Two widths, and the tape only gives you one
Pull a tape straight across a back glass and you have the chord — the shortest line between the two edges. Run a piece of film over the same glass and it takes the long way, following the curve, and that path is longer. On the glass in the form, 52 inches straight with a 2.5 inch bow under the straightedge, the film travels 52.32 inches. The extra 0.32 of an inch is what has to disappear before the film lies flat, and it disappears in one of two ways: it comes out with heat, or it gathers into fingers.
Under a third of an inch does not sound like much, and stated as 0.62 percent it sounds like nothing. Stated as a third of an inch of loose film spread along a curved edge, everyone who has laid a back glass knows exactly what it looks like.
The bow is the whole story, and it is squared
The excess for a shallow arc is close to eight thirds of the bow squared over the width. The width is on the bottom and it appears once; the bow is on the top and it appears twice. That is why the page prints a block showing the same glass with the bow doubled: at 5 inches instead of 2.5 the excess goes from 0.32 inches to 1.27, and the page prints the ratio as 3.98 to 1.
The practical consequence is that estimating from the look of a glass fails in a specific direction. Two back glasses that both read 52 inches across and look similarly curved can be an inch of excess apart, and the more curved one is disproportionately worse rather than proportionately worse. The straightedge takes ten seconds and settles it.
The roll check nobody does until it is too late
The piece has to be cut to the developed size plus trim, not to the tape measurement plus trim. At 52.32 over the curve with an inch of trim each side, the piece is 54.32 wide — which does not come off a 40 inch roll across, and does come off it turned, using 4.53 feet of roll for the piece. The page reports both orientations because the answer flips depending on which way round the glass is, and finding out after the piece is cut is expensive.
What this cannot tell you
Whether your film will take the excess up is not in here and cannot be. It is a property of the specific film, changed by glass temperature, by the weather that day and by the person holding the heat gun. The form asks for the figure so it can be printed next to the geometry and subtracted, and the subtraction is arithmetic — it is not a judgement that a job will work or a warning that it will not.
The model also treats the two directions as independent arcs, which understates the corners. A doubly curved surface cannot be covered by a flat sheet without distortion at all, which is the reason the technique exists. Take the numbers as the size of the problem rather than as the shape of it, and let the scrap of film on the actual glass settle the rest.
Questions people ask
How do I measure the bow on a back glass?
Lay a straightedge across the glass at its widest point, resting on both edges, and measure the biggest gap between the straightedge and the glass. That gap is the bow, or sagitta. Do it again from the top edge to the bottom edge for the second direction. A flat side window reads zero and needs no allowance at all, which is why side glass is quick and back glass is not.
Why does the film need to be bigger than the glass measures?
Because a tape pulled straight across a curved glass takes a shortcut the film cannot take. The film follows the surface, which is longer than the straight line between the same two edges. On a 52 inch glass with a 2.5 inch bow the film has to cover 52.32 inches, and the 0.32 inch difference is the excess that either shrinks out or gathers up.
Does twice the curve mean twice the excess?
No, it means about four times. The excess goes roughly as the square of the bow, so the same 52 inch glass with a 5 inch bow instead of 2.5 carries 1.27 inches of excess instead of 0.32. This is the single most useful thing on the page, because it is the opposite of how people estimate from looking at a glass.
What roll width do I need for a back glass?
Compare the roll against the developed size plus your trim, not against the tape measurement. The default glass here needs a piece 54.32 by 24.03 including an inch of trim all round, so a 40 inch roll takes it turned but not straight on. The page prints both orientations and the roll length each one uses, because which way round a piece runs changes the answer completely.
Will my film shrink enough?
This page cannot tell you, and does not try. How much excess a film will take up depends on the film itself, the glass temperature, the humidity and the technique, and the honest test is a scrap of the same roll on the same glass. The form asks for your own figure so it can be printed next to the geometry and subtracted; the difference it prints is a subtraction, not a verdict.