Isolated, combined, strip, mat, or pile cap: what actually decides footing type
A single footing type doesn't fit every column — the right choice usually comes down to how much room there is on site, how close neighboring loads are, and whether the soil near the surface can carry the load at all.
An isolated (pad) footing under a single column is the default starting point whenever there's enough room to spread a square or rectangular base under that column without its edge hitting a property line or another footing — the simplest and usually the most economical option, sized mainly to keep the applied pressure at or below the allowable bearing pressure. Once two adjacent footings would physically overlap, or a footing would have to be built right up against (or past) a property line, an isolated footing stops being workable — the practical alternative is a combined footing, one larger base supporting two or more columns together, avoiding the badly eccentric loading an isolated footing squeezed against a boundary would otherwise carry.
Strip, mat, and pile cap: when it stops being about one or two columns
A strip (continuous) footing runs beneath a line of closely spaced columns or a load-bearing wall — a natural choice once separate isolated footings along that line would nearly touch anyway. A mat (raft) foundation goes further still, spreading the entire building's load over one continuous slab covering the whole footprint; it becomes the practical choice once individual footings would already cover most of the site, or once the soil's bearing capacity is low enough that spreading load as widely as possible is the only way to keep pressure within the allowable value. A pile cap belongs to a genuinely different foundation strategy: rather than spreading load onto the soil at the surface, it transfers load down into a group of piles that carry it to a deeper, stronger layer — the right call once the soil near the surface simply can't support the load, no matter how wide a shallow footing gets. Foundation Design supports all five footing types directly, applying the same flexure, shear and development-length checks but built around each type's own real geometry (see the related tool below).