Soffit calculator
Soffit area is the eave length times the overhang depth: 118 ft of eave with an 18 in overhang is 177 sq ft. Panels are counted by the run, because each one is cut into strips as deep as the overhang. Type the eaves, rakes and overhang, and this calculator gives the panels, F-channel and J-channel, fascia pieces or trim coil, and the vent area of the vented soffit. It starts with our sample house, measured by Measured Bid. Free, no account.
Eaves and overhang
Type the feet of eave and gable end (rake), and how far the overhang runs from the wall to the back of the fascia.
Level roof edges
Sloped gable ends
0 if no soffit under it
Per sq ft, vented
Alside Alliance T4, vented: 12 in × 12 ft 6 in, 7 sq in of net free area per square foot (Alside catalog, April 2026).
Share of soffit vented
- Eaves: 118 ft × 18 in
- 177 sq ft
- 8 cuts a panel at 18 in, + 10% waste
- 17 panels
- Vented 17, solid 0
- 177 sq ft vented
- Channel: 118 ft at the wall and at the fascia
- 11 + 11
- Fascia line: 118 ft eaves + 35 ft rakes
- 153 ft
- Vent area: 177 sq ft × 7 sq in
- 1,239 sq in
As intake, that's enough for an attic floor up to 2,581 sq ft at the 1/150 ratio, or 5,163 sq ft at 1/300, with half the vent area at the soffit. Check the attic.
Counts by the run: each panel is cut into strips as deep as the overhang. Add H-bar or channel at each eave corner, and starter or trim where your system needs it.
Worked out in your browser from the numbers you type. Nothing is looked up or saved. No prices are shown here. Confirm every number on site.
How to measure soffit and fascia
Soffit runs along the eaves, under the overhang. Fascia runs along every roof edge: the eaves and the gable ends. Measure them separately.
- Measure every eave: the level roof edges where gutters hang. Then measure the gable ends (rakes) along the slope.
- Measure the overhang from the wall to the back of the fascia. Measure the rake overhang too if it has soffit under it.
- Soffit area is eave length times overhang depth. Panels: divide the eave length by the panel width, then by how many strips one panel cuts into.
- Channel: one run along the wall and one along the fascia, in 12 ft 6 in lengths.
- Fascia: eaves plus rakes, in pieces or in strips cut from trim coil. Add waste to each.
ExampleOur sample house, vinyl triple 4 soffit, all vented
- Eaves: 118 ft × 18 in
- 177 sq ft
- 118 ft ÷ 12 in panels = 118 strips, 8 from each 12 ft 6 in panel
- 15 panels
- + 10% waste
- 17 panels
- F-channel at the wall and J-channel at the fascia, 12 ft 6 in each
- 11 + 11
- Fascia line: 118 ft eaves + 35 ft rakes
- 14 pieces
- Vent area: 177 sq ft × 7 sq in
- 1,239 sq in
The eaves and gable ends are the engine's measurements of our sample house, which has several roof sections, so its gable-end length is an estimate. The 18 in overhang is Measured Bid's starting overhang for wall area.
Soffit panels and their vent area
Vented soffit lets air into the attic at the eaves: the low half of a balanced intake and exhaust system. Each maker rates its vented panels for net free area: the open area air can pass through. The numbers here are from Alside's April 2026 catalog (Alside catalog), checked October 2026.
A deeper overhang cuts fewer strips from each panel, so the panel count rises faster than the square feet. Pick the panel length that wastes least at your overhang.
Need to know how much intake the attic needs? The attic ventilation calculator works it out from the attic floor.
Panels on our sample house, by overhang
| Panel | Vent area per sq ft | 12 in | 16 in | 18 in | 24 in |
|---|---|---|---|---|---|
| Vinyl triple 4 12 in × 12 ft 6 in | 7 sq in | 11 | 16 | 17 | 22 |
| Vinyl double 5 10 in × 12 ft | 6.4 sq in | 14 | 18 | 20 | 27 |
| Aluminum triple 4 12 in × 12 ft | 11.6 sq in | 11 | 16 | 17 | 22 |
| Aluminum quad 4 16 in × 12 ft | 5.8 sq in | 9 | 11 | 14 | 17 |
Panels with 10% waste for 118 ft of eave. Vinyl triple 4 is Alside's Alliance T4, vinyl double 5 its punched D5, aluminum triple 4 its fully aerated HT T4 and aluminum quad 4 its center-aerated HT Q4.
Channel, fascia and trim coil
CertainTeed's measuring guide has you measure F-channel along the wall under the eave, and J-channel along the wall and along the fascia board, with H-bar at the eave corners (CertainTeed). The calculator counts one run of each.
Alside lists its J-channel, F-channel and aluminum fascia in 12 ft 6 in lengths, and its trim coil 24 in wide and 50 ft long. A coil gives as many strips as fit across its width, each 50 ft long.
What goes where, on our sample house
| Piece | Where | Feet | To order |
|---|---|---|---|
| F-channel | Wall, under the eave | 118 | 11 |
| J-channel | Along the fascia | 118 | 11 |
| Aluminum fascia | Eaves and rakes | 153 | 14 |
| Trim coil, 8 in strips | Eaves and rakes | 153 | 2 coils |
With 10% waste. Three 8 in strips fit across a 24 in coil, so one coil covers 150 ft of fascia. The 8 in strip is a starting value for a 6 in board plus its bends; set your own.
Skip the tape: eaves and gable ends from an address
Enter an address and Measured Bid uses aerial imagery and property data to trace the eaves, number each run and add the gable ends, then prices fascia, soffit, wrap or paint with your per-foot rates.
On roofs with several sections the gable-end length is an estimate, and the estimate says so. Measure the overhang and check the boards on site before you order.
Price it with your rates
Panels and channel are the order. In Measured Bid you price fascia, wrap, soffit and paint per foot at each story, with removal, a steep-roof adjustment and a job minimum.
FAQ
How do I calculate soffit square footage?
Multiply the eave length by the overhang depth in feet. 150 ft of eave with an 18 in overhang is 225 sq ft. Add the gable ends the same way when they have an overhang with soffit under it.
How many soffit panels do I need?
Count them by the run. Panels run from the wall out to the fascia, so divide the eave length by the panel width for the number of strips, then by how many strips one panel cuts into. Alside's Alliance T4 panel is 12 in wide and 12 ft 6 in long, so it cuts 8 strips for an 18 in overhang.
How much vent area does vented soffit give?
It depends on the panel. Alside's April 2026 catalog lists 7 sq in of net free area per square foot for its vented Alliance T4 vinyl soffit, 11.6 for its fully aerated aluminum T4 and 3.9 for its center-aerated aluminum T4. Use the number for the panel you install.
How much vented soffit do I need?
Enough for at least half the attic's vent area. At the code's 1/150 ratio that's 0.48 sq in of intake for each square foot of attic floor. A 1,200 sq ft attic needs 576 sq in of intake: about 83 sq ft of vented T4 vinyl soffit at 7 sq in per square foot.
Do I need J-channel and F-channel for soffit?
Most vinyl soffit jobs use both. CertainTeed's measuring guide has you measure F-channel along the wall under the eave, and J-channel along the wall and along the fascia board. The calculator counts one run at the wall and one at the fascia, in the 12 ft 6 in lengths Alside lists.
How much fascia or trim coil do I need?
Fascia runs along every eave and gable end. Alside's aluminum fascia comes 6 or 8 in tall in 12 ft 6 in lengths, and its trim coil is 24 in wide and 50 ft long. Cut three 8 in strips from the coil and one roll covers 150 ft of fascia, before waste.
Related pages and tools
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