In a typical home about 2/3 of all heat loss is from leakage around doors and windows and loss from opening and closing doors, so any true calculation needs to calculate in all that.

In most cases though the hvac folks just say....hmmmm, 2800 sf about average windows etc....about 2.5 tons ac and 150K btu heat. (as an example)

And it works.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.