How wide and thick does a footing need to be?
Footings must be at least 12 inches wide and about 6 inches thick, with the exact width taken from Table R403.1 (IRC R403.1).
The IRC sets minimum footing width in Table R403.1 based on the number of stories the footing carries and the soil's load-bearing value. For a typical light-frame home on average soil, 12 inches (305 mm) is the practical minimum width, and wider footings are required for heavier loads or weaker soil.
Footing thickness is conventionally a minimum of 6 inches (152 mm). Three-story construction, poor soils, or concentrated loads push the required width up, so check the table against your story count and the presumptive soil bearing value.
These are minimums only. Site-specific soil conditions or an engineered design can require larger footings.
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