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BuyIRC Deck Footing Size Tables
Bookmarkable IRC 2021 Table R507.3.1 grids — square, round, and thickness by load, tributary area, and soil
These grids are the same cells the deck footing size calculator returns. Values are IRC 2021 Table R507.3.1. Each cell is side of a square footing (in), diameter of a round footing (in), and thickness (in) for plain concrete. Columns are soil bearing. Rows are tributary area in square feet. They are not local law by themselves.
How to use a cell: guide. Pad vs post: comparison. Existing settlement: troubleshooting. Beam above the post: deck beam span calculator.
Look up one combination.Load, tributary area, soil column.
Open Calculator →40 psf live load (standard residential)
IRC 2021 Table R507.3.1 — Minimum footing size for decks. 10 psf dead load (highest of dead+live or dead+snow). Cells are square side / round diameter / thickness (inches). Starting point only.
| Tributary area (sq ft) | 1,500 psf sq / rd / th | 2,000 psf sq / rd / th | ≥ 3,000 psf sq / rd / th |
|---|---|---|---|
| 5 | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ |
| 20 | 10″ / 12″ / 6″ | 9″ / 9″ / 6″ | 7″ / 8″ / 6″ |
| 40 | 14″ / 16″ / 6″ | 12″ / 14″ / 6″ | 10″ / 12″ / 6″ |
| 60 | 17″ / 19″ / 6″ | 15″ / 17″ / 6″ | 12″ / 14″ / 6″ |
| 80 | 20″ / 22″ / 7″ | 17″ / 19″ / 6″ | 14″ / 16″ / 6″ |
| 100 | 22″ / 25″ / 8″ | 19″ / 21″ / 6″ | 15″ / 17″ / 6″ |
| 120 | 24″ / 27″ / 9″ | 21″ / 23″ / 7″ | 17″ / 19″ / 6″ |
| 140 | 26″ / 29″ / 10″ | 22″ / 25″ / 8″ | 18″ / 21″ / 6″ |
| 160 | 28″ / 31″ / 11″ | 24″ / 27″ / 9″ | 20″ / 22″ / 7″ |
50 psf live or ground snow
IRC 2021 Table R507.3.1 — Minimum footing size for decks. 10 psf dead load (highest of dead+live or dead+snow). Cells are square side / round diameter / thickness (inches). Starting point only.
| Tributary area (sq ft) | 1,500 psf sq / rd / th | 2,000 psf sq / rd / th | ≥ 3,000 psf sq / rd / th |
|---|---|---|---|
| 5 | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ |
| 20 | 11″ / 13″ / 6″ | 10″ / 11″ / 6″ | 8″ / 9″ / 6″ |
| 40 | 15″ / 17″ / 6″ | 13″ / 15″ / 6″ | 11″ / 13″ / 6″ |
| 60 | 19″ / 21″ / 6″ | 16″ / 18″ / 6″ | 13″ / 15″ / 6″ |
| 80 | 21″ / 24″ / 8″ | 19″ / 21″ / 6″ | 15″ / 17″ / 6″ |
| 100 | 24″ / 27″ / 9″ | 21″ / 23″ / 7″ | 17″ / 19″ / 6″ |
| 120 | 26″ / 30″ / 10″ | 23″ / 26″ / 8″ | 19″ / 21″ / 6″ |
| 140 | 28″ / 32″ / 11″ | 25″ / 28″ / 9″ | 20″ / 23″ / 7″ |
| 160 | 30″ / 34″ / 12″ | 26″ / 30″ / 10″ | 21″ / 24″ / 8″ |
60 psf live or ground snow
IRC 2021 Table R507.3.1 — Minimum footing size for decks. 10 psf dead load (highest of dead+live or dead+snow). Cells are square side / round diameter / thickness (inches). Starting point only.
| Tributary area (sq ft) | 1,500 psf sq / rd / th | 2,000 psf sq / rd / th | ≥ 3,000 psf sq / rd / th |
|---|---|---|---|
| 5 | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ |
| 20 | 12″ / 14″ / 6″ | 11″ / 12″ / 6″ | 9″ / 10″ / 6″ |
| 40 | 16″ / 19″ / 6″ | 14″ / 16″ / 8″ | 12″ / 14″ / 6″ |
| 60 | 20″ / 23″ / 7″ | 17″ / 20″ / 6″ | 14″ / 16″ / 6″ |
| 80 | 23″ / 26″ / 9″ | 20″ / 23″ / 7″ | 16″ / 19″ / 6″ |
| 100 | 26″ / 29″ / 10″ | 22″ / 25″ / 8″ | 18″ / 21″ / 6″ |
| 120 | 28″ / 32″ / 11″ | 25″ / 28″ / 9″ | 20″ / 23″ / 7″ |
| 140 | 31″ / 35″ / 12″ | 27″ / 30″ / 10″ | 22″ / 24″ / 8″ |
| 160 | 33″ / 37″ / 13″ | 28″ / 32″ / 11″ | 23″ / 26″ / 9″ |
70 psf ground snow
IRC 2021 Table R507.3.1 — Minimum footing size for decks. 10 psf dead load (highest of dead+live or dead+snow). Cells are square side / round diameter / thickness (inches). Starting point only.
| Tributary area (sq ft) | 1,500 psf sq / rd / th | 2,000 psf sq / rd / th | ≥ 3,000 psf sq / rd / th |
|---|---|---|---|
| 5 | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ | 7″ / 8″ / 6″ |
| 20 | 12″ / 14″ / 6″ | 11″ / 13″ / 6″ | 9″ / 10″ / 6″ |
| 40 | 18″ / 20″ / 6″ | 15″ / 17″ / 6″ | 12″ / 14″ / 6″ |
| 60 | 21″ / 24″ / 8″ | 19″ / 21″ / 6″ | 15″ / 17″ / 6″ |
| 80 | 25″ / 28″ / 9″ | 21″ / 24″ / 8″ | 18″ / 20″ / 6″ |
| 100 | 28″ / 31″ / 11″ | 24″ / 27″ / 9″ | 20″ / 22″ / 7″ |
| 120 | 30″ / 34″ / 12″ | 26″ / 30″ / 10″ | 21″ / 24″ / 8″ |
| 140 | 33″ / 37″ / 13″ | 28″ / 32″ / 11″ | 23″ / 26″ / 9″ |
| 160 | 35″ / 40″ / 15″ | 30″ / 34″ / 12″ | 25″ / 28″ / 9″ |
Source and limits
Primary source: 2021 International Residential Code, Table R507.3.1 — Minimum Footing Size for Decks, as reproduced in the Town of Topsham, Maine Deck Building Permit (ICC-licensed extract, Order #101689801). The 40 psf block matches Southington, CT Residential Wood Deck Construction Guide (rev. 02/11/2025). The 70 psf block matches Island County, WA 2021 IRC Deck Handout “60 Live or 70 Ground Snow Load” combined row.
Footnote a: interpolation permitted, extrapolation not permitted. This chart prints tabulated cells only. Thickness (footnote f) applies to plain concrete footings. Footing must fully bear the post; brick/CMU piers need a 2-inch projection on all sides. Soil columns are presumptive — verify actual bearing locally. Frost depth is not in this table.
Starting point only. Not a substitute for the adopted local code, a soils report, frost-depth requirements, or a licensed engineer outside ordinary residential decks.
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Layout & Dig Gear
Confirm the post grid and the hole. The table does not replace a square on layout or a tape on frost depth.
16×24 Framing Square
Mark post layout after the beam table says the span reaches. Tributary length starts there.
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Swanson Speed Square
Quick 90s and post-plumb checks over a fresh pad. Soil column still has to be a number you can defend.
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48" Box Beam Level
Existing settlement is a field measurement first: dish in the deck, gap at the ledger. Compare that to the tabulated pad second.
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7-1/4" Circular Saw
Posts get cut to Table R507.4 height, not to the pad diameter. Different table, same post.
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Johnson Stair Gauges
When the deck also needs stairs, lock the square. Footing size and stringer math are separate.
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Look Up the Table Cell
Load, tributary area, soil column. Starting point only — verify local soil, frost, and code.
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