The soffit width is not the overhang
People measure the overhang from the edge of the roof sheathing and then order panel to that number. The soffit is narrower than that, because the fascia board stands at the outer end and the panel dies into a channel behind it. Measure horizontally from the face of the wall, or the face of the siding if the siding is already on, to the back of the fascia. On a house with 18 inches of roof overhang the soffit is commonly 15 or 16 inches. An inch of error over 140 feet of eave is not a rounding problem, it is a box of panel.
Why the panel count is not just area divided by panel size
Soffit panels run perpendicular to the eave. A 12 foot panel does not cover 12 feet of eave, it gets cut into pieces the width of the soffit, and each piece covers only its own locked width of run. That is what this calculator is doing: it works out how many pieces you need along the run, then how many pieces come out of one panel, and only then how many panels to buy. With a 16 inch soffit you get nine pieces from a 12 foot panel with no drop at all, because nine times 16 inches is exactly 12 feet. With a 17 inch soffit you get eight and lose 8 inches off every panel, which is a five percent penalty for an inch of width.
That relationship is worth checking before you buy, because it is one of the few places in construction where a slightly different dimension is free money. If the soffit width divides evenly into the panel length, the drop is zero.
Fascia goes on the rakes too
Soffit may not run up the rakes, but fascia almost always does, which is why this page counts the two runs separately. If your gables are open with exposed rafter tails there is no rake fascia and no rake soffit, and both figures drop out. If the gables have boxed returns, both apply and the rake cuts are all angles.
Fascia length also matters more than most trim, because a splice in a fascia board sits at eye level along a straight line and it will be the thing you notice from the driveway for the next twenty years. Buying 16 foot stock instead of 12 to reduce the number of joints is usually worth the handling.
The eave is where ventilation gets in
Continuous vented soffit is the standard intake path for a vented attic, and the number that matters is not how much panel is vented but the net free area of the specific product, which varies from about five to over twenty square inches per linear foot. That figure comes from the manufacturer sheet and nowhere else. The output above turns your vented run into a total so you can carry it into the attic ventilation calculator and see whether the intake actually balances the exhaust.
Two things kill soffit intake regardless of how good the panel is: insulation stuffed into the eave, which blocks the path at the top plate, and paint. If you are replacing rotted soffit, look up into the bay while it is open and check whether air can actually get from the vent into the attic.
A word about getting up there
Soffit and fascia work happens at the roof edge, which is exactly where falls happen. Falls from roofs and from the edges of roofs kill and disable people every year, and the risk does not scale with the size of the repair. Replacing three feet of rotted fascia puts you in the same place as replacing all of it. Steep roofs, wet roofs and high roofs are not a do-it-yourself job. A page can tell you how many panels to buy and it cannot make an eave safe to work from. If any part of that gives you pause, the material list still has value and someone else does the installation. Ongoing checks that keep the eave from getting to this state in the first place are in the home maintenance schedule.
Questions people ask
How do I measure the soffit width without a ladder?
From the ground you can measure the horizontal distance from the wall to the outer face of the fascia by holding a tape out at arm level and sighting up, but it will be approximate. A better trick is to measure the roof overhang at a gable end, where the rake board is at a reachable height at the low corner, then subtract the thickness of the fascia and any subfascia. If the soffit is already panelled, count the number of panel locks visible across the width and multiply by the coverage width. Where the number is close to a boundary that changes how many pieces come out of a panel, it is worth getting it exactly.
Do I need channel on both ends of the panel?
That depends on the product and the detail. The wall side almost always gets a channel, either fastened to the wall or to a nailer, because the panel has to be held without being face-nailed. At the outer end the panel may sit in a channel behind the fascia, or the fascia cover may capture it, or it may nail to a subfascia. Vinyl soffit needs room to move with temperature, so the detail that captures it without pinning it is not optional. Read the installation sheet for the panel you bought.
Can I put vented soffit everywhere?
Physically yes, but it does nothing where there is no ventilated space above it, and it may be actively unhelpful. Over a porch ceiling with a closed cavity, or over a section where the roof above is a cathedral ceiling with no airway, vented panel just opens a hole into a dead space that insects can find. It also costs more than solid panel. The usual approach is vented over attic space and solid elsewhere. Where the roof assembly is unvented by design, the whole intake question changes and it is worth confirming what you actually have before you buy.
What is the fascia board width field for?
It does not change the linear footage, which is what you actually buy fascia by. It is there so you can sanity-check against what is on the house: if the existing fascia is a 1x8 and you buy 1x6, the new board will not cover the sub-fascia and the gutter will not land where the old one did. Measure the exposed face of the existing board before you order. If the roof edge is being redone at the same time, the drip edge and its overlap are handled by the existing underlayment and drip edge calculator.
Why is the waste allowance so high for the rakes?
Because every rake piece is cut at the roof angle, and the offcut from the left side of a gable is the mirror image of what the right side needs, so it does not fit. On an eave the cuts are square and a leftover piece can start the next run. On a rake it goes in the bin. If most of your run is rake, ten percent is the floor rather than the answer, and fifteen is more realistic.