US Footing Sizes & Frost Depth (IRC R403)

The requirement

A concrete footing must be at least 12 inches (305 mm) wide and 6 inches (152 mm) thick (R403.1.1). Exterior footings must sit at least 12 inches (305 mm) below undisturbed ground (R403.1.4). In cold country you dig deeper, below your local frost line. That frost depth is local. It is not in the national code.

In the United States, houses are built to the International Residential Code (IRC), Section R403.1 “Footings.” That is the model code. Your town or state adopts it. Some places use an older edition. Some add their own changes. So always confirm the numbers with your local building department.

The minimum size. The IRC sets a hard floor on how small a footing can be. No footing goes smaller than 12 inches wide and 6 inches thick (R403.1.1). You size the real footing off Table R403.1(1). But you never go below that 12 × 6 minimum.

The minimum depth. Exterior footings sit at least 12 inches (305 mm) below the undisturbed ground surface (R403.1.4). That is the frost-free minimum. It is the shallowest you are ever allowed to go.

1. 1 footing
2. 2 reinforcement
3. 3 ground

Don’t pour on frozen ground. R403.1.4.1 says footings shall not bear on frozen soil unless that frozen condition is permanent. Frozen dirt thaws and moves. Dig it out and get to solid, unfrozen bearing.

The frost line is not a national number. This is the bit that trips people up. In cold country, 12 inches is not enough. The footing must reach below the frost line, so winter frost can’t heave it. But the IRC does not print a frost depth. Table R301.2(1) is left blank on purpose. Each local jurisdiction fills it in. It runs from near zero in the far south to 40 inches or more up north. There is no single right number, so we won’t print one. Call your local building department, the Authority Having Jurisdiction, and ask for your frost depth. Then dig below it.

The width is table-driven too. The real width comes off Table R403.1(1). It uses your soil’s load-bearing value, the number of stories, and the snow and roof load. The soil value comes from Table R401.4.1, or a soils report where the ground is questionable (R401.4). Heavier or engineered loads go to a registered design professional. Read the width from the adopted code and your soils info. Don’t assume it.

Work it out on site

The depth and width are code. The volume of concrete you order is just math. It feeds straight into the calculator. Say you have a strip footing to pour. These are example numbers, not code minimums. Your real sizes come off the plan and your local frost depth:

• Total length of trench: 80 ft
• Footing width: 16 in = 1.33 ft
• Footing depth: 36 in (below a northern frost line) = 3 ft

Volume is length times width times depth. So 80 × 1.33 × 3 = 319 cubic feet. Divide by 27 to get cubic yards. That gives 319 ÷ 27 ≈ 11.8 cubic yards.

So you’d order about 12 yards. Then add a little for spillage and an uneven trench bottom. Most crews tack on 5 to 10 percent. Swap in your own numbers and the calculator does the sum for you.

Do it in 5 seconds

Related

• Part of the building codes for US builders — the hub, with the state-by-state adoption picture.
• Working out the pour? Use the concrete volume calculator for slabs & footings.

Sources

• 2021 IRC, Section R403.1 “Footings” — minimum footing size 12 in × 6 in (R403.1.1); minimum exterior footing depth 12 in below undisturbed ground (R403.1.4); no bearing on frozen soil (R403.1.4.1); frost-line depth taken from the locally filled Table R301.2(1). Confirm at codes.iccsafe.org or a verbatim reproduction such as up.codes.
• Table R403.1(1) — minimum footing width by soil load-bearing value, stories, and snow/roof load. Table R401.4.1 — presumptive soil load-bearing values; a soils investigation under R401.4 where the ground is questionable. Read the width and soil value from the adopted code, not from a website.
• Adoption caveat: The IRC is a model code with no legal force on its own. It applies only where your state or local jurisdiction adopts it, and many places adopt an older edition (2018 / 2015) or add their own amendments. The frost-line depth is inherently local — Table R301.2(1) is blank in the model code. Always confirm the governing edition, the exact numbers, and your frost depth with the local Authority Having Jurisdiction before you pour.