Footing Calculator – Foundation Footing Size Calculator
Calculate footing dimensions for foundations
How to Use
- Enter the wall load (lbs or kN)
- Enter soil bearing capacity (psf or kPa)
- Enter wall length and thickness
- Enter desired footing depth
- Click calculate to get footing dimensions
Purpose of Footings in Construction
Footings distribute the weight of a structure over a larger area of soil, preventing the building from settling unevenly or sinking. They serve as the critical connection between the building and the ground, transferring loads safely into the earth.
Without properly sized footings, concentrated loads can exceed the soil's bearing capacity, leading to differential settlement, cracked walls, and structural failure. Every permanent structure — from a garden wall to a multi-story building — requires footings designed for the specific soil conditions and loads involved.
Types of Footings
Strip footings (also called continuous footings) run along the length of a wall and are the most common type for residential construction. Spread footings (or pad footings) are isolated square or rectangular pads that support individual columns or posts.
Stepped footings are used on sloped sites where the footing follows the terrain in a stair-step pattern. Mat or raft foundations are large slabs that cover the entire building footprint and are used when soil is weak or loads are heavy enough that individual footings would overlap.
Footing Sizing Guidelines
The required footing width depends on the load it must carry and the bearing capacity of the soil beneath it. As a general rule, residential footings for a one-story structure are at least 12 inches wide, while two-story structures typically require 15–18 inch wide footings.
Footing depth (thickness) should be at least equal to the projection beyond the wall on each side, and never less than 6 inches for residential work. Local building codes specify minimum dimensions, and a soil bearing test can confirm whether the default values are adequate for your site.
Frequently Asked Questions
- How deep should footings be to get below the frost line?
- Frost line depth varies by region — from 12 inches in southern climates to 48 inches or more in northern states. Footings must extend below the frost line to prevent frost heave, which can lift and crack foundations. Check your local building code or contact your building department for the required frost depth in your area.
- Do footings always require rebar?
- Most building codes require at least two horizontal rebar bars (#4 or #5) running continuously through strip footings, placed in the lower third of the footing. Rebar reinforces the concrete against tension forces caused by uneven soil support. For simple, lightly loaded structures like garden walls, some codes allow unreinforced footings, but adding rebar is always good practice.
- When are footings required for a project?
- Footings are required for any permanent structure that bears significant weight, including houses, garages, retaining walls, and deck posts. Small, lightweight structures like garden sheds under a certain size may be exempt in some jurisdictions. Always check local building codes — most require a permit and inspection for any footing work.
- What concrete strength should I use for footings?
- Standard residential footings use 2,500 PSI concrete, though 3,000 PSI is more commonly specified because the cost difference is minimal. In areas with sulfate-rich soil, use sulfate-resistant concrete (Type II or Type V cement). For commercial or heavily loaded footings, 3,500–4,000 PSI concrete may be required by the structural engineer.