The piece is not the frame
A 30.5 by 26.5 inch frame with a 1 inch overhang all round is a 32.5 by 28.5 inch piece of mesh. Off a 36 inch roll, one of those fits across the width and the roll gives up 28.5 inches of length for each. Turn the piece the other way — 28.5 across, 32.5 along — and it still only fits one across, but now each costs 32.5 inches of roll. Four screens is 114 inches one way and 130 the other, for identical screens. The page tries both orientations and reports the cheaper one.
Where the overhang bites
The 34 by 22 kitchen screen becomes a 36 by 24 piece at a 1 inch overhang, which fits a 36 inch roll exactly and with nothing to spare. Go to a 1.25 inch overhang and the piece is 36.5 wide, does not fit that way round, and has to be cut as 36.5 along the roll instead — half again the roll length for one screen. That step is invisible until it happens at the bench.
Spline and stock are separate orders
The four default lines are eight screens: 94.8 feet of groove, plus 6 inches of waste each, which is 98.8 feet of spline and four 25 foot packs. The frame stock is a different problem again — 32 pieces packed onto 8 foot sticks, longest first, with a kerf at every cut. Neither of those follows from the mesh, and running out of one of them mid-afternoon is the usual way this job takes two days.
Related pages
For a screened porch bay rather than a window frame, the porch screen panel calculator works one panel per opening and adds the stop. For the openings themselves, the rough opening calculator covers framing sizes, and the gap survey page ranks openings by open area. For sealing around a window rather than screening it, the weatherstrip calculator and the flashing take-off are the two.
Questions people ask
Do I measure the frame or the opening?
The frame, on its outside, in both directions. The opening is a different dimension and using it produces mesh that is short. If the frame is bent or the corners have opened up, straighten it before measuring, because a frame measured out of square gives a piece that is right on paper and wrong at three corners.
How much overhang should I leave?
Enough to hold on to while the spline goes in, which for most people is around an inch, and no more than that. The overhang is added at both ends of both dimensions, so it costs four times what it looks like it costs, and it is the thing that decides whether a piece fits across the roll. Try a quarter inch either side of your usual figure in this page before ordering a roll.
Why does the page say a screen is too wide when the frame is narrower than the roll?
Because the cut piece is the frame plus twice the overhang in each direction, and a piece has to be no wider than the roll in at least one of its two directions to come off the roll in one part. A 46 inch frame with an inch of overhang is a 48 inch piece and will not come whole off a 36 inch roll either way round. The options are a wider roll or a seam, and a seam in a screen is rarely worth it.
What does the frame stock section assume about corners?
It follows the selector. Square cut with corner keys means the two rails run between the stiles, so their length is the outside width less twice the frame profile width, and the stiles run the full height. Mitred means every piece is cut to the full outside dimension and the angle takes care of the corner. The two produce noticeably different cut lists from the same set of screens.
Does the stick count assume perfect cutting?
It packs the longest piece first into each stick and starts a new one only when nothing fits, with your kerf taken at every cut. That is close to what a careful person does at a chop saw and better than dividing total length by stick length, which always reads low. On a very long list the page falls back to the division and says so, because packing thousands of pieces individually is not worth the wait.