square footage?

OK sports fans. Useful square footage of my 5er or my stick home has always stymied me. We're not dumb. I have a Master's Degree. The more we know, the more complex it becomes. Of course I can measure square footage for flooring I need. Living space with slideouts becomes another question. Sure, I can use the Pythagorean Theorem to measure what roofing material I have for my stick home. While I know my 5er is 40 feet long and can multiply my width to give me a total square footage, that's not useful in my living space, or the exact number of people I need to throw out before ascertaining bed space by alcohol levels. We all have space questions, and it's unfair to judge what those are without being the one to ask.