Soffit area is the eave length times the overhang width. Enter those as the rectangle above — a house with a 12 inch overhang and 140 feet of eave has 140 square feet of soffit.
Vented and solid
Soffit panels come vented, solid, or in a centre-vent pattern. The attic needs continuous intake air at the eaves, so most of the run should be vented.
The usual target is one square foot of net free vent area for every 300 square feet of attic floor, split roughly half low and half high. Vented vinyl soffit provides far less net free area than its face area suggests — check the manufacturer's figure.
Fascia is measured separately
Fascia runs along the outer edge of the eave and is ordered by linear foot, in the same length as the eave measurement.
Aluminium fascia wrap goes over existing wood fascia. It is not structural — the wood behind it still has to be sound, and wrapping rotten fascia just hides the problem for a few years.
Do not block the airflow
Insulation pushed out into the eave blocks soffit vents from the inside. Baffles at each rafter bay keep a clear air channel from the soffit up to the attic.
This is invisible once the soffit goes on, so it has to be checked before the panels are installed.
How much venting the attic actually needs
The usual rule is one square foot of net free vent area for every 150 square feet of attic floor, halved to 1 in 300 when the venting is balanced between low and high and a vapour retarder is present.
The words that matter are net free area. A vented soffit panel is mostly solid material with small perforations, and its net free area is a fraction of its size — often stated in square inches per linear foot on the specification sheet.
So the panel count from this calculator tells you the material. Whether it vents enough is a separate calculation using that net free area figure, and it is worth doing.
Intake and exhaust have to be balanced
Attic ventilation works as a system. Cool air enters low at the soffit and leaves high at the ridge, and the flow depends on both ends being open.
A ridge vent with blocked or insufficient soffit intake does not draw from the soffits — it pulls conditioned air out of the house through every ceiling penetration instead, which costs money all winter.
If anything, intake should slightly exceed exhaust. It is the cheaper half to add and the half most often found blocked by insulation shoved into the eaves.