What does the percentage on windshield tint refer to and how is it calculated?

What the percentage means

The number on a tint film — like 5%, 15%, 35%, 50%, or 70% — refers to Visible Light Transmission (VLT). VLT is the percentage of visible light that can pass through the window film and/or the glass [1][8][16][23][29][31][38][48][58]. In plain words, it tells you how much light actually comes through the window [29][56][58].

• A 25% tint lets only 25% of outside light enter the cabin [2].
• A 50% tint lets half the visible light through, and a 5% tint blocks 95% of the light [17].
• 60% VLT means 60% of light passes through [51].

Lower numbers are darker; higher numbers are lighter [3][9][24][40][49]. Think of the scale from 0% (fully opaque, nearly blackout) to 100% (fully transparent) [7][20][32]. The less light that passes through, the darker the window looks [3][24][52].

How the final percentage is calculated

The percentage is not just the film's number. Since factory glass already blocks some light, the final VLT combines the glass and the film together [4][18][25][41][48][50][53].

The formula is:

Final VLT (%) = (Tint Film VLT / 100) × (Original Glass VLT / 100) × 100 [10]

In simpler steps: turn each percentage into a decimal, multiply them, then turn the result back into a percentage. For example, 0.80 × 0.35 = 0.28, which is 28% [13][53]. The math is multiplicative, not additive [12].

Examples from the evidence:

• Factory glass at 60% VLT + 50% film = 30% total VLT [5].
• Factory glass at 80% VLT + 35% film = 28% total VLT [14].
• Factory glass at 78% VLT + 20% film = about 15.6% total VLT [19].
• Factory glass at 75% VLT + 50% film = about 37.5%, not 50% [27].
• Two layers of 20% film = 20% × 20% = 4% [22].

Factory tint on original glass already reduces VLT by about 3–8% on most vehicles [37].

How VLT is measured

After installation, VLT is measured with a specialized device called a tint meter, a light transmission meter, or a spectrophotometer [28][30][33]. The meter shines visible light through the window and measures how much comes out the other side. It then reports the ratio as a VLT percentage [28][34]. Professional tint meters are calibrated to measure visible light wavelengths between 380 and 780 nanometers — the range human eyes can see [35]. The reading covers both the glass and the film together [36].

Why this matters for the law

Tint laws are written using this percentage, and they usually regulate the final combined VLT of glass plus film, not just the film alone [15][26][41]. For example, California requires front side windows to allow at least 70% light transmission [39][46][44]. Officers check this with a tint meter [45]. Because factory glass already blocks light, the film itself often has to be about 88% VLT or lighter to keep the combined reading above 70% [42][43]. California also restricts windshield tint to the top 4 inches and allows 35% tint only on rear windows [46][47].

Also worth knowing: the VLT percentage does not measure heat rejection or UV protection — those depend on the film's construction, not the VLT number [21].