Footing Calculations: What's Different from a Slab
A footing is a continuous or isolated pad of concrete below grade that distributes load into the soil. The calculation is the same as a slab — length × width × depth — but footings are measured differently. Width and depth are typically specified in inches (because footings are relatively narrow), while length is in feet for a continuous foundation footing.
This calculator uses inches for width and depth, feet for length — which matches how footings are typically specified and how most contractors discuss them. Use the Footing tab above.
Footing Size Rules of Thumb
- Minimum depth — Footings must extend below the frost line to prevent heaving. Frost line depth ranges from near zero in South Florida to 60 inches in northern Minnesota and Michigan. Check your local building code or call your building department.
- Width for continuous wall footings — A general rule: the footing should be at least as wide as the wall it supports, and ideally twice as wide. A 6-inch concrete wall typically sits on a 12-inch-wide footing; an 8-inch wall on a 16-inch footing.
- Depth for continuous footings — Minimum 8 inches deep for light residential walls; 12 inches is the most common standard for foundation walls. For structural applications, follow the engineer's specification.
- Isolated pad footings — Point loads (columns, posts) use square or rectangular pad footings sized by load calculation. For a deck post, a common residential standard is 12×12 inches at minimum, often 18×18 or 24×24 depending on the tributary load area and soil bearing capacity.
- Reinforcement — Continuous footings typically use 2 bars of #4 rebar running longitudinally, with #3 stirrups at 18–24 inch spacing. Pad footings use a grid of rebar. Consult your local code for requirements in your jurisdiction.
These guidelines are for typical residential construction. Structural footings, retaining wall footings, and footings in poor soil conditions require engineering review.
Need a different shape? The full Concrete Calculator covers slabs, footings, round columns, and post holes in one place.