Bearing capacity: ultimate vs allowable, and why the tools take it as an input
The ground beneath a footing has a real capacity limit, just like a steel or concrete section does — but unlike a section's capacity, that number can't be read off the footing's own geometry. It comes from the soil actually on site.
Ultimate bearing capacity is the pressure at which the soil beneath a footing fails in shear — the ground itself gives way, punching or heaving rather than the footing's concrete or steel failing. Classical bearing capacity theory, originally developed by Karl Terzaghi in the 1940s and later refined by Meyerhof, Hansen and others, estimates this ultimate pressure from the soil's own strength parameters (cohesion and friction angle), the footing's width and shape, and how deep it's embedded below the surrounding ground surface. This theory is general soil mechanics, developed across the 20th century — it isn't an Eurocode or ACI 318 formula, in the same way Euler's buckling formula isn't a steel-code formula.
Ultimate capacity on its own isn't what a real design compares applied pressure against, though — a safety margin is applied first, either as a straightforward factor of safety (dividing ultimate capacity down to an allowable value, common in US practice) or through EC7's partial-factor approach (applying reduction factors directly to soil strength parameters or resistance before comparing). Either way, the number that actually gets used day to day is a design or allowable bearing pressure, not the raw ultimate value.
Why Foundation Design and Retaining Wall Design take it as an input, not an output
Both tools compare the applied pressure under a footing — computed from the real loads and the footing's own geometry — against an allowable bearing pressure value you provide, rather than deriving that value themselves from soil strength parameters. That's a deliberate scope boundary, not a missing feature: the real bearing capacity of a specific site depends on an actual subsurface investigation — borings, standard penetration tests, soil classification, groundwater level — that only a geotechnical report can supply. Treat the allowable bearing pressure you enter as coming from that report, never as a general assumption typed in to get a number.