Window Tint: Net VLT Of Film Over Glass

All versions

The number on the film roll is not what ends up on the window. Factory glass already blocks some light, and tint multiplies against it. Multiply the two percentages to get the net VLT — the figure a meter (and a police officer) will read.

Quick formula: Film VLT times glass VLT, divided by 100 to stay in percent:

=ROUND(B2*C2/100,1)

A 35% film on 78% factory glass nets 27.3%, not 35% — the difference is what keeps a job legal.

Functions used (tap for the full reference guide):

PRODUCT ROUND

The example

Three vehicles, each with a chosen film VLT and the factory glass VLT it is going over.

	A	B	C	D
1	Vehicle	Film VLT	Glass VLT	Net VLT
2	Storefront	35	78	27.3
3	Sedan	20	88	17.6
4	SUV	50	80	40.0

The formula

Two transmittances multiply; dividing by 100 keeps the answer a percent:

=ROUND(B2*C2/100,1) // film% x glass%, /100 to keep it in percent

How it works

Transmittances stack by multiplication, not subtraction:

B2*C2 multiplies the two percentages — 35 x 78 = 2730.
/100 brings it back to a percent because both inputs were already percentages.
ROUND(...,1) gives a clean net VLT to compare against the local legal limit.
Never add or subtract the two numbers — light that passes the film then has to pass the glass, which is a product, not a sum.

Check the net number, not the film number, against your state's window-tint law before you cut.

Try it: interactive demo

Interactive

Enter the film's VLT and the factory glass VLT.

Film VLT (%)

Glass VLT (%)

Variations

Two films stacked

Security film under a tint? Multiply all three transmittances.

=ROUND(B2*C2*D2/10000,1)

Legal check

Flag whether the net VLT clears a 25% limit.

=IF(B2*C2/100>=25,"OK","Too dark")

Pitfalls & errors

Do not subtract the percentages. 35% film on 78% glass is 27.3% net, not 13% (35 minus 22). Subtraction will make you promise a legal job that fails a meter.

Ask for the factory glass VLT, not the assumption that it is clear. Privacy glass on rear windows is already deeply tinted, so even a light film can put the net well below the limit.

Practice workbook

📊

Download the free Window Tint: Net VLT Of Film Over Glass practice workbook

Edit the yellow film and glass VLT cells; the net VLT recalculates.

Frequently asked questions

Why multiply instead of subtract?

VLT is the fraction of light that passes through. Light has to pass the film and then the glass, so the fractions multiply. A 35% film passes 35% of what reaches it, and the glass then passes 78% of that — 27.3% overall.

Does the legal limit apply to the film or the net?

Tint laws are written against what a meter reads on the finished window, which is the net of film and glass together. Always check the net VLT, because a legal-looking film can fail once it is on already-tinted factory glass.

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Function references: PRODUCT ROUND