Frost Depth Calculator

1. Pick the method. Use “below the local frost line” for a conventional footing, or “frost-protected shallow foundation” for a heated, insulated building.
2. For the first method, enter the local frost depth in inches from your building department. For the second, enter the site air-freezing index in F-days.
3. Optionally enter your planned footing depth, then read the minimum depth and whether your plan meets it.

Here is what each result line means:

Frost depth is how far below the ground surface the soil freezes in a normal winter, and it is the number that decides how deep a footing has to go. A footing that sits above it can be lifted by freezing, wet soil and then settle unevenly in spring.

The depth is not one national figure. In the 2021 International Residential Code the footing rule refers to the frost line in Table R301.2, and the figure for your lot comes from your building department. This tool does not guess it. What it adds is the arithmetic around that number: the 12 inch minimum depth the code sets for exterior footings, and a published table that lets a heated building with insulation use a much shallower footing based on the air-freezing index.

The air-freezing index is a measure of how severe a winter is: cumulative degree days below 32 degrees Fahrenheit. NOAA describes it as a common metric of freezing severity. For the footing itself, the concrete footing calculator turns a chosen depth into cubic yards.

The tool has two modes and never mixes them. Each reads one published rule.

1. Below the local frost line. Take the frost depth you entered. If it is under 12 inches, use 12 inches, because the 2021 IRC says exterior footings go not less than 12 inches below undisturbed ground.
2. Frost-protected shallow foundation. Look up the air-freezing index in Table 3 of the HUD builder guide. At 1,500 or less the footing depth is 12 inches; at 2,000 it is 14; from 2,500 to 4,500 it is 16. The guide says interpolation between values is permissible, so the tool interpolates in a straight line between rows.
3. Compare. If you entered a planned depth, the tool subtracts the minimum from it and shows the difference.

The interpolation step is just proportion: between 1,500 and 2,000 F-days the depth rises 2 inches over 500 F-days, so each 250 F-days adds 1 inch.

First, a conventional footing. Your building department tells you the frost depth is 42 inches, and you plan a footing bottom at 48 inches.

Calculation: the larger of 42 and 12 is 42 in, which is 3.5 ft. Your plan is 48 – 42 = +6 in deeper than the minimum, so the tool shows it as meeting the minimum.

Second, a heated building on a shallow foundation at 1,750 F-days, with a planned depth of 14 inches. Table 3 gives 12 inches at 1,500 and 14 inches at 2,000. The index is halfway between: 12 + (1,750 – 1,500) / (2,000 – 1,500) x (14 – 12) = 12 + 0.5 x 2 = 13 in. The vertical insulation interpolates the same way between R-4.5 and R-5.6: 4.5 + 0.5 x 1.1 = R-5.05. A 14 inch plan is +1 in over the minimum.

The lesson: the shallow foundation is only about a foot deep because insulation, not depth, is doing the frost protection, so the insulation value matters as much as the depth.

Your Local Frost Depth

The single biggest factor. Two towns a short drive apart can have different frost depths in their codes. Always use the figure from the jurisdiction where you build.

Heated or Unheated Space

The shallow foundation table is for buildings kept at a monthly mean temperature of at least 64 degrees Fahrenheit. The IRC does not allow that approach for unheated spaces such as porches, garages or carports.

Insulation, Not Just Depth

A 12 to 16 inch footing is only acceptable with the vertical, and at higher indexes horizontal, insulation the table requires. Dropping the insulation voids the table.

Frozen or Permanently Frozen Ground

The code says footings shall not bear on frozen soil unless the frozen condition is permanent, so the bottom of the trench must be on unfrozen, undisturbed soil when you pour.

Energy Code Needs

HUD notes the insulation values are for frost protection and greater values may be required to meet energy conservation standards.

The two approaches protect against the same heave in different ways, which changes what you dig and what you buy.

If you are still choosing, the guide on how deep footings should be walks through the decision, and the foundations and footings cluster lists the sizing tools.

Checking a Plan Before You Apply for a Permit

Enter the frost depth the building department gave you and your planned depth. A red result tells you to fix the drawing before it goes in for review.

Weighing a Shallow Foundation for a Heated Addition

If your air-freezing index is within the table, the result shows the footing depth and the insulation R-value to ask a designer about.

Sizing the Dig for Piers and Posts

Once you know the depth, the rebar calculator and the footing calculator turn it into materials.

1. Using a Number from a Map or a Forum

The code refers to a local frost line. A figure from another county or an online chart may not be what your inspector enforces.

2. Treating 12 Inches as the Frost Depth

The 12 inch figure is a minimum depth below undisturbed ground, not a frost depth. Where frost applies, the footing must also reach the local frost line or use an approved alternative.

3. Using the Shallow Foundation Table for an Unheated Space

The table is for heated buildings. Porches, garages and carports are excluded in the IRC text.

4. Mixing Feet and Inches

Entering 3.5 for a 42 inch frost depth returns 12 inches. The field is in inches; a value over 240 is blocked as a likely unit mix-up.

5. Skipping the Insulation

Copying a 16 inch depth from the table without the required insulation is not a frost-protected foundation.

6. Reading the Result as a Permit Approval

The result is an estimate of a published rule. Only your building official approves a footing depth.

The arithmetic is exact. The numbers it relies on are a code minimum, a user-supplied local figure and one published table, so the result is only as good as those.

It should reach the frost depth your local building department sets, and never less than 12 inches below undisturbed ground under the 2021 IRC. There is no single national number, so enter your local figure above. A heated building with insulation can use a shallower footing from the HUD table.

Ask your local building department. The 2021 IRC refers to the frost line in Table R301.2, and your jurisdiction supplies the figure. A permit office, inspector or published local code can give it. This tool does not look it up and does not guess one.

It is the cumulative degree days below 32 degrees Fahrenheit over a winter, a common measure of freezing severity. NOAA says it can help estimate the maximum depth of frost penetration. The HUD table uses it to choose footing depth and insulation for heated buildings.

No. The IRC text for frost-protected shallow foundations covers buildings kept at a monthly mean of at least 64 degrees Fahrenheit and does not allow the approach for unheated spaces such as porches, garages or carports. Use a conventional footing below the local frost depth there.

Yes. The HUD guide says interpolation between table values is permissible. Between 2,000 (14 inches) and 2,500 (16 inches), 2,250 gives 15 inches with R-6.15 vertical insulation. Table 3 lists horizontal insulation from the 2,500 row, so check the full table.

Only where no frost depth applies. The 12 inch figure is a minimum below undisturbed ground, and it sits alongside the frost rule. Where the ground freezes, the footing must go below the frost line or use an approved alternative such as a frost-protected shallow foundation.

Table 3 in the HUD guide ends at 4,500. Above that the guide points to a detailed design method, and the tool will not extrapolate past a published table. In a very cold location, follow your local rule or a licensed designer.

Maps and charts estimate frost depth in general; the code uses the figure your jurisdiction adopts. Your building department can set a different or more conservative number. Use the local figure, since it is what the inspector checks.

No. It applies a code minimum and a published table. Soil type, drainage, expansive clay and the structure above the footing are outside it. For anything load-bearing, confirm with the building official or a licensed engineer.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Revised Builder’s Guide to Frost Protected Shallow Foundations (U.S. Department of Housing and Urban Development, 2004; Table 3).
• 2021 IRC Chapter 4, Foundations (Virginia edition) (UpCodes; sections R403.1.4, R403.1.4.1 and R403.3).
• Frost Protected Shallow Foundations and the air-freezing index (NOAA National Centers for Environmental Information).
• Minnesota Rules 1303.1600, footing depth due to freezing (Minnesota Revisor of Statutes; an example of a locally set frost depth).