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Soffit Calculator
Quick answer: Soffit area = (eave length + rake length) × overhang width in feet. A 12″-wide, 12-ft panel covers 12 sq ft. Allow J-channel for both the wall side and the fascia side — roughly twice your eave length.
Work out soffit square footage, how many panels that is, and the fascia and J-channel to go with it — for both the eave runs and the sloped gable rakes.
Results update automatically as you type.
How soffit is estimated
Soffit is the horizontal underside of a roof overhang. Estimating it comes down to one area calculation and three linear runs.
- Eave soffit runs along the horizontal edges of the roof, the sides where the gutters hang.
- Rake soffit runs up the sloped gable edges. Many houses have little or none, but on a gable-end overhang it can be a significant area.
- Fascia is the vertical board at the outer edge, covering the rafter tails. It runs the full perimeter of both eave and rake.
- J-channel receives the cut edge of the soffit panel. You need it on both the wall side and the fascia side, so roughly twice the eave length.
Panel coverage
Vinyl and aluminium soffit is normally sold in 12″ wide panels, 12 ft long, so one panel covers 12 sq ft. Some profiles come 16″ wide. The calculator divides your area by the actual panel coverage rather than assuming a standard, because the difference across a whole house is several panels.
Vented, solid, or a mix
| Type | Net free area | Use |
|---|---|---|
| Fully vented | ~5–9 sq in per lineal ft | Where you need intake ventilation — most eave runs |
| Centre-vented | ~2–5 sq in per lineal ft | A common compromise; vent strip down the middle |
| Solid | None | Rake/gable soffit, porch ceilings, and where there is no attic behind |
Order of installation
- Fascia board first if you are replacing it, so the soffit tucks behind.
- J-channel or F-channel against the wall, level, screwed to the wall or to a nailer.
- Receiving channel on the fascia side.
- Panels cut about 1/4″ short of the full span so they can expand — vinyl moves noticeably with temperature.
- Fascia cap or aluminium coil over the fascia board last.
Cut panels 1/4″ under the measured span. Vinyl soffit fitted tight buckles in summer heat, and a buckled run cannot be fixed without pulling the whole section.
Frequently asked questions
How do I calculate how much soffit I need?
Multiply the total eave length plus rake length by the overhang width in feet. A house with 120 ft of eave, 40 ft of rake and an 18 in overhang needs (120 + 40) × 1.5 = 240 sq ft, before waste.
How wide is a soffit panel?
Most vinyl and aluminium soffit is 12 in wide in 12 ft lengths, so each panel covers 12 sq ft. Some profiles are 16 in wide. Check the actual coverage on the box — the difference is several panels across a whole house.
Should soffit be vented or solid?
Vented wherever there is attic space behind it, because soffit vents supply nearly all the intake air for attic ventilation. Solid soffit is for porch ceilings and gable-end overhangs with no attic behind. Blocking soffit ventilation is a leading cause of ice damming and roof-deck rot.
How much should I cut soffit panels short?
About 1/4 inch under the measured span. Vinyl expands significantly with temperature, and panels fitted tight will buckle in summer heat.