Deck Footing Size Calculator

Minimum tabulated footing size from IRC 2021 Table R507.3.1 — a starting point, not a soils report

Starting point only — not a substitute for local soil, frost depth, code, or an engineer. These sizes come from the standard IRC 2021 prescriptive table for deck footings: Table R507.3.1, keyed to live/snow load, tributary area, and assumed soil bearing (1,500 / 2,000 / ≥3,000 psf). Typical soil bearing capacities vary enormously by site. Many jurisdictions require a soils report or inspector sign-off. Actual capacity should be verified locally against Table R401.4.1 and the adopted code. Frost depth is a different requirement (R507.3.3 / R403.1.4). This is not a substitute for local code or a licensed engineer, especially for anything outside standard residential deck scenarios (hot tubs, roofs, poor or unknown soils, areas past the table).

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Layout & Dig Gear

The table sizes the pad. The square, tape, and level still have to match the post grid you actually framed, and the hole still has to hit frost depth and bearing soil.

16×24 Framing Square

Layout post bays after the beam table says the span reaches. Tributary length starts from those bays. Same square for the deck stairs.

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Swanson Speed Square

Fast 90s when you are checking post plumb over a fresh pour. Not a substitute for reading the soil column.

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48" Box Beam Level

An existing deck that has settled shows up here: a dish in the surface, a gap at the ledger. Sight it, then compare to the tabulated pad.

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7-1/4" Circular Saw

Posts get cut to the height Table R507.4 allows — a different table than this pad size. Do not confuse the two.

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Johnson Stair Gauges

When the same deck gets stairs, lock rise and run on the square. Footing size and stringer layout are different jobs.

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How Tributary Area Works

Measure the area a post actually carries, then pick a soil column you can defend.

Read the Footing Size Guide →

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How to Use This Deck Footing Size Calculator

Finish the framing math first: joists, then the beam, then the post grid. Tributary area is the deck surface that one post and its footing actually carry — half the bay to each adjacent support, including any cantilever. Enter that area, the 40 psf live-load row unless you are in snow country, and a soil column you can defend. The result is the minimum tabulated square side, round diameter, and thickness from IRC 2021 Table R507.3.1. Read the yellow flag: it is a starting point. Confirm soil, frost depth, and the adopted code before you order tubes.

Tributary area = tributary length × tributary width · Footing size = IRC 2021 Table R507.3.1 · Post height = IRC 2021 Table R507.4 (different table)

How to measure tributary area and how to find local soil bearing: footing size guide. Settling, a deck pulling away from the house, frost: troubleshooting. Why a big pad does not size the post: footing size vs post height. Full grid: IRC deck footing size tables.

The beam that dumps load onto this post: Deck Beam Span Calculator. Joists that dump load onto that beam: Joist Span Calculator. Same deck, stairs: Deck Stair Calculator. Interior stringers: Stair Stringer Calculator. Commons: Rafter Length & Roof Pitch Calculator. Buying the bunk: Board Foot Calculator.

Deck Footing Size Calculator