Every state has its own law pertaining to the degree of tint allowed on each window on a car. You can find your state's law through the link in the Resources section. It is important to understand the degree of window tinting your state allows.

Percent Light In

"Percent Light in" is a common measure of how dark a tinted window is. The lower the percentage, the darker the tinting allowed.

Percent Reflectivity

"Percent reflectivity" refers to the window tinting that reflects like a mirror. A higher percentage indicates a more reflective sticker. Some states have a maximum percent allowed, while many states have banned reflective tinting altogether.

Windshield

The first thing you will read are the laws regarding tinted stickers on the windshield. States can restrict how far from from the top of the windshield a sticker is allowed to extend.

Front Side Windows

The "front side windows" are the windows on the driver's and front-seat passenger's doors. States will often have more scrutiny for these windows, to promote greater visibility, both for the driver looking out, and for the law-enforcement officer looking in.

Back Side Windows

The "back side windows" are the windows on the doors to the car's back seat. This restriction is usually more lenient than the front side windows because the shade provides greater passenger comfort.

Back Window

Finally, some states have separate laws regarding the rear window (behind the back seat). Usually this is where the laws are the most lenient.

Source:

TintCenter.com: Laws