Does a cracked windscreen fail an MOT?

It depends on the size of the damage and where it is on the windscreen. The area directly in front of the driver, a strip about 290mm wide called Zone A, has the strictest rule: damage bigger than 10mm across typically fails the test. Anywhere else within the area the wipers clear, damage bigger than 40mm across typically fails.

Even below those sizes, a tester can still fail the car if the damage genuinely affects the driver's ability to see the road clearly - the measurements are thresholds to check against, not an automatic pass just because a chip happens to be smaller than 10mm or 40mm.

For a buyer, a small chip well away from the driver's eyeline is often not urgent, but the same size chip sitting right in front of the driver is a different matter. Cracks also tend to spread over time, especially in cold weather, so it's rarely something that gets better on its own.

Source: gov.uk: MOT inspection manual, visibility