Fascia is the board that closes off the ends of the rafters along the roof edge. It carries the gutter, it hides the rafter tails, and because it lives directly under the drip line it is the first exterior trim on a house to fail.
Measuring the run
Measure along every roof edge that gets a fascia board. On a simple gable house that is the two eaves, and on a hipped roof it is the whole perimeter.
Rake boards — the sloped trim running up the gable end — are usually a different profile and are measured separately. If you are doing them at the same time, remember the sloped length is longer than the horizontal span; a 6/12 roof rake is about twelve percent longer than the run beneath it.
Ten percent waste covers the mitres at corners and the offcuts. Corners on fascia are mitred, and like all mitres the first cut is often a test.
Buy long boards
Sixteen foot boards are the default here because a joint in a fascia run is a place for water to sit. Every joint you avoid is a failure point you avoid.
Where a joint is unavoidable, it should land on a rafter tail so both ends are supported, and it should be a scarf rather than a butt where the profile allows.
Nail joints tight and seal them. An open fascia joint wicks water into the end grain of both boards, which is the fastest way to rot a board from the inside.
Drip edge goes on with it
Drip edge is the metal trim that kicks water off the roof edge and out over the gutter instead of letting it run back along the underside of the sheathing.
It comes in ten foot sticks and overlaps a couple of inches at each joint, which is why the calculator allows about five percent over the raw length.
At the eave, drip edge goes under the underlayment. At the rake, it goes over. Getting that order backwards is common and it channels water into the roof deck instead of away from it.
Material choice
Primed finger-jointed pine is the cheapest and it is fine if it is sealed on all sides and kept painted. It is unforgiving of neglect.
Cellular PVC and fibre cement cost more up front and do not rot. On a house where getting a ladder to the fascia is difficult, that difference pays for itself.
Whatever you use, the back face and the cut ends need sealing before installation. Painting the front after it is up protects the one side that was never the problem.
Gutter loads
The fascia carries the gutter, and a full gutter is heavy — water weighs about eight pounds a gallon, and a forty foot run of five inch gutter holds a lot of it.
That load goes into the fascia and through it into the rafter tails. Hangers should be screwed into the rafter tails where possible, not just into the fascia board.
Fascia that has softened at all will not hold a gutter through a heavy rain, and a gutter that pulls away takes a section of board with it.