Enter the total length of your eaves, how deep the soffit is, and the size of your soffit panels. The calculator works out the panels, J-channel and fascia you need, and how much intake venting your attic should have.
Extra and ventilation
Venting rules depend on your local code and roof design. The figure here uses common code ratios, with half the venting at the soffits.
How to measure soffit
- Length: measure along every eave that gets soffit, and add the rakes too if they’re boxed in.
- Depth: measure from the house wall to the back of the fascia board.
How soffit panels are counted
Soffit panels run across the overhang, from the wall to the fascia. Each long panel is cut into short pieces as long as the soffit is deep.
- Pieces from each panel = panel length ÷ soffit depth, rounded down
- Pieces needed = eave length ÷ panel coverage width
- Panels = pieces needed ÷ pieces from each panel, plus extra
Worked example
150 ft of eaves, 16 inches deep, with 12 ft panels that cover 12 inches:
- Each panel cuts into 9 pieces 16 inches long
- 150 pieces are needed, so 17 panels, or 19 with 10 percent extra
- Soffit area: 200 square feet
- J-channel or F-channel: 300 ft, along the wall and the fascia
- Fascia: 150 ft
Trim
- F-channel or J-channel along the wall holds the inside edge of the panels.
- J-channel or a fascia lip holds the outside edge.
- Fascia cover wraps the fascia board, usually in aluminum coil or fascia trim.
Soffit venting
Attics need fresh air coming in at the soffits and going out near the ridge. A common US code rule is 1 square foot of net free vent area for every 150 square feet of attic floor, or 1 per 300 with balanced high and low venting. About half of that should be intake at the soffits.
Example: a 1,500 square foot attic at 1 per 150 needs 10 square feet (1,440 square inches) of venting, so about 720 square inches at the soffits. Vented soffit panels list their net free area on the package, so use that to work out how many need to be vented.
Frequently asked questions
How many soffit panels do I need?
Work out how many pieces each panel gives at your soffit depth, then divide the number of pieces needed along the eaves by that. Add about 10 percent.
How do I measure for soffit?
Measure the total length of the eaves and the depth from the wall to the back of the fascia.
How much soffit venting do I need?
A common rule is 1 square foot of vent area per 150 square feet of attic floor, with about half at the soffits. Check your local code.
Should all my soffit be vented?
Not always. Many homes mix vented and solid panels to get the right amount of intake.
What size are soffit panels?
Many vinyl and aluminum soffit panels are 12 ft long and cover 12 or 16 inches. Check the product sheet.
Results are estimates for planning. Venting rules depend on your local code. See How We Calculate for the formulas we use.
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