Footing depth: the deeper of 12 inches or the frost line

Every exterior footing goes at least 12″ below undisturbed ground (IRC R403.1.4) AND below your local frost line (R403.1.4.1). Whichever is deeper governs, measured from finish grade to the bottom of the footing.

Two side-by-side cross-sections of a foundation wall on a footing, drawn to the same vertical scale. Left, a mild climate: the frost line is 6 inches down, so the 12-inch code minimum governs and the footing bottom sits 12 inches below finish grade. Right, a cold climate: the frost line is 42 inches down, so the footing must go below 42 inches. In both, depth is measured from finish grade to the bottom of the footing, and the footing bears on undisturbed soil. — Source: 2021 IRC R403.1.4, R403.1.4.1; frost line per the jurisdiction’s Table R301.2

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Two side-by-side cross-sections of a foundation wall on a footing, drawn to the same vertical scale. Left, a mild climate: the frost line is 6 inches down, so the 12-inch code minimum governs and the footing bottom sits 12 inches below finish grade. Right, a cold climate: the frost line is 42 inches down, so the footing must go below 42 inches. In both, depth is measured from finish grade to the bottom of the footing, and the footing bears on undisturbed soil.

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    <img src="https://remodelcalculators.com/api/figure-embed/frost-footing-depth-rule" alt="Two side-by-side cross-sections of a foundation wall on a footing, drawn to the same vertical scale. Left, a mild climate: the frost line is 6 inches down, so the 12-inch code minimum governs and the footing bottom sits 12 inches below finish grade. Right, a cold climate: the frost line is 42 inches down, so the footing must go below 42 inches. In both, depth is measured from finish grade to the bottom of the footing, and the footing bears on undisturbed soil." width="800" height="520" style="width:100%;height:auto;border:0" loading="lazy">
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    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/frost-footing-depth-rule" target="_blank" rel="noopener">Footing depth: the deeper of 12 inches or the frost line</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
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