Soffit Calculator: How Many Soffit Panels Do I Need?

Measure the total length of every eave where soffit will go and type it into Total eave length. Walk the house with a tape and add up each run along the fascia, including gable overhangs if they get soffit too. Then measure the Overhang width: the horizontal distance from the outside wall to the back of the fascia board, in inches. Press Calculate and you get the number of standard 12 in x 12 ft soffit panels to buy, already rounded up and padded with a 10% waste allowance.

Open More options if you are buying 16 in or 10 in panels, want metric units, need a different waste factor, or want a cost estimate. The calculator also tells you how many feet and 12-ft pieces of J-channel or F-channel to order for the wall side and fascia side of the soffit.

Soffit is the material that closes off the underside of your roof overhang, between the wall and the fascia. Vinyl and aluminum soffit is sold in panels that are almost always 12 feet long, in widths of 10, 12 or 16 inches. Because the panel length is fixed, the coverage of one panel in square feet equals its width in inches: a 12 in panel covers 12 sq ft and a 16 in panel covers 16 sq ft.

The total soffit area is simply eave length times overhang depth. A house with 120 feet of eaves and a 12 in overhang has 120 sq ft of soffit. Installers cut each panel into short pieces that span the overhang, so some material is lost at the ends, at outside corners and around hip roof transitions. That is why a waste allowance of about 10% is standard, rising to 15% on hip roofs with lots of corners.

Soffit panels also need trim. A J-channel or F-channel runs along the wall to receive the inner end of each piece, and another channel or the fascia trim holds the outer end. A good rule of thumb is two feet of channel for every foot of eave, bought in 12-ft sticks.

Soffit area (sq ft) = eave length (ft) x overhang (in) / 12
Order area = soffit area x (1 + waste %)
Panels = order area / panel coverage, rounded up
Panel coverage (sq ft) = panel width (in) x 12 ft / 12 = panel width in inches
J-channel (ft) = 2 x eave length
J-channel pieces = J-channel feet / 12, rounded up

Metric inputs are converted first: metres x 3.28084 = feet, centimetres / 2.54 = inches.

Example 1: 150 ft of eaves with a 16 in overhang. Area = 150 x 16 / 12 = 200 sq ft. Add 10% waste: 200 x 1.10 = 220 sq ft. With 12 in panels (12 sq ft each): 220 / 12 = 18.33, so buy 19 panels. J-channel = 2 x 150 = 300 ft, or 25 twelve-foot pieces.

Example 2: a 60 ft detached garage with a 12 in overhang. Area = 60 x 12 / 12 = 60 sq ft. With waste: 66 sq ft. Using 12 in panels: 66 / 12 = 5.5, so 6 panels. Switch to 16 in panels and you need 66 / 16 = 4.13, so 5 panels. J-channel = 120 ft, or 10 pieces.

Example 3: how much soffit for a 1,500 sq ft ranch house? A 30 x 50 ft ranch with a hip roof has soffit on all four sides: 160 ft of eaves. With an 18 in overhang the area is 160 x 18 / 12 = 240 sq ft. At 15% waste for the hip corners that is 276 sq ft, or 276 / 12 = 23 panels of 12 in soffit.

Measure to the back of the fascia, not the drip edge. The soffit sits between the wall and the fascia board, so that is the depth that counts.

Check cut yield on deep overhangs. A 12 ft panel cut for a 24 in overhang gives exactly 6 pieces, but a 26 in overhang only gives 5 with a lot of offcut. If your overhang does not divide evenly into 144 inches, choose the 15% waste setting.

Mix vented and solid panels. Building codes generally ask for balanced attic ventilation, so many installers use vented soffit along the eaves and solid soffit on gable overhangs and porch ceilings. Order the two types separately using each run length.

Buy one extra box or a few spare panels. Panel color and texture can vary between production runs, and a matching spare makes future repairs easy.

Do not forget fascia trim. Soffit usually goes in at the same time as fascia wrap, which is ordered by the same eave length.