How Deep Do Footings Need to Be?

The two rules

1. At least 12 inches deep. "Exterior footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface" (2021 IRC, R403.1.4). This applies everywhere, including places with no frost.
2. Protected from frost. Foundations must be protected from frost (R403.1.4.1). The usual way is to extend the footing below the frost line listed in Table R301.2. The code also allows a frost-protected shallow foundation, which uses buried insulation (R403.3, or an engineered design to the ASCE 32 standard), or building on solid rock. Footings also "shall not bear on frozen soil unless the frozen condition is permanent", so don't pour on ground that froze overnight.

In Dallas, for example, the city lists 6 inches, so the 12 inch minimum governs.

How the depth is measured

Table R301.2 asks each jurisdiction for the "minimum depth of footing below finish grade" (footnote b). Measure from finished grade, the final ground level beside the footing, to the bottom of the footing, not the top. Finish grade is the ground level when the work is done, so if you plan to dig the ground down next to the footing, measure from that lower level.

The two rules start from different levels. The 12 inch minimum is measured from undisturbed ground, so fill you add doesn't count toward it. The frost depth is measured from finished grade.

Frost depth by state

These states set one depth for the whole state:

Washington, DC sets 30 inches. Florida's code lists the frost line as "NA", so only its 12 inch minimum applies there, measured below finished grade. A few states set the depth by zone or county:

Most other states leave the number to each city or county. These are official values we read in city and county documents:

The frost line depth by state page has the full list, including every county in the county-based states. If your town isn't listed, your building department has the number, usually in a handout or its copy of Table R301.2.

When the frost line doesn't apply

The 2021 IRC drops the frost protection rule for small free-standing accessory structures (R403.1.4.1, exceptions):

• Light-frame (stick-built) accessory structures, such as a wood shed, of 600 sq ft or less with an eave height of 10 ft or less.
• Accessory structures of other construction of 400 sq ft or less with an eave height of 10 ft or less.

The 12 inch minimum depth still applies to their footings. Your state or city may use a different IRC edition or be stricter, so confirm the shed rule locally before you build on a shallow footing.

Deck footings

Deck footings follow their own section (R507.3). They go at least 12 inches into undisturbed soil, and a deck attached to a house that is protected from frost needs footings below the frost line, on solid rock, or protected another approved way. Ask your building department about free-standing decks. The deck framing planner lays out the posts, and the post hole concrete calculator gives the bags per hole once you know the depth.

Frequently asked questions

Calculators for this

Published 2026-11-01. Sources: 2021 International Residential Code, Chapter 4 (R403.1.4, R403.1.4.1); 2021 International Residential Code, Table R301.2; State and local codes, listed on the frost line depth by state page. See how we calculate.