Soffit Calculator: Panels, Channel and Attic Vent Area

This soffit calculator works out three things at once: how many square feet of soffit you have, how many panels to buy once you account for how they're cut, and how much vented soffit your attic needs. Enter the total length of your eaves and how deep the soffit is from the wall to the back of the fascia. Add gable rakes if they're boxed in too.

How the panel count works

Soffit panels run from the wall out to the fascia, so each panel is cut into pieces as long as the soffit is deep. A 12-foot panel yields eight pieces for an 18-inch soffit (144 ÷ 18 = 8), but only six for a 24-inch soffit, with the leftover wasted. That's why two houses with the same square footage can need different numbers of panels. The calculator counts pieces along the run, divides by the pieces each panel yields, and then adds your waste allowance for corners, mistakes and damaged pieces. Ten percent is typical; use 15% on houses with many hips and corners.

Common vinyl soffit panels are 12 feet long and cover 12 inches (often sold as triple 4-inch) or 10 inches (double 5-inch). Aluminum soffit often covers 16 inches. Check the exposure on the product label.

Measuring

1. Measure each eave from corner to corner along the wall, then add them up.
2. Measure soffit depth at several spots, from the wall to the back of the fascia. Use the largest number.
3. For boxed gable rakes, measure their length along the slope and their depth separately.
4. Note inside and outside corners; each one adds a little waste.

Vented or solid soffit

Most houses with vented attics use vented soffit as the intake for attic ventilation, paired with a ridge vent or other high exhaust. The 2021 International Residential Code (section R806.2) asks for a net free vent area of 1/150 of the attic floor area, or 1/300 when 40 to 50 percent of the venting is high on the roof (within 3 feet of the ridge) and the rest is low, at the soffits. The calculator uses the 1/300 rule, since soffit-and-ridge systems are built to meet it.

Net free area (NFA) is the open area air can actually pass through, and it's much smaller than the panel's size. Manufacturers list it on the spec sheet, usually in square inches per square foot or per linear foot of panel. Enter that number to see how many square feet of vented panel you need.

Some practical points:

• Intake should equal or exceed exhaust. A ridge vent with too little soffit intake pulls conditioned air from the house instead.
• Vented soffit only works if air can get from the soffit into the attic. Check that insulation isn't stuffed into the eaves; install baffles (rafter vents) to keep a channel open.
• In very cold climates or with unvented (sealed) attics, the venting rules are different. Follow your local code.

Materials and trim

• Vinyl soffit: the most common and least expensive; won't rot; can sag over very deep overhangs without extra support.
• Aluminum soffit: stiffer and fire resistant; dents more easily.
• Wood (often beadboard or tongue and groove): traditional look; needs paint and good ventilation details.
• Fiber cement: durable and paintable; heavier and solid unless you buy vented panels.

Panels sit in F-channel or J-channel along the wall, and the outer end is covered by fascia trim. The calculator gives the channel length along the wall in 12-foot pieces. For the roof side of the ventilation, see the roof ridge cap guide, and for plastic siding and trim, see PVC siding.