A proper load calculation is not a rule of thumb about tons per square foot — it is a room-by-room accounting of heat gain and loss. The reason people fall back on rules of thumb is that gathering the inputs means measuring every room in the house.
The inputs a scan delivers
- Floor area per room, the basis for every per-room calculation
- Ceiling height per room, which turns area into the volume that drives air changes
- Window area per room, measured rather than estimated — the single biggest gain variable
- Exterior wall area per room, which is the conduction surface
- Ceiling area under the attic, where the other half of the load lives
That is most of the geometry side of a Manual J, collected in a single walkthrough instead of a morning with a clipboard.
Why window area especially
Solar gain through glass often dominates a room's cooling load, and it is also the number most likely to be guessed. A room with a wall of west-facing glass behaves nothing like a room with one window, and a measured opening area is what separates the two in the calculation instead of in a complaint next July.
Orientation matters as much as area — note which way the glass faces as a voice note while you are in the room.
Volume for duct and air changes
Ceiling height is where square-footage shortcuts fall apart. A vaulted great room has half again the volume of its footprint, which changes CFM, register placement, and return sizing. The scan reports real wall heights, including rooms where the height changes across the space.
What the scan does not know
Insulation values, window U-factors, air tightness, duct condition, equipment age. Those are inspection and testing, and they are what you pin voice notes about while you are in the mechanical room.
Why it pays
Right-sized equipment is quieter, cheaper to run, and does not short-cycle. Getting there needs real geometry.
Rules of thumb exist because measuring was expensive. It stopped being expensive.