How deep does a footing need to be?
A footing's bottom must be at least 12 inches below the undisturbed ground surface, and always below the local frost line (2018 IRC §R403.1.4, §R403.1.4.1). Frost depth is set by your jurisdiction — a few inches in the South to 48 inches or more in cold climates — so the required depth varies widely by location.
Source: 2018 IRC §R403.1.4 · Verify against your AHJ and the adopted code edition.
Two rules set how deep a footing goes, and you take the deeper of the two:
- At least 12 inches below the undisturbed ground surface (IRC §R403.1.4) — below topsoil and disturbed fill, onto soil that can actually bear.
- Below the frost line (§R403.1.4.1). When wet soil freezes it expands and heaves; a footing below the frost depth stays put through the winter.
The catch is that the frost line is local. It ranges from a few inches in the Deep South to 48 inches or more in northern states, and your building department publishes the figure for your area — so “how deep” has no single national answer.
Related requirements often travel with depth: minimum footing thickness and width for the load and soil, and provisions for frost-protected shallow foundations (§R403.3) as an alternative. This is preliminary guidance — verify the frost depth and footing design with your AHJ and, where required, an engineer.
Work it out: the concrete takeoff gives the cubic yards and bag count once you’ve set the footing’s width, length, and depth.
Sources
- ICC — International Codes (IRC) — 2018 IRC §R403.1.4, §R403.1.4.1