Measure the run, then the depth
Soffit covers the underside of the eave overhang. Walk the house with a tape and total every eave run — front, back, both sides, plus any porch or bay overhangs — then multiply by the eave depth (usually 16–24 inches from wall to fascia). Enter the run as length and the depth as width above.
Solid or vented — the 1:150 rule
Attics need 1 sq ft of net free ventilation per 150 sq ft of attic floor, and the soffit supplies roughly half of that at the intake. In practice that means every other panel is vented on most houses — order a vented/solid mix rather than all-solid, or the attic will breathe only through the ridge.
Aluminum or vinyl
Both ship as 12-ft strips, 12 or 16 inches wide. Aluminum costs more and dents less; vinyl is cheaper and cuts with a utility knife. Either way, nail the J-channel at the wall and fascia first — the panels themselves take zero nails, they float in the channels so the metal can move with the seasons.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- National Roofing Contractors Association — installation standards for steep- and low-slope roof systems.
- FEMA Building Science — wind and snow load guidance for residential roofs.
- American Society of Civil Engineers — ASCE 7, the source of the ground snow load maps used in roof design.
Figures, formulas and reference tables last verified: 19 September 2026.