Foundation Footing Size Calculator

About the Foundation Footing Size Calculator

Foundation footings transfer the building's weight to the soil. The footing must be wide enough to spread the load so that the soil bearing pressure doesn't exceed the soil's capacity. An undersized footing leads to settlement, cracking, and structural failure.

This calculator determines the required footing width based on the load per linear foot, the soil bearing capacity, and then computes the concrete volume needed. It also checks against IRC (International Residential Code) minimum footing widths for standard residential construction.

Soil bearing capacity varies significantly: soft clay may support only 1,000 PSF, while dense gravel can support 6,000+ PSF. A geotechnical report provides the actual bearing capacity for your specific site. Without one, local building departments prescribe presumptive values based on soil type.

When This Page Helps

Correct footing sizing prevents costly foundation settlement. This calculator sizes footings based on actual loads and soil conditions rather than guesswork, and cross-checks against code minimum requirements to ensure compliance.

How to Use the Inputs

1. Enter the total load per linear foot of footing (dead load + live load).
2. Enter the soil bearing capacity in pounds per square foot (PSF).
3. Enter the footing length for volume calculation.
4. Enter the desired footing thickness (8" min for residential).
5. Review the required width and concrete volume.
6. Compare to IRC minimums and use the larger value.

Example Calculation

Tips & Best Practices

• Always get a soil test (geotechnical report) for precise bearing capacity values.
• Footing thickness should be at least equal to the projection beyond the wall on each side.
• Place footings below the frost line (varies by climate — check local codes).
• Use #4 rebar longitudinally (2 bars minimum) in continuous footings.
• Pour footings on undisturbed soil — never on fill, organic material, or frozen ground.
• Check IRC Table R403.1 for minimum footing widths based on building stories and soil type.

IRC Minimum Footing Widths

The International Residential Code (IRC) Table R403.1 prescribes minimum footing widths based on the number of stories and soil bearing capacity. For 2,000 PSF soil: 1-story = 12", 2-story = 15", 3-story = 23". For 1,500 PSF soil: 1-story = 16", 2-story = 21", 3-story = 32". Always use the larger of the calculated width or code minimum.

Load Estimation

A typical 2-story wood-frame house produces 2,000–3,000 lbs per linear foot of exterior footing. Interior bearing walls carry about 50–75% of the exterior wall load. Roof, floor, wall, and occupancy loads all contribute to the total.

Stepped Footings on Sloped Sites

When the grade slopes, footings must step down to maintain the minimum depth below grade at all points. Each step should be no taller than 2 feet, and the horizontal distance between steps should be at least 2 feet. Rebar must tie steps together continuously.

Frequently Asked Questions

• How wide should a house footing be?

  Residential footings typically range from 12–24 inches wide. IRC minimums: 12" for 1-story on 2,000+ PSF soil, 15" for 2-story, 23" for 3-story. Actual width depends on load and soil bearing capacity.

• What is soil bearing capacity?

  Soil bearing capacity is the maximum pressure soil can support without excessive settlement. Values range from 1,000 PSF (soft clay) to 12,000+ PSF (bedrock). A geotechnical engineer determines this through soil testing.

• How thick should a footing be?

  Minimum 6–8 inches for residential per IRC. The thickness must be at least equal to the projection beyond the wall. A 24" wide footing under an 8" wall has 8" projection on each side, requiring at least 8" thickness.

• What are presumptive soil bearing values?

  When no soil test is available, building codes provide default values: soft clay 1,500 PSF, medium clay 2,000 PSF, dense sand 3,000 PSF, gravel 3,000–5,000 PSF, bedrock 12,000+ PSF. These are conservative estimates.

• Do I need rebar in footings?

  The IRC requires at least 2 continuous #4 rebar in footings. Footings on poor soil, expansive clay, or in earthquake zones may need more. Rebar is placed in the bottom third of the footing thickness.

• What happens if a footing is too small?

  Undersized footings cause differential settlement — some parts sink more than others. This creates cracks in walls, sticking doors, and sloping floors. In severe cases, structural failure can occur. It's much cheaper to build correct footings than to fix foundation problems later.

• How deep should footings be?

  Below the local frost line (12" in warm climates, 48"+ in northern regions). The bottom of the footing, not just the top, must be below frost depth. Check the local building department for the frost depth in your area.