No. The table stops at 160 ft², and footnote a permits interpolation but not extrapolation. Split the load with another post, or have an engineer size the footing.