Manual J
The ACCA room-by-room load calculation a contractor must run before installing HVAC equipment — the real sizing method our square-footage screen approximates.
Manual J is the load-calculation standard published by the Air Conditioning Contractors of America (ACCA). It sizes heating and cooling equipment room by room from your home's actual windows, orientation, insulation, air leakage, and duct losses — details a square-footage screening estimate can't see. Many building departments now require a Manual J (or an equivalent) on the permit for a full system replacement.
Our calculator runs a "Manual-J-lite" screen — square footage times a climate-zone BTU factor, adjusted for insulation, age, ceiling height, windows, and sun exposure — which gets you within about a ton for budgeting and comparing quotes. It is not a substitute for a contractor's real Manual J, which is what the installed equipment should actually be sized from.