Strip Footing Calculator

Strip Footings vs. Individual Footings

Understanding when to use a strip footing versus an individual footing is fundamental to foundation design. Each type serves a different structural purpose, and most buildings use a combination of both.

A typical residential foundation combines strip footings around the perimeter and under interior bearing walls with individual footings under any posts or columns in the basement or crawl space. The strip footing calculator on this page handles the continuous footing portions. Use our individual footing calculator for the pad footings under isolated columns and posts.

Strip Footing Sizing Per IRC Code

The International Residential Code (IRC) Table R403.1(1) provides minimum footing widths based on the number of stories supported and the soil bearing capacity. These are minimums; your local building official or structural engineer may require larger footings based on site-specific conditions. Here are the standard minimums for soil with a 2000 PSF (pounds per square foot) bearing capacity:

On weaker soils (1500 PSF or less), these minimums increase significantly. For example, a two-story home on 1500 PSF soil requires a 20-inch wide footing instead of 15 inches. Always obtain a soils report or geotechnical investigation for your building site, especially if the soil appears soft, wet, or has been previously disturbed. The footing thickness (depth) must be at least 6 inches per the IRC, but 8 to 12 inches is standard practice for residential strip footings.

Depth Requirements: Below the Frost Line

One of the most critical requirements for strip footings is that the bottom of the footing must be placed below the local frost line depth. When water in the soil freezes, it expands and pushes upward with tremendous force, a process called frost heave. If a footing is above the frost line, this heaving can crack the foundation, shift walls, and cause serious structural damage that is extremely expensive to repair.

• 1. Southern states (FL, TX, LA, AZ): Frost line is 12 inches or less. Some areas have no frost requirement at all, but footings still need to reach undisturbed soil below any topsoil or organic layer.
• 2. Mid-Atlantic and Midwest (VA, OH, IN, MO): Frost line ranges from 24 to 36 inches. This is the most common range for residential construction in the United States and typically requires a trench depth of 30 to 42 inches to accommodate both the frost depth and the footing thickness.
• 3. Northern states (MN, WI, ME, MT): Frost line can reach 48 to 72 inches. Deep footings significantly increase concrete volume and excavation costs. Some builders in these regions opt for frost-protected shallow foundations (FPSF) that use rigid insulation to reduce the required depth.

Contact your local building department to confirm the frost line depth for your area. This information is also available from the National Weather Service and is included in your local building code amendments. Never guess on frost depth because the consequences of getting it wrong are severe and irreversible.

Reinforcement: Rebar in Strip Footings

Concrete is strong in compression but weak in tension. Rebar (reinforcing steel) handles the tensile forces in a footing, preventing cracks from spreading and holding the footing together if the soil settles unevenly. Here is the standard reinforcement approach for residential strip footings:

• Longitudinal rebar: A minimum of two #4 rebar bars running continuously along the full length of the footing, placed in the bottom third. For footings wider than 18 inches, add a third bar. Position the bars with 3 inches of concrete cover from the bottom and sides of the footing.
• Lap splices: Where rebar sections meet, they must overlap by at least 40 bar diameters. For #4 rebar (1/2-inch diameter), that is a minimum 20-inch overlap. For #5 rebar (5/8-inch diameter), the minimum overlap is 25 inches. Tie all splices securely with wire ties.
• Corner and intersection bars: At corners and T-intersections where footings meet, add L-shaped or U-shaped bars that extend at least 24 inches into each leg of the footing. This ties the footings together structurally and prevents the corners from separating.
• Dowels for walls: Vertical rebar dowels (typically #4 at 48-inch spacing) are set into the wet footing concrete to tie the foundation wall to the footing. These dowels extend up into the wall above and create a mechanical bond between the two pours.

Step Footings on Sloped Sites

When building on sloped ground, the footing must step down to follow the grade while maintaining the required depth below the frost line at every point. Stepped footings are more complex to form and pour but are essential for hillside construction. Here is how they work:

To calculate concrete for a stepped footing, break the footing into individual level sections between each step. Calculate the volume of each section separately (length x width x depth), then add the volume of each vertical step riser. Sum all the sections for the total concrete volume. Our calculator handles this for simple configurations, but for complex stepped footings with many elevation changes, you may want to calculate each section individually and add the totals.

Stepped footings require careful formwork to ensure each step is level and at the correct elevation. Use plywood or dimensional lumber for the step risers, braced securely to prevent blowout when concrete is placed. Pour the entire stepped footing in one continuous operation if possible to avoid cold joints between steps.

Example: Perimeter Footing for a 30x40 Garage

Let's walk through a real example. You are pouring a continuous strip footing around the perimeter of a 30-foot by 40-foot detached garage. The footing will be 16 inches wide and 8 inches deep (standard for a single-story structure on 2000 PSF soil). Here is the calculation:

You would order approximately 5.5 cubic yards of 3000 PSI concrete from your ready-mix supplier. At $130 to $160 per cubic yard, the concrete material cost would be roughly $715 to $880 plus delivery. You would also need approximately 600 linear feet of #4 rebar (two bars running continuously plus lap splices and corner bars) and enough rebar chairs to support the steel 3 inches off the bottom of the trench. Our calculator does all of this math for you instantly.