Footings must be located on undisturbed soil below the frost line. Check your local building codes for requirements regarding size, depth, and metal reinforcement. (See Preparing Sites for Wall Footings, Related Projects for how to lay out and excavate for wall footings.) You may not need to build forms; in some situations a simple trench is enough.
Make footings wide enough to distribute the weight of the wall adequately. Usually this means making the footing twice as wide as the wall is thick. A footing also must be thick enough so it will not crack. As a rule, the thickness of a footing should equal the width of the wall, but it should be no less than 8 inches thick.
To protect against structural damage that could result when a footing cracks and shifts because of unstable soil conditions, install two or three 1/2-inch reinforcing rods the full length of the footing (see Adding Reinforcement, Related Projects). Also, be sure to provide for drainage (see Getting to Know Your Structures and Preparing Basement Leaks, Related Projects). For more information on building forms, see Building Forms for Slabs, Related Projects.