How Deep Should Footings Be?

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Featured image with the headline How Deep Should Footings Be, beside a pale grey block at the bottom of a deep trench with layered brown earth walls

How deep should footings be? The answer has two layers, and only one is the same everywhere. The 2021 IRC sets a model-code floor, and your local building department sets the frost number that applies to your site.

Quick Answer

Where frost protection applies, put the bottom of an exterior footing below the frost line your local building department sets. The 2021 IRC model code adds a floor of 12 inches below undisturbed ground. Minneapolis, for example, lists 42 inches. Footings must not bear on frozen soil.

Footing Depth Means Two Different Things

In footing rules, depth names two measurements: how thick the concrete is, and how far below the ground its bottom sits.

Section R403.1.1 of the 2021 IRC sets a footing of at least 12 inches wide and 6 inches thick, and it calls that thickness depth. Section R403.1.4 is the other one: exterior footings go at least 12 inches below the undisturbed ground surface. That 12 inches is a floor. It is not a frost depth.

Our Concrete Footing Calculator has a depth field too. It means footing thickness, not trench depth, and it does not check code, frost depth or soil.

ICC labels its 2021 IRC pages as a historical version, so newer editions exist. We did not check which edition your town uses.

Who Sets the Frost Line for Your Site?

Your local jurisdiction sets it. The model code leaves the frost line depth column of Table R301.2 blank, and each jurisdiction fills it in.

The table note says the entry is the minimum depth of footing below finish grade. Two published examples show how local numbers read. A City of Minneapolis bulletin (updated 05/13/2025, citing the 2020 Minnesota Residential Code) says the frost depth there is 42 inches, measured to the bottom of the footing. Minnesota rule 1303.1600 gives 5 feet in Zone I and 3 1/2 feet in Zone II when no engineer’s soil determination exists, and it allows lesser depths with an engineer’s evidence.

The rule page we read says it was published electronically on January 23, 2015, so check that it is current. These are local examples, not national figures. Other towns may measure to a different point; that is not verified here.

Footing depth below ground: model-code floor and local examples Horizontal bars at 8 pixels per inch. IRC model-code floor 12 inches (96 px). Minneapolis bulletin 42 inches (336 px). Minnesota rule 1303.1600 Zone I 5 feet or 60 inches (480 px). Zone II 3 1/2 feet or 42 inches (336 px). Depth below ground in inches (scale: 8 px per inch) IRC model-code floor 12 in Minneapolis bulletin 42 in Minnesota, Zone I 60 in (5 ft) Minnesota, Zone II 42 in (3 1/2 ft)
The amber bar is the model-code floor. The teal bars are local examples, not rules for your site.

Worked Example: One City’s Number in Inches

Minneapolis lists 42 inches as its frost depth, measured to the bottom of the footing. Assume a footing 8 inches thick.

  1. Bottom of footing. 42 inches below undisturbed ground.
  2. Top of footing. 42 – 8 = 34 inches below undisturbed ground.
  3. Against the floor. The model-code floor is 12 inches, so the local number is 42 – 12 = 30 inches deeper.
This is an example using one city’s published number, not a rule for your site.
Footing section for the Minneapolis example Vertical scale 6 pixels per inch. The 12 inch model-code floor is 72 px below the ground surface. The footing top is 34 inches (204 px) down. The footing is 8 inches thick (48 px), and its bottom sits at the 42 inch frost line (252 px). Footing width is not to scale. Footing section (vertical scale: 6 px per inch) Undisturbed ground surface 12 in model-code floor Frost line: 42 in (example) footing, 8 in thick 12 in 42 in 34 in to top 8 in Footing width is not drawn to scale.
The footing top sits 34 inches down and its bottom reaches the 42 inch line.

Ways to Build Above or Around the Frost Line

The 2021 IRC lists four ways to protect footings from frost: go below the local frost line, use a frost-protected shallow foundation, follow ASCE 32, or sit on solid rock.

A frost-protected shallow foundation is for buildings kept at a monthly mean temperature of at least 64 F. With insulation per Section R403.3, the minimum footing depth is 12 inches at an air-freezing index of 1,500 or less, 14 inches at 2,000, and 16 inches from 2,500 to 4,000. It is not for unheated spaces such as porches, utility rooms, garages or carports.

The model code also exempts free-standing accessory structures from frost protection. For light-frame construction the limit is 600 sq ft, and for other construction it is 400 sq ft, with eave height 10 ft or less in both cases. Local rules may differ.

Piers fall under the same frost rule. For round pier forms, see our round pier forms calculator.

Which frost rule applies to an exterior footing Start with an exterior footing. If it is a free-standing accessory building, light-frame at 600 sq ft or less or other construction at 400 sq ft or less, with eave height 10 ft or less, the model code does not require frost protection and local rules still apply. If not, protect it by going below the local frost line, by a frost-protected shallow foundation for heated buildings only, by ASCE 32, or by building on solid rock. Footings never bear on frozen soil unless it is permanently frozen. Which frost rule applies? (2021 IRC model code) Exterior footing: start here Small free-standing accessory building? Light-frame 600 sq ft or less, or other 400 sq ft or less, with eave height 10 ft or less Yes Frost protection not required by the model code. Check local rules. No Protect from frost with one of these: Below the local frost line, or a frost-protected shallow foundation (heated buildings only), or ASCE 32, or solid rock. In every case: no bearing on frozen soil unless it is permanently frozen.
Check the exemption first; otherwise pick one of the four protection methods.

What Soil Can a Footing Sit On?

Footings sit on undisturbed natural soil or engineered fill. They must not bear on frozen soil unless the frozen condition is permanent.

In place of a full geotechnical evaluation, the 2021 IRC says to assume the load-bearing values in Table R401.4.1. Where soil-science data show that expansive, compressible, shifting or otherwise questionable soils are likely, the building official decides whether a soil test is required. An approved agency does the test using an approved method.

Presumptive load-bearing values, 2021 IRC Table R401.4.1
Soil or rock Bearing value (psf)
Crystalline bedrock 12,000
Sedimentary and foliated rock 4,000
Sandy gravel and gravel (GW, GP) 3,000
Sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC) 2,000
Clay, sandy clay, silty clay, clayey silt, silt, sandy silt clay (CL, ML, MH, CH) 1,500

The table notes that a soils investigation sets the capacity where the official expects in-place soils under 1,500 psf.

How Wide and Thick Should a Footing Be?

No footing is less than 12 inches wide and 6 inches thick. Tables R403.1(1) to R403.1(3) set the size from the soil bearing value, stories, basement, wall type and roof or snow load.

The rows below come from Table R403.1(1), which covers conventional light-frame walls, in its block for a 20 psf roof live load or 25 psf ground snow load. The table assumes a 32 ft building width, 9 ft wall height and an 8 ft basement wall. Over 32 ft wide, it adds 2 inches of width and 1 inch of thickness per 4 ft of added width.

Footing width x thickness in inches, Table R403.1(1), 20 psf roof or 25 psf snow block
House 1,500 psf soil 2,000 psf soil 2,500 psf soil
1 story, slab-on-grade 12 x 6 12 x 6 12 x 6
2 story, plus basement 19 x 6 14 x 6 12 x 6
3 story, plus basement 22 x 7 16 x 6 13 x 6

Read these as a model-code example, not a design. Your plan and your building official decide the size.

When Is the Footing Inspected?

Under the model code, the foundation inspection comes after the trenches are dug, the forms are up and any required reinforcing steel is in place, and before concrete is placed.

  1. Permit. Section R105.1 says an owner who intends to construct a building must first apply to the building official and get the required permit.
  2. Dig and form. Excavate the trench, build the forms, and place and support any required reinforcing steel.
  3. Inspection. Section R109.1.1 puts the foundation inspection here. It includes excavations for thickened slabs.
  4. Pour. Concrete is placed after the inspection.

These are model-code texts, and a city or state may amend them. Ask your department how it schedules the inspection.

Know your depth and size? Estimate the concrete next.

Once the plan sets the numbers, estimate the concrete for your strip footing. You can also browse the foundations and footings calculators or all concrete calculators.

FAQs About Footing Depth

Is 12 inches deep always enough for a footing?

No. Twelve inches below undisturbed ground is only the model-code floor in Section R403.1.4. Where frost applies, the footing must also meet the frost protection rule, such as the local frost line in Table R301.2.

Who sets the frost line depth?

The local jurisdiction does. The IRC leaves the frost line column of Table R301.2 for each jurisdiction to fill in, so ask your building department for the number that applies to your site.

Can a shed skip frost protection?

The 2021 IRC exempts free-standing light-frame accessory structures of 600 sq ft or less with eaves 10 ft or less. Other construction is exempt at 400 sq ft or less. Local rules may differ, so check first.

What is a frost-protected shallow foundation?

It is a footing protected from frost by insulation, under Section R403.3. The model code allows it only for heated buildings, with minimum footing depths of 12 to 16 inches depending on the air-freezing index.

Can a footing sit on fill?

The model code allows footings on undisturbed natural soil or engineered fill. Footings must not bear on frozen soil unless it is permanently frozen. Ask your building official about any fill on your site.

Does the inspector come before or after the pour?

Before. Under the model code, the foundation inspection comes after excavation, forms and required rebar are in place, and before concrete is placed. Your city may amend this, so confirm its schedule.

Is 6 inches thick enough for a footing?

Six inches is the model-code minimum thickness, with a 12 inch minimum width. Tables R403.1(1) to R403.1(3) can call for more, such as 22 x 7 inches for a three-story house with basement on 1,500 psf soil.

Sources and Further Reading

References Used in This Article

This is a model-code overview with two local examples, not a design for your site. Codes change and each town sets its own frost line, so confirm the depth with your building department. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-19.