Plumbing Sizing Calculators
Both of these size a vessel from household demand rather than from floor area, which is why square footage never appears in either one.
There are 2 calculators here: Water Heater Sizing & Operating Cost Calculator (tank size from household demand, and what it costs to run) and Septic Tank & Drain Field Sizing Calculator (tank size, drain field area and system cost by bedroom count).
They all work the same way: quantity first, then a waste allowance you can change, then a price built from verified figures and adjusted for your state. Cost is always a range rather than a single number, and every assumption behind it is printed on the page with its source and the date it was last checked.