Window Tint Guide: Understand
What VLT Means on Window Tint (and How to Do the Math)
Every tint shade is sold by a percentage: 50, 35, 20, 5. That number is the film's visible light transmission, or VLT, and it is the number most people use to choose a shade. It is also the number that causes the most confusion, because what a police officer's meter reads on your window is not the number on the box.
This guide explains what VLT measures, why the film and the glass multiply rather than add, and how to work out what film shade keeps your car legal in South Carolina, with examples you can redo for your own vehicle.
The short answer
VLT (visible light transmission) is the percentage of visible light that passes through a window. Lower means darker. When film goes on glass, the two multiply: a 35% film on side glass that already passes 80% gives about 28% combined (0.35 x 0.80). South Carolina's 27% limit applies to that combined figure, so the film you can use depends on your factory glass.
Key points
- VLT is the share of visible light that gets through; 70% is nearly clear and 5% is limo dark.
- Film and glass multiply: combined VLT is roughly film VLT times glass VLT.
- South Carolina's 27% minimum applies to the combined reading, not the film's label.
- Shade names are nominal: a film sold as a 35 may measure a few points above or below 35% on its own.
- On a pickup's factory privacy glass, which is often already darker than 27%, any added film puts the window under the limit.
The definition
What VLT Measures
South Carolina's tint statute defines light transmission as the ratio of the total visible light that passes through a product to the total light falling on it. Tint makers and the law use the same idea, expressed as a percentage. A 70% film lets 70% of visible light through; a 5% film lets through only 5%.
The International Window Film Association puts it in practical terms: a 70% film is nearly clear to the naked eye, while a 15% film looks nearly black from the outside and makes it difficult to see through the glass. VLT measures darkness only. It says nothing about heat or UV, which are separate numbers on a spec sheet.
The math
Why Film and Glass Multiply
Light passes through the glass and then through the film. If the glass lets through 80% and the film lets through 35% of what reaches it, the window passes 35% of 80%, which is 28%. You multiply the two fractions: 0.80 x 0.35 = 0.28.
It never works by subtraction or by taking the darker of the two. Stacking two 50% films gives 25%, not 0% and not 50%. Real meters read a little differently from the pure math because of surface reflections and glass tint color, so treat the calculation as a close estimate, not a guarantee.
South Carolina writes this directly into the law. Subsection (C) of S.C. Code § 56-5-5015 requires that the combined light transmission of the film with the factory or manufacturer installed material must not be less than twenty-seven percent.
Worked examples
Five Examples You Can Redo for Your Car
35% film on clear front glass that measures 80%
0.35 x 0.80 = 0.28, or 28%. That is just above 27%. It works on paper, but there is very little margin.
The same 35% film on glass that measures 75%
0.35 x 0.75 = 0.2625, or about 26%. Now the window is under 27%. Same film, different car, different result. Front side glass on new cars is generally made to transmit at least 70% under federal glazing rules, but where it lands above that varies.
A film labeled 35 that actually measures 37% on clear glass
LLumar's published performance sheet lists its CTX 35 ceramic film at 37% VLT and its IRX 35 at 39%, measured on clear glass. On 75% glass, 0.37 x 0.75 is about 28%, which is legal. The label is a nominal shade; the measured number is what counts.
50% film on 75% glass
0.50 x 0.75 = 0.375, or about 38%. Comfortably legal, noticeably darker than untreated glass, and still easy to see through at night.
Any film on a pickup's rear privacy glass at 20%
Factory glass alone is not regulated for light transmission in South Carolina, but once film is added, the combined number must be at least 27% behind the driver on a pickup. 0.20 x 0.90 is already 18%. Even a nearly clear film puts that window under the limit.
Glass values in these examples are illustrations. Your car's glass has to be measured with a meter to know its real VLT.
Working backward
What Film Keeps You at 27%?
To find the darkest film your glass can take, divide 27 by your glass VLT. On 80% glass, 27 / 80 = 0.3375, so the film needs to measure about 34% or lighter. On 75% glass you need about 36% or lighter, and on 70% glass about 39% or lighter.
That is why a shop that cares about the law meters your glass first. The same shade name can be legal on one car and illegal on the next, and the difference is invisible until you measure it. The table below runs the numbers for common glass values.
Factory tint
Privacy Glass and the SUV Exception
Many SUVs, vans and pickups come with darker privacy glass behind the front doors. Glass makers produce it by changing the glass composition; AGC, a major automotive glass maker, lists its privacy glass at 4.5% to 40% light transmission, against more than 70% for normal vision glass.
On most SUVs and vans this does not matter legally, because South Carolina's light limit does not apply to windows behind the driver on multipurpose passenger vehicles. Pickups are the exception: subsection (I) applies the limit behind the driver on pickup trucks. The front side windows on every vehicle are still held to 27% combined, and on an SUV that is where the math matters most.
- Multipurpose passenger vehicleMost SUVs and vans
- The statute defines it as a vehicle designed to carry ten persons or less, built on a truck chassis or with special features for occasional off-road operation. Check the vehicle type on your door-jamb certification label. SUVs, vans and pickups
Beyond darkness
VLT Is Not Heat, and Not UV
A low VLT number does not guarantee a cool car. On LLumar's published sheet, its IRX 50 ceramic film transmits 54% of visible light and is listed at 50% total solar energy rejection, while its dyed ATC 05, at 5% VLT, is listed at 44%. The lighter film rejects more total heat.
UV is separate again. Every automotive film on that same sheet, from the darkest to the nearly clear, is listed at more than 99% UV protection. You can choose a legal, lighter shade without giving up UV protection, and with a ceramic film, without giving up much heat control either.
Watch for
Common VLT Mistakes
- Asking for a shade by its label alone, without knowing what your glass already blocks.
- Assuming the rear windows of a sedan follow the SUV rule. Sedans are not multipurpose passenger vehicles; their rear side windows and back glass are held to 27% combined.
- Matching front windows to factory privacy glass by eye. Getting the fronts that dark is usually well under the limit.
- Forgetting that the windshield follows a different rule: film only above the AS1 line, regardless of VLT.
- Believing an out-of-state limit applies here. A car registered in South Carolina is held to South Carolina's rules.
| Factory glass VLT | 35% film gives | 50% film gives | Film needed for 27% |
|---|---|---|---|
| 85% | about 30% | about 43% | about 32% or lighter |
| 80% | 28% | 40% | about 34% or lighter |
| 75% | about 26% | about 38% | 36% or lighter |
| 70% | about 25% | 35% | about 39% or lighter |
| 20% (privacy glass) | 7% | 10% | None: already under 27% |
Calculated as film VLT x glass VLT. Real meter readings vary slightly; only a meter on your car gives the actual figure.
Get the Shade Checked Before It Goes On
The Auto Show checks the legal shade for your vehicle before installing, accounting for factory glass, and puts the South Carolina compliance sticker on every tinted window.
Common questions
Is a lower VLT percentage darker or lighter?
Darker. VLT is the percentage of visible light that gets through, so a 5% film lets through very little light and looks nearly black, while a 70% film is nearly clear. People sometimes say 5% tint as if it describes how much is blocked, but the number always means how much passes through.
Does 35% tint read 35% on a tint meter?
Almost never. The meter reads the film and the glass together. A 35% film on glass that passes 80% reads about 28%, and on glass that passes 75% it reads about 26%. Shade names are also nominal, so the film alone may measure a few points off its label.
How do I find my factory glass VLT?
The reliable way is to have it measured with a tint meter before any film goes on. Glass varies between makes, models and even windows on the same car, and rear privacy glass is much darker than the front glass. Ask the installer to show you the reading.
Can I stack a second film over old tint?
Stacking multiplies again: two layers of 50% film give about 25%, and old film over factory glass is already part of the combined number. South Carolina's statute also refers to a single sunscreening device on side windows, so layering films is a bad plan. Old film is normally removed before new film goes on.
Is VLT measured the same way on the windshield?
South Carolina does not use a VLT limit for windshield film. Film may only go along the top of the windshield, no lower than the AS1 line, and if the AS1 line is not visible, no film is allowed at all. Clear films are still subject to that rule.
Sources
- S.C. Code Ann. § 56-5-5015, South Carolina Legislature (https://www.scstatehouse.gov/code/t56c005.php), accessed October 2026
- Automotive, International Window Film Association (https://iwfa.com/automotive/), accessed October 2026
- LLumar Window Film Performance Data (LL0116NAEN), Eastman Performance Films (https://www.llumar.com/content/dam/eastman/performance-films/llumar/documents/LL0116NAEN_AutoSpecs_screen.pdf), accessed October 2026
- Privacy Glass, AGC Automotive (https://www.agc-automotive.com/products-and-solutions/privacy-glass), accessed October 2026
- Fleetwood Enterprises, Inc.; Action on Application for Decision of Inconsequential Noncompliance (FMVSS No. 205 light transmittance), Federal Register Vol. 63 No. 43, NHTSA (https://www.govinfo.gov/content/pkg/FR-1998-03-05/html/98-5743.htm), accessed October 2026
- 49 CFR § 571.205, Glazing materials, Legal Information Institute, Cornell Law School (https://www.law.cornell.edu/cfr/text/49/571.205), accessed October 2026
Prices on this page are The Auto Show’s published starting prices. Film ratings and warranty terms come from the manufacturer. This guide explains South Carolina law in plain words; it is not legal advice.