About HVAC Calculations
HVAC sizing has a quick answer and a right answer, and it pays to know the difference. The quick answer is square-footage rules of thumb: so many BTU, or so many square feet per ton of cooling. That's fine for a ballpark when you're planning a budget or sanity-checking a quote, and it's what these calculators give you.
The right answer is a Manual J load calculation, the industry-standard method that accounts for your climate, insulation, windows, orientation, and air sealing. Two houses the same size can need very different equipment. Oversize the system and it short-cycles, never dehumidifies, and wears out early; undersize it and it runs flat out and still can't keep up. So treat a square-footage number as a starting point, not a spec to buy from.
Ducts get sized to move a target airflow without too much friction or noise. Each room needs a certain CFM based on its load, and the duct has to carry that without choking the system. The duct calculator gives a size for a given airflow, but the full design is a Manual D, the companion to Manual J. Undersized ducts are a common reason a correctly sized unit still underperforms.
Use these to get oriented and ask better questions. Get a rough tonnage, see roughly what duct a room wants, then have a pro run the real load calc before you spend money on equipment. The numbers here are planning estimates, not a substitute for a licensed HVAC contractor's design.