Light does not subtract
Each layer in the way passes a fraction of the light that reaches it, and the second layer only ever sees what the first one passed. Glass at 78 percent passes 0.78 of what hits it; a 35 percent film passes 0.35 of that; 0.78 times 0.35 is 0.273, so the window reads 27.3. Nothing is subtracted anywhere, and there is no version of the arithmetic in which 78 and 35 give 35.
This is worth stating flatly because both of the common wrong answers are plausible. Reading the film number as the window number gives 35, which is 7.7 points off. Subtracting the film darkening from the glass — 78 minus 65 — gives 13, which is 14 points off in the other direction. The page prints both, next to the right one.
Privacy glass is the case that catches people
Put the same 35 film on factory privacy glass metering 26 and the window reads 9.1. That is not a subtle difference from the 27.3 on the front doors; it is three times darker, from the same roll, on the same vehicle, in the same hour. The ratio between the two windows is exactly the ratio between the two glasses, because the film cancels out of the comparison entirely.
Which also means the rear windows of a car with privacy glass are already at 26 before anything is applied. If you are working to a number below that, no film gets you there, because every film takes it further down. The page says so in words rather than printing a negative.
Working backwards
Give it a target and it divides instead of multiplying: the film needed is the target over the glass fraction. On 78 glass, landing on 30 needs a film of 38.5 or lighter. On 26 glass, landing on 30 is not reachable at all. The same division works on a meter reading — read a finished window and divide by the bare glass and you have what the film is actually doing, which is frequently a few points away from what the box says once meter tolerance and a curved pane are in the mix.
What this page will not do
It will not tell you what number to aim for. Tint limits are set by state, they differ between the windshield, the front sides, the rear sides and the back glass within a single state, and they change. There is no limit anywhere in this page, no default that hints at one, and no verdict on any result it prints. The target field exists so the page can subtract against a figure you brought with you, and the subtraction is arithmetic.
It will also not substitute for a meter. Every number it prints assumes your two readings were exact, and glass readings move with where on the pane you take them. Meter the finished window.
Questions people ask
If I put 35 percent film on my car, what will the window read?
Less than 35, and how much less depends entirely on the glass. On door glass metering 78 the window reads 27.3. On factory privacy glass metering 26 the same film reads 9.1. The film figure on the box describes the film measured on its own, and film is never fitted on its own — there is always glass under it, and automotive glass is not 100 percent to start with.
Do the film and the glass percentages add up or multiply?
They multiply. Each layer passes a fraction of what reaches it, so 0.78 of the light gets through the glass, 0.35 of that gets through the film, and 0.78 times 0.35 is 0.273. Subtracting gives 13 and reading the film number gives 35, and both are wrong in opposite directions. The page prints all three so the gap is visible.
What film do I need to land on a specific net VLT?
Divide the target by the glass fraction. Aiming for 30 through glass metering 78 needs a film of 38.5 or lighter. If the division comes out above 100 the target is not reachable on that glass at all, which is common on factory privacy glass — glass already at 26 is below a target of 30 before any film exists, and every film moves it further away.
Why does my meter disagree with this calculation?
Because the calculation assumes your two inputs were exact and nothing else is in the light path. Meter tolerance, a curved pane, the frit band near the edge, dirt on either face and the angle the meter is held at all move a reading, and two meters on one window routinely differ. Enter the finished reading and the bare glass reading and the page divides them to show what the film is actually doing.
Is this tint legal?
This page cannot answer that and does not try. Limits are set state by state, they differ between the windshield, the front side glass, the rear side glass and the back glass inside a single state, medical exemptions and vehicle class change them again, and they get amended. No limit appears anywhere in this page. Get the figure from the authority that publishes it for your state and that specific window, and confirm the finished glass with a meter.