Foundation Depth Calculator

Foundation depth must be sufficient to reach below the frost line, rest on stable soil, and support the building load safely. Too shallow and you risk frost heave, where freezing water in the soil expands and pushes the foundation upward, cracking it.

Minimum Depth Rule: Foundation Depth = Frost Line Depth + Safety Margin (typically 6–12 inches / 15–30 cm)

Frost Line Depths by Region (United States):

International Frost Depths:

Soil Type Considerations: Different soils have different bearing capacities and frost susceptibility:

Foundation Types by Depth:

• Slab-on-grade: 4–6 inches thick, used where frost line is shallow (warm climates). Edges thickened to 12–18 inches.
• Crawl space: Foundation walls extend 18–24 inches below grade minimum, below frost line.
• Full basement: Foundation walls extend 7–8+ feet below grade, well below any frost line. Most common in cold climates.
• Pier/post: Individual footings at frost depth, supporting beams.

Footing Size Rule of Thumb: The footing width should be at least twice the wall thickness. For a standard 8-inch (200 mm) foundation wall, the footing should be at least 16 inches (400 mm) wide. Footing thickness is typically equal to the projection beyond the wall (minimum 6 inches / 150 mm).

Checking the width against the soil

That rule of thumb is a starting point, not the answer. The footing has to be wide enough that the building’s weight, spread along its length, stays under what the soil can carry:

Required width (inches) = (Wall load in pounds per foot ÷ Soil bearing capacity in PSF) × 12

Typical wall loads for light-frame construction, per linear foot of bearing wall including roof, floors, occupants and snow:

Run a 2-story house at 3,000 lb per foot on soft clay at 1,250 PSF and you need 28.8 inches of footing, not the 20 the rule of thumb suggests. On bedrock the same house needs 3 inches, and the 20-inch minimum wins instead. The calculator does this comparison and reports whichever is larger, because a footing sized only by the rule of thumb is the classic way a house on poor soil ends up settling unevenly.

Soil Testing: For any significant structure, a geotechnical soil test is recommended. The test determines soil bearing capacity, water table level, and composition at your specific site. Costs range from $500–$3,000 depending on the number of bore holes and depth required.

Building Code Note: Local building codes always take precedence. Check with your local building department for the specific frost depth and minimum foundation requirements in your area. Codes may require deeper foundations near slopes, in expansive soil areas, or for commercial structures.