A footing is the wide concrete base at the bottom of a foundation wall, pier or column. Soil can only support so many pounds per square foot, so the footing spreads the building’s weight over a larger area than the wall above it — the same way snowshoes keep you from sinking. Continuous (strip) footings run under foundation walls; spread or pad footings sit under individual posts and columns.
Under the IRC, footings must be at least 6” thick (R403.1.1), with widths set by Table R403.1(1) based on the number of stories, type of construction and soil bearing capacity — commonly 12”–24” for houses. The bottom must rest on undisturbed soil at least 12” below grade and below the local frost line (R403.1.4) so freeze-thaw cycles can’t heave it. In higher seismic design categories the code also requires reinforcing steel, and many builders run two continuous #4 rebar bars regardless.
To estimate concrete for a strip footing, multiply length × width × depth in feet and divide by 27. A 100 ft footing 16” wide and 8” deep is 100 × 1.333 × 0.667 = 88.9 cu ft, or 3.3 yd³ — order about 3.5 with waste.