Here is how to calculate VLT after tinting: multiply the light transmission of the glass by the light transmission of the film. 80% glass with 70% film gives you about 56%, not 70%, and not 80 minus 30 either. The result is an estimate; to know the exact number on your car, you have to measure.
Plenty of owners pick a "70" film and assume their glass is still at 70%. Then a meter reads much lower and they think the shop installed the wrong film. Usually the shop got it right. Both sides just forgot about the glass.
Why multiply instead of add?
VLT is the share of visible light, roughly 380 to 780 nm, that gets through a layer of material. With two layers, light passes the first one before it reaches the second, and the second only lets through part of what is left. It works like a discount on a discount: 20% off and then another 30% off is not 50% off.
Written out, the formula is:
VLT after tinting ≈ glass VLT × film VLT
Convert percentages to decimals before multiplying. According to one guide on VLT calculation, car glass usually has a VLT of around 70 to 85%, so factory glass is not the "100%" pane many people imagine. That is the common calculation method, not a number for any specific car.
How to calculate VLT after tinting: a worked table
The numbers below are just multiplication, with glass down the side and film across the top. They are not a recommendation for which film to choose.
| Factory glass | 70% film | 50% film | 35% film |
|---|---|---|---|
| 85% | 59.5% | 42.5% | 29.75% |
| 80% | 56% | 40% | 28% |
| 75% | 52.5% | 37.5% | 26.25% |
Calculated from assumed numbers, with no tolerance applied. Not measured values and not tied to any regulatory threshold.
Two things jump out from the table. The result is always lower than both the glass number and the film number. And with the same film, the darker the factory glass, the further the result drops, so asking "what percentage is this film?" without asking what your car's glass is only asks half the question.
To go the other way, when you know the level you want and need to find the film, divide that level by the glass VLT. To end up at 50% on 80% glass, the film needs to be about 50 ÷ 80 = 62.5%. How to choose a level for each window position is in our article on window tint percentage; here we only deal with the math.
On regulations, this article gives no numbers. Check current local regulations before you decide.
Where do you get the glass VLT?
This is the hardest part, because nobody prints that number on a sticker for you.
The sure way is to measure. Many shops have a handheld light transmission meter. Before tinting, ask them to measure each window you plan to do, and note the numbers on your phone. It takes a few minutes and gives you exactly the number you need.
If you can't measure, use the low end of the common range for your calculation. That way you won't be too optimistic. The final result still has to be measured again after tinting.
Cases where people get the math wrong
Laminated windshields. A car windshield is usually laminated glass: two panes sandwiching an interlayer. Many people hear "two layers" and want to multiply again. No need. The measured VLT of a windshield already covers the whole pane; you only multiply by the film once. Some windshields also have a coating or a built-in green tint, so they are not as clear as they look, which is one more reason to measure.
Factory privacy glass. Many SUVs and MPVs come with dark rear glass from the factory. That glass already has color in it, so the first number in the multiplication is fairly low. Suppose a pane measures 25%, purely for the sake of the calculation: 70% film leaves you at about 17.5%, and 35% film at about 8.75%. Numbers like that are very noticeable to people inside the car, especially when reversing at night. In many cases a very light film makes more sense on this glass, mainly to handle heat and UV rather than to darken it further.
Two layers of film. Every extra layer is one more multiplication. 80% glass with two layers of 70% film comes to about 0.8 × 0.7 × 0.7 = 39.2%. With two 50% layers, it is 20%. Extra layers also bring adhesive issues and the risk of trapping heat, as a newspaper article has analyzed, so we don't recommend it.
Old film left on. Tinting over old film is also two layers, even if you didn't mean it to be. If you don't know whether the glass already has film on it, a calculation starting from the "factory" number is wrong from the start.
Why do calculated and measured numbers differ?
The multiplication is an estimate, and the gap comes from a few places.
First, it ignores light bouncing back and forth between the glass and film surfaces. Second, there is the VLT on the spec sheet. A serious sheet usually states its test conditions: one international brand's specification measures under ASTM E-903, on 6 mm clear glass. Another sheet calculates under NFRC, also on 6 mm clear glass. In other words, the VLT on the sheet already includes a pane of clear glass.
That leads to a point few articles mention. If the sheet number already includes the test glass, then multiplying it by your car's glass VLT counts glass twice. So the calculated result tends to come out a little lower than reality. We have no measurements to say by how much, and every sheet needs its test conditions read carefully, but this is why you shouldn't be surprised if the meter reads a bit higher than the math.
Third, the film's name is not its real VLT. The number in a film code may be a rounded label; the number to calculate with has to come from the spec sheet. Our article on window film specifications covers how to read it in more detail.
Fourth, the meter and where you measure. Every meter has its own tolerance. The edge of the glass, the black dot band around it, dirt or adhesive marks all make the reading jump.
How to measure properly
For the number to be useful, measure the same way before and after tinting.
- Clean the glass at the spot you will measure.
- Pick an area in the middle of the glass, away from the black dot band and the film edge.
- Measure a few points and note each reading.
- After tinting, measure the exact same spot again once the film has dried.
Freshly installed film still holds some moisture and can look hazy, so a reading taken then doesn't reflect its settled state. Waiting until the film has dried, as the shop advises, gives a fairer reading. Ask the shop to write the readings on your paperwork if you want a record.
If the measured number is much lower than the calculation, don't jump to conclusions. Ask what kind of meter was used, whether it measured the same spot, and whether any old film was left behind, then check the film code on the spec sheet against the film that was installed.
Next step
Ask the shop to measure the VLT of each window before you choose a film, then use the multiplication above to narrow down the options. Reference setups are on our automotive window film solutions page, and if you have a question about your own car, send a request through the VEGO contact page.
