Soffit Calculator
A soffit calculator multiplies eave perimeter by overhang width: 140 ft of eave with an 18 in overhang needs 210 sq ft of soffit before any waste.
Soffit calculator
A soffit calculator finds the area under your roof’s eaves from two measurements: the eave perimeter and the overhang width. The area equals the perimeter times the overhang in feet. A house with 140 ft of eave and an 18 inch overhang has 140 × 1.5 = 210 sq ft of soffit.
That is the net area. Order extra for cuts and corners.
The formula in plain words
Soffit area (sq ft) = eave perimeter (ft) × overhang width (in ÷ 12).
Think of the soffit as a long, narrow strip that runs all the way around the house. Its length is the perimeter and its width is the overhang. The fascia, the board at the front edge, and the J-channel or trim are measured separately.
Worked examples
The default, 140 ft of eave and an 18 in overhang:
- Overhang in feet: 18 ÷ 12 = 1.5
- Area: 140 × 1.5 = 210 sq ft
- With 10% extra: 231 sq ft
- In 12 sq ft panels: 231 ÷ 12 = 19.25, rounded up to 20
A larger house, 180 ft of eave with a 24 in overhang:
- Overhang in feet: 2
- Area: 180 × 2 = 360 sq ft
- With 10% extra: 396 sq ft
A narrow overhang on a shed or garage, 200 ft with 12 in:
- Area: 200 × 1 = 200 sq ft
Soffit area per 100 ft of eave
| Overhang | Area per 100 ft | Notes |
|---|---|---|
| 12 in | 100 sq ft | Tight eave |
| 18 in | 150 sq ft | Common on many homes |
| 24 in | 200 sq ft | Wider overhang |
| 30 in | 250 sq ft | Deep eave |
| 36 in | 300 sq ft | Porches and gables |
Multiply by your perimeter divided by 100. The product follows from the formula, so you can check any value by hand.
From square feet to panels
Soffit panels come in different widths and lengths, so check your product. As an example, a panel 12 in wide and 12 ft long covers 12 sq ft. Divide your area by that and round up.
- 210 sq ft ÷ 12 = 17.5, rounded up to 18 panels
- With 10% extra: 20 panels
Also count the trim. J-channel or F-channel runs along the wall side, and a channel along the fascia holds the other edge, so add linear feet for those.
How to measure the eave perimeter
- Measure each side of the house at the wall, then add the overhang at each corner.
- If the house is an L shape or has bump-outs, add each extra run.
- Decide whether the rakes (the sloped gable edges) are boxed in with soffit. If so, include them.
- For a hip roof, the eave runs all the way around.
A tape and a helper make this quick from the ground. Use a ladder only when needed, and follow fall safety rules.
Measuring the overhang
Hold a tape against the wall and measure out to the fascia. Overhangs often vary, especially around porches, bays and additions. If you see a difference, measure each section.
Ventilation
Attic ventilation relies on intake low at the soffit and exhaust high at the ridge. If you replace solid soffit with vented soffit, you add intake. The common code ratio is about 1 sq ft of vent area per 150 sq ft of attic floor, and rules vary. Look at the product’s net free area rating and your local code. Do not block existing vents with insulation.
Common mistakes
- Entering the overhang in feet. The box takes inches.
- Using the wall perimeter. The roof perimeter is larger because of the overhangs.
- Skipping the trim channels in the order.
- Ignoring the rakes when they are boxed.
- Ordering exactly the net area and running short after cuts.
Related calculators
The roof area calculator covers the roof surface, the gutter cost calculator prices the gutter along the same eave, and the vinyl siding calculator handles the walls below. The gutter size calculator helps pick a gutter that fits the roof.
Frequently asked questions
What is soffit and where is it on my house?
Soffit is the underside of the roof overhang, the flat strip you see when you look up at the eaves from outside. It closes the gap between the wall and the roof edge. Vented soffit also lets air flow into the attic.
How do I measure my eave perimeter?
Add up the length of every eave and, if they are boxed in, every rake, along the outer edge of the roof. A rectangular house 50 ft by 20 ft with overhangs on all four sides has about 140 ft of perimeter. Measure from the ground with a long tape, or use the building's wall lengths plus the overhang at each corner.
Does the overhang width need to be the same all around?
The calculator uses one overhang figure. If the overhang differs between sides, run each side separately and add the results. A front porch with a deeper overhang is a common example.
Do I need extra soffit for vents?
Vented soffit panels replace solid panels, so they do not add area. They do change your ventilation. A common code ratio is 1 sq ft of net free vent area per 150 sq ft of attic floor, with exceptions, so check your local code and the vent product's rating.
How much extra should I order?
Add roughly 10 percent for cuts, corners and mistakes. The calculator shows net area. At 210 sq ft, 10 percent more is 231 sq ft, and panels sold at 12 sq ft each come to 20 panels.
Estimates only. Verify quantities with your supplier before purchasing.