Soffit Calculator: Panels, Trim and Intake Vents

How to use the soffit calculator

1. Measure the eaves along the wall and add them up, with any boxed-in rakes.
2. Measure the overhang depth from the wall to the back of the fascia board, in inches. If it changes around the house, run the calculator once per depth.
3. Count outside corners. A plain hip roof has four. Each one needs a square of mitered panels that a straight eave measurement misses.
4. Pick the panel: 12 in wide Triple 4 or 10 in wide Double 5, both 12 ft long. The net free area changes with it; replace it with the figure on your product’s spec sheet.
5. Enter the attic floor area and the code ratio to see how many of the pieces should be vented.

You get panels and cartons, the cut length, vented and solid pieces, and the channel and fascia footage.

Why the count comes from a cut list

Soffit panels run across the overhang, from the wall to the fascia, so each 12 ft panel is cut into short pieces. The Vinyl Siding Institute’s installation manual, now published by PEPA, says to measure wall to fascia and subtract ½ in for expansion. On a 16 in overhang that is a 15½ in piece, and a 144 in panel yields nine of them with 4½ in left over. Dividing area by 12 sq ft per panel undercounts, because the offcuts are waste.

A 100 ft eave takes 100 pieces of a 12 in panel or 120 of a 10 in panel. Corners add more: the panels meeting at a hip corner are miter-cut from both sides, so the calculator adds two pieces per panel width of depth at each outside corner. Over 16 in, PEPA calls for nailing strips or extra framing so every panel is fastened, unless the soffit maker’s instructions say otherwise.

Worked example: a hip-roof ranch

• 120 ft of eave, 16 in overhang, four outside corners, Triple 4 panels.
• Straight runs: 120 ft × 12 ÷ 12 in = 120 pieces. Corners: 4 × 2 × 2 = 16 pieces. Total 136.
• Panels: 136 ÷ 9 = 15.1, so 16; with 5% for breakage, 17 panels, two cartons of 16.
• Trim: 120 ft of wall channel and about 131 ft of fascia cover, two overhang depths more per corner.
• Ventilation: a 1,500 sq ft attic at 1/150 needs 1,440 sq in of net free area, half of it, 720 sq in, from the soffit. Each 15½ in vented piece at 5.9 sq in per sq ft supplies about 7.6 sq in, so 95 of the 136 pieces should be vented.

Attic intake: what IRC R806.2 asks for

Section R806.2 reads the same in the 2021 and 2024 International Residential Code: the net free ventilating area has to be at least 1/150 of the vented attic floor area. It drops to 1/300 only when both conditions are met: 40 to 50% of that area sits in upper vents no more than 3 ft below the ridge, with the rest in the bottom third of the attic, and in climate zones 6, 7 and 8 the ceiling has a Class I or II vapor retarder. The calculator defaults to 1/150, the stricter case, because whether the exception applies depends on details only you and your inspector can confirm.

The soffit is the intake half of the system. IIBEC, the building-enclosure consultants’ institute, puts it plainly: intake should equal or exceed exhaust, or the attic pulls air from the house through light fixtures and the hatch. Under the 1/300 exception that means 50 to 60% low.

Shallow eaves are where the rule bites. A gable house with two 40 ft eaves, a 12 in overhang and a 1,200 sq ft attic needs 576 sq in of intake at 1/150, but a fully vented Triple 4 soffit there supplies about 452. At 1/300 it needs 288 and passes. A 4.0 sq in panel, the figure DuraSpan prints for its 12 in full vent, would leave the hip-roof example above short as well. When the soffit cannot do it, add other low vents rather than skimping on intake.

Vents only count if air reaches them. R806.3 requires at least 1 in between the insulation and the roof sheathing at each vent, which is what rafter baffles are for; the insulation calculator works out the attic insulation behind them.

Channel, fascia and the rest of the eave

Panels slide into a receiving channel at the wall, either J-channel or F-channel, and are fastened at the fascia, which is then wrapped with an aluminum fascia cover lapped ¾ in at the joints. The calculator gives both runs in linear feet; divide by the length of the trim you buy and add a piece per corner for the laps and miters.

This is also the moment to look at the wood behind it. Gutters hang on the fascia board, and rot there is cheaper to fix while the soffit is open. The gutter installation cost calculator prices new gutters and fascia repair, and the siding calculator covers the walls below.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Panels run from wall to fascia; cut length = depth − ½ in (VSI/PEPA). Pieces per 12 ft panel = floor(144 ÷ cut length).
• Straight pieces = eave length × 12 ÷ panel coverage (12 in Triple 4 or 10 in Double 5), rounded up. Each outside corner adds 2 × ceil(depth ÷ coverage) mitered pieces.
• Panels = pieces ÷ pieces per panel, rounded up, plus the breakage allowance. Cartons hold 16 Triple 4 or 20 Double 5 panels (DuraSpan; CertainTeed 16 Triple 4 panels, 192 sq ft).
• Required ventilation = attic floor area ÷ 150 (or ÷ 300 when both R806.2 exception conditions are met) × 144 sq in. R806.2 is worded the same in the 2021 and 2024 IRC; the 1/150 default is the stricter case.
• The soffit supplies the intake share, 50% by default (IIBEC: intake at least equal to exhaust; under the 1/300 exception, upper vents are 40–50%, so 50–60% stays low).
• Net free area per sq ft of vented panel from spec sheets: CertainTeed Universal Triple 4 fully vented 5.9 sq in, Double 5 fully vented 6.4; DuraSpan 12 in full vent 4.0 per lineal ft. Use the figure printed for your product.
• Channel = eave length; fascia cover = eave length + 2 × depth per outside corner. Spans over 16 in need nailing strips (VSI/PEPA).

Sources

1. UpCodes, 2021 International Residential Code, R806.2 Minimum vent area (Colorado DFPC edition). 1/150 of the vented space; 1/300 when both conditions are met: Class I or II vapor retarder in climate zones 6–8, and 40–50% of the area in upper vents not more than 3 ft below the ridge, the rest in the bottom third.
2. International Code Council, via O’Hagin, 2024 International Residential Code, Section R806 Roof ventilation (page 468 reprint). Same R806.2 wording as 2021; R806.3 requires a 1 in space between insulation and roof sheathing at the vent.
3. IIBEC, Attic Ventilation 101. Required area = attic floor area ÷ 150 or ÷ 300; intake area should be equal to or more than exhaust area.
4. Polymeric Exterior Products Association (formerly VSI), Soffit Installation, Vinyl Siding Installation Manual. Measure wall to fascia and subtract ½ in; ¼ in at corners, miter-cut corner panels; nailing strips over 16 in spans; soffit individually fastened per the IRC.
5. CertainTeed, Vinyl Carpentry Universal Soffit specification sheet. Net free air space: Triple 4 fully vented 5.9 sq in, center vented 2.0, Double 5 fully vented 6.4 sq in per sq ft.
6. Cameron Ashley / DuraSpan, DuraSpan Vinyl Soffit and Porch Ceiling brochure. 12 ft panels; 12 in: 16 per carton, 192 sq ft, full vent 4.0 sq in per lineal ft; 10 in Double 5: 20 per carton, full vent 3.8; InVent 9.3 and 7.1 sq in per sq ft.

Last reviewed October 29, 2026. Product figures vary by brand; the label on what you buy wins.