Why the tape length is not the perimeter
The obvious way to estimate flashing tape is to work out the perimeter of the opening and buy that. It is short by a third or more, for two reasons that both come from how flashing sheds water rather than how it is measured.
The first is that jamb and head pieces run past the corners. A jamb piece stops flush with the top of the opening does nothing at the corner, which is the single place water concentrates. Running it six inches past gives the head piece something to lap over and covers the corner twice. The second is that the sill piece runs further still, because it turns up the jambs on each side to make a tray rather than a flat strip. Add both of those and a 36 by 60 inch opening needs about nineteen linear feet of tape rather than the sixteen its perimeter suggests.
Then there is waste, which is higher on flashing tape than on almost any other material because the tape is aggressive, it grabs on contact, and a piece that folds onto itself is finished. Twelve percent is a reasonable default on straightforward openings and low if the weather is cold or windy.
The sill pan is the part that does the work
Everything above the sill is protection against water that runs down the face. The sill is protection against water that has already got in, and some always does. A sill pan is a tray under the unit with the back and both ends turned up, so that anything arriving there is caught, held away from the framing and directed back outward. It is the difference between a leak that shows up as a stain on a ceiling in year six and one that never happens.
The bottom corners are the hardest part of the tray and they are where failures concentrate. Tape does not fold into a three-dimensional corner without either bunching or leaving a pinhole, which is why preformed corner pieces exist and why this calculator counts two per opening as a separate line. Two extra pieces per window is a trivial cost against having to open a wall up.
The sill joint is left open on purpose
The perimeter sealant field defaults to sealing the jambs and the head and leaving the sill alone, and that is not an oversight. A wall assembly is designed to shed most water and drain the rest. If the perimeter of a window is sealed all the way round, the small amount of water that does get behind the unit has no way out, and it sits on the sill and on the framing under it. Leaving the sill joint open gives that water a path back to the outside.
There are assemblies where a fully sealed perimeter is what the manufacturer specifies, which is why the checkbox exists. What is not defensible is sealing all round because it looked unfinished otherwise. If you do not know which one you are building, that is a question for the unit installation instructions, and it is worth ten minutes.
Getting the bead cross-section right
Sealant quantity comes out of geometry: a joint that is three eighths of an inch wide and a quarter inch deep holds 0.09 cubic inches per inch of run, which is about 1.13 cubic inches per foot, so a 10.1 fluid ounce cartridge at roughly 18 cubic inches covers about sixteen feet. That is far less than the coverage figures printed on cartridges, which are quoted for a small bead.
The depth is set by the backer rod, and the rod is there for that reason rather than to save sealant. A sealant joint that is deeper than it is wide cannot stretch when the joint moves and tends to fail at the back. Rod diameter runs about twenty five percent over the joint width so it stays compressed and does not creep out, and it also gives the sealant a surface it will not bond to, which lets the bead stretch across the joint instead of being anchored on three sides. For general bead maths beyond openings, the caulk coverage calculator handles joints of any shape.
What this page will not tell you
It gives you no installation sequence, and that omission is the most important thing on the page. Flashing works by lapping, each piece over the one below it, so that water running down the wall stays on the outside of every layer it passes. Reverse two of those laps and the assembly looks exactly the same once the trim is on and channels water into the wall instead of out of it. The sequence depends on whether the unit has an integral nailing fin, on the weather barrier product, and on whether the wall has a drainage gap behind the cladding, and the manufacturers of both the unit and the barrier publish it. Follow theirs.
Nor does it decide what tape to use, and that matters more than it sounds like it should. Adhesives are chemistry, and some flashing tapes are incompatible with some sealants, some sheathing facings and some housewraps, in ways that show up months later as tape lifting off in sheets. Compatibility is published, and the wall around the opening is the subject of the house wrap calculator. If you are here because something is already leaking, start with the water leak guide rather than with tape, and if the opening itself is being changed, size it on the rough opening calculator before ordering any of this.
Questions people ask
How much flashing tape does one window need?
For a 36 by 60 inch rough opening with six inch overhangs, about nineteen linear feet: a 48 inch sill piece, two jamb pieces of 66 inches each, and a 48 inch head piece. Add waste and call it twenty two feet bought. The perimeter of that opening is only sixteen feet, so estimating from the perimeter leaves you short on every single opening, which is exactly how people end up splicing a jamb piece halfway up.
What width of flashing tape should I use?
Wide enough to cover the nailing fin or frame edge and land properly on the sheathing or weather barrier beyond it, which usually means four inches at a minimum and six inches as the common working choice. Nine and twelve inch tapes get used at sills where the piece has to form a pan and turn up the jambs, because a narrow tape simply cannot make that shape. Wider tape costs more per foot and covers mistakes; narrow tape is cheaper and less forgiving.
Do I need a sill pan if the window has a nailing fin?
The fin sheds water off the face of the wall. It does not catch water that gets past the unit itself, past a failed sealant joint or around a fastener, and something always eventually does. A sill pan is what catches that and sends it back out. Whether one is required in your work is set by the adopted code and the manufacturer instructions, and whether the warranty survives without one is a manufacturer question. In terms of what keeps a wall dry over decades, the sill detail does more than any other part of the assembly.
How many tubes of sealant for a house full of windows?
Work it from the bead. A three eighths by quarter inch joint uses about 1.13 cubic inches per foot, so a standard 10.1 ounce cartridge covers roughly sixteen feet of that joint. Ten windows at about thirteen feet of joint each, jambs and head with the sill left open, is 130 feet, which is eight or nine cartridges. Buy ten. Cartridges skin over once opened and the last third of a partly used tube is usually wasted anyway.
Can I flash over the housewrap or does it go under?
It depends on the piece and that is precisely why this page gives no sequence. Some pieces lap over the barrier and some tuck behind it, and getting one of them backwards creates a funnel into the wall. The correct arrangement is published by the weather barrier manufacturer and by the window manufacturer, they sometimes differ in detail, and where they differ the installation instructions for the unit generally govern for warranty purposes. Read both before the first piece goes on, because none of it is visible or fixable once the cladding is up.