The short answer
The front windscreen must let through at least 75 per cent of visible light. Both front side windows must let through at least 70 per cent. Rear side windows and the rear windscreen have no legal minimum, so you can go as dark as you like back there. Break the front rules and the police or DVSA can stop you driving the car until the tint comes off.
Those numbers come from the Road Vehicles (Construction and Use) Regulations 1986 and are confirmed on the GOV.UK tinted windows guidance page. They are not forum rumour. They are the spec sheet.
Where the rules come from
It is an explainer of existing law. The legal framework sits in the Road Vehicles (Construction and Use) Regulations 1986, Regulation 32, which governs glazing on vehicles used on public roads in Great Britain. The same thresholds apply in Northern Ireland, as confirmed by nidirect.gov.uk.
Here are the numbers, window by window.
Windscreen: 75 per cent VLT minimum. That is the tightest limit on the car. Factory glass on a modern car typically sits around 85 to 88 per cent, which leaves almost no margin before a film tips the combined assembly under the legal threshold.
Front side windows (driver and passenger): 70 per cent VLT minimum. A little more room, but still tight once you factor in what the factory glass is already absorbing.
Rear side windows: No legal minimum. Any shade from a light privacy film down to full limo black is permitted on a standard passenger car.
Rear windscreen: Also no legal minimum, provided you have working mirrors on both wings. GOV.UK confirms this.
Older vehicles and special cases
Vehicles first used before 1 April 1985 work slightly differently. Both the front windscreen and front side windows must meet 70 per cent VLT, rather than 75 per cent for the screen on later cars. Same source, same page. If you are building a classic, you get a fractionally wider margin on the screen, but the front side windows sit at the same 70 per cent floor either way.
The critical detail most people miss is the stacking problem. The legal test measures the combined assembly: factory glass plus any film applied on top. The film’s rated VLT is not the figure the law cares about. If your front side windows already transmit 80 per cent from the factory and you apply a film rated at 90 per cent, the combined result might land around 72 per cent. Still legal. But if your factory glass is already at 75 per cent and you put any tinted film at all on the front side window, you are very likely under the 70 per cent limit before you have even paid the installer.
Always get the baseline VLT of your factory glass measured before ordering film for front windows. A decent installer will do this before they quote you.
Northern Ireland and enforcement
The law applies across Great Britain under the Construction and Use Regulations. In Northern Ireland, the same VLT thresholds are enforced, but MOT administration is handled exclusively by the DVA at official test centres rather than private garages.
Enforcement works like this. The police and DVSA vehicle examiners carry calibrated light-measuring equipment specifically for tint checks. GOV.UK is clear on the consequences: if your front windows are over the limit you can receive a prohibition notice, which stops you using the vehicle on the road until the tint is removed. You can also receive a penalty notice or a court summons. It is also illegal to fit or sell glass, or a vehicle already fitted with glass, that breaks the tint rules. That applies to installers and dealers, not just the person behind the wheel.
GOV.UK does not specify a fixed penalty amount on its current guidance page, and third-party sources quote conflicting figures. Do not rely on anything you have read elsewhere about the precise fine. The prohibition notice is the immediate practical problem: the car stops being drivable until the glass is brought back into spec.
The DVSA MOT Inspection Manual lists excessive tinting on the windscreen or windows beside the driver’s seat as an inspectable item. If tinting is excessive but the driver’s view is not seriously affected, that is a minor defect. The car passes. If tinting seriously affects the driver’s view or the view of a required mirror, that is a major defect and the car fails.
Here is the part that catches people out: a pass at the MOT does not mean your tint is road legal. The MOT tester makes a visual assessment. The police and DVSA use calibrated photometers at the roadside, and those devices do not care what your MOT certificate says. The Construction and Use Regulations are independently enforceable at any time. Pass your MOT, drive away, get pulled over, fail the meter test. It happens.
Who this applies to
Anyone running a modified car in the UK who wants to tint and stay legal. The rear windows are your playground. The front windows are where you need to be precise.
If you are thinking about how tint rules fit alongside other exterior modifications, the logic is similar to what we cover in the underglow and aftermarket lighting guide and the number plates explainer: there is a defined line, and crossing it has consequences that follow the car around.
What is still uncertain
Two topics come up constantly in the modified scene and deserve an honest note.
Chameleon and colour-shift windscreen films. These are popular because they look subtle in certain light. The 75 per cent VLT limit applies regardless of film type. Many chameleon films drop the combined VLT below the legal threshold. The current GOV.UK guidance does not address them specifically. Verify with your installer, get a meter reading on the combined assembly, and do not assume the colour makes it compliant.
Sunstrip across the top of the windscreen. Some sources describe a permissible band above the wiper-swept area. The current GOV.UK page does not confirm this. Treat it as unverified and check with your installer before fitting anything across the screen.
Exact penalty amounts and points. GOV.UK lists a penalty notice or court summons but does not quote a specific fine or mention penalty points. Third-party figures vary widely. Until the government page confirms a number, treat any specific amount you see online as unverified.
What to do next
If you are planning front window tint, get your factory glass measured first. Any professional installer should offer this before fitting. Ask them to confirm the combined VLT after installation and get that figure in writing.
If you already have film on and you are not sure where you sit, a tint meter reading from an installer or a mobile tint checker will tell you quickly. If you are under the limit, the film has to come off the front windows before the car goes near a main road. There is no grey area once the photometer is out.
For the rear windows, go as dark as you want. That is one place where the law genuinely gives you room to build.