Soffit Calculator
Forty feet of eave at a 12 inch overhang is 40.00 ft². Triple 4 inch full-vent soffit on that area is 4 pieces, 1 carton, and 573.60 in² of net free area.
Figures checked against the Georgia-Pacific vinyl spec. Eave opens at 40 ft, rake at 0, overhang at 12 in, panel on 151-02.
A soffit calculator for the flat area under the eaves and rakes, the carton count from the spec, and the vented panel’s net free area.
Wall and gable siding squares: siding calculator. The attic’s required net free area: attic ventilation calculator.
Soffit area, pieces, and vent area
Measure the eave and the rake along the fascia, then the overhang from the wall to the fascia. Carton counts and net free air are the printed Georgia-Pacific rows. The attic’s required vent area is calculated on the attic ventilation page, not here.
Results
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Cartons and net free air from one spec
Source: Georgia-Pacific Vinyl Siding Spec Guide, GP21 Rev D, file GP21_Georgia-Pacific-Spec-Guide_4101099991101_RevD_MS_0221. The soffit packaging pages are the guide pages numbered 15 through 19. Each vented row below uses the net free air printed on that row, in square inches per square foot. Solid rows have no net free air line in the spec.
| Panel | Pieces | Squares | Net free air, in²/ft² |
|---|---|---|---|
| Triple 4" Full-Vent (151-02) | 16 | 1.92 | 14.34 |
| Triple 4" Center Vent (152-02) | 16 | 1.92 | 4.78 |
| Triple 3-1/3" Vented (134) | 20 | 2 | 9.19 |
| Premium Double 5" Perforated (141) | 20 | 2 | 6.2 |
| Double 5" Perforated (145) | 20 | 2 | 6.2 |
| Premium Triple 4" Center Vent (231) | 16 | 1.92 | 1.956 |
| Premium Triple 4" Full Vent (232) | 16 | 1.92 | 5.867 |
| Beaded Hidden-Vent (158) | 28 | 2 | 2.66 |
| Economy Triple 4" Center Vent (2319216) | 16 | 1.92 | 1.956 |
| Economy Triple 4" Full Vent (2329216) | 16 | 1.92 | 5.867 |
| Economy Double 5" Full Vent (1459216) | 20 | 2 | 6.2 |
| Triple 4" Solid (150-02) | 16 | 1.92 | not printed |
| Triple 3-1/3" Solid (133) | 20 | 2 | not printed |
| Premium Double 5" Solid (140) | 20 | 2 | not printed |
| Standard Double 5" Solid (144) | 20 | 2 | not printed |
| Premium Triple 4" Solid (230) | 16 | 1.92 | not printed |
| Beaded Solid (157) | 28 | 2 | not printed |
| Economy Triple 4" Solid (2309216) | 16 | 1.92 | not printed |
| Economy Double 5" Solid (1449216) | 20 | 2 | not printed |
Area is the eave length plus the rake length, times the overhang depth. Pieces are that area divided by the square feet in one piece, rounded up. Cartons are the pieces divided by the pieces in one carton, rounded up. Net free area, when the row has one, is the soffit area times the printed rate. 40 × 14.34 = 573.60.
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Worked areas
- 40 ft eave, no rake, 12 in overhang, panel 151-02: 40.00 ft², 4 pieces, 1 carton, 573.60 in².
- 80 ft eave, 36 ft rake, 2 ft overhang, same panel: 232.00 ft², 20 pieces, 2 cartons, 3,326.88 in².
- 100 ft eave, 2 ft overhang, beaded hidden vent 158: 200.00 ft², 28 pieces, 1 carton, 532.00 in².
Take the net free area to the attic page and compare it with the intake the code section there requires. This soffit line is only the product’s free area on the area you measured. It does not decide whether the attic is balanced.
Soffit questions
What does a 40 foot eave at 12 inches of overhang order?
Leave the eave at 40 ft, the rake at 0, the overhang at 12 in, and the panel on Triple 4" Full-Vent (151-02). Soffit area reads 40.00 ft². Piece coverage reads 12.0000 ft² per piece · 1.92 squares / 16 pieces. Pieces reads 4. Cartons reads 1. Net free area reads 573.60 in².
How many cartons cover 80 feet of eave plus 36 feet of rake at 2 feet deep?
Set the eave to 80 ft, the rake to 36 ft, and the overhang to 2 ft. Keep Triple 4" Full-Vent. Soffit area reads 232.00 ft². Pieces reads 20. Cartons reads 2. Net free area reads 3,326.88 in². The run is 116 feet, times 2 feet is 232. 232 ÷ 12 rounds up to 20 pieces, and 20 ÷ 16 rounds up to 2 cartons.
What does the beaded hidden-vent row do with 200 square feet?
Set the eave to 100 ft, the rake to 0, the overhang to 2 ft, and the panel to Beaded Hidden-Vent (158). Soffit area reads 200.00 ft². Piece coverage reads 7.1429 ft² per piece · 2 squares / 28 pieces. Pieces reads 28. Cartons reads 1. Net free area reads 532.00 in². Two squares are 200 ft², and 200 ÷ 28 pieces is 7.1429 ft² each, so 200 ft² is one full carton.
Where do 1.92 squares and 2 squares come from?
The Georgia-Pacific spec prints 16 pieces per carton / 1.92 squares on the triple 4 inch soffit rows, and 20 pieces per carton / 2 squares on the triple 3-1/3 inch and double 5 inch rows. The beaded row is 28 pieces per carton / 2 squares. A square in that packaging is 100 square feet, so 1.92 squares is 192 square feet and 16 pieces are 12 square feet each.
Is the net free area the attic requirement?
No. The square-inch line is the area of this soffit times the net free air the spec prints for that vented panel. A solid panel has no such line. How much net free area the attic needs, and how it splits between intake and exhaust, is the attic ventilation calculator.
Does a solid panel show zero vent area?
It shows the sentence “Solid panel. No net free air is printed.” The spec simply does not list a net free air number on the solid rows, so this page does not type a zero into that field and call it a measurement.