The rules for tinted front windscreens and front side windows depend on when the vehicle was first used. There are no rules for tinting the rear windscreen or rear passenger windows.
Vehicles first used on 1 April 1985 or later
The front windscreen must let at least 75% of light through and the front side windows must let at least 70% of light through.
Vehicles first used before 1 April 1985
The front windscreen and front side windows must both let at least 70% of light through.
Penalties for having wrongly tinted windows
It's illegal to fit or sell glass (or a vehicle already fitted with glass) that breaks the rules on tinted windows.
The police or the Driver and Vehicle Standards Agency (DVSA) vehicle examiners use light measuring equipment to measure window tint.
If your windscreen or front side windows are tinted too much you could get:
The law on tinted windows from GOV.UK this is governed under the Road Traffic Act 1988 and The Road Vehicles (Construction and Use) Regulations 1986.