A footing spreads a concentrated column or wall load over enough soil that the ground does not over-stress and settle. This calculator sizes the footing plan area, the square dimension, and a minimum thickness from the load and the soil’s allowable bearing pressure, then checks the depth against your frost line.
How it works
The governing rule is that soil pressure under the footing must not exceed the allowable bearing capacity:
required area = total service load / allowable bearing pressure
square side = sqrt(required area)
soil pressure = load / provided area (must be <= allowable)Frost protection requires the footing bottom to sit below the frost line, so the tool flags any footing depth shallower than your entered frost depth. Minimum thickness is taken as the larger of 6 inches or the footing projection beyond the supported member, which keeps one-way shear in check for small footings.
Example and tips
A deck post carrying 8,000 lb on soil rated at 2,000 psf needs 4 square feet of footing, or about a 24-inch square pad. In a region with a 42-inch frost depth, the bottom of that pad must be at least 42 inches below grade. Always use the lowest credible bearing value when soil is unknown, round footing dimensions up to a convenient size, and remember that eccentric or moment loads need a larger footing than pure axial load.