Window Tint Percentage (VLT): Why Films Multiply, Not Add
Tint percentage is visible light transmission — the share of light that passes through. A lower number means darker glass. When film is applied to factory-tinted glass the percentages multiply, which is how a legal-looking film ends up failing a roadside meter.
The multiplication rule
- Net VLT = glass VLT × film VLT, using decimals. 0.78 × 0.35 = 0.273, so 27.3%.
- Adding a second layer multiplies again.
- Darkness blocked = 100 − net VLT.
- Most factory side glass sits around 70-80% VLT before any film.
Reading tint numbers
5% is limousine dark, 20% is common on rear glass, 35% is a popular all-round choice, and 70% is close to clear and often used for heat-rejecting ceramic films. Because VLT measures light let through, the intuition runs backwards from most percentages you deal with: smaller means darker.
Legality and performance
- Limits vary by jurisdiction and usually differ between front side, rear side and windshield.
- Enforcement measures the total assembly, glass plus film, exactly as this calculator does.
- Heat rejection depends on infrared performance, not darkness — good ceramic film can be light and still cool.
- Medical exemptions exist in some places but need documentation.
Key takeaways — Window Tint Percentage Calculator
- Net VLT = glass VLT × film VLT; percentages multiply.
- Lower VLT means darker glass.
- Check local limits before choosing a film — the meter reads the whole assembly.