You can check the depth of the tread, i.e. the top, grooved surface of the tyre, using a special tread depth gauge. You can also use everyday objects such as a coin or card. Place the gauge in one of the tyre grooves and mark the point where it starts to protrude above the rubber. The measured height is the current tread depth.
Inside the grooves there are tabs marked with the acronym TWI, which indicate that the tread has worn down to the legally required minimum height of 1.6 mm. If the tread has worn down to these, the tyres must be replaced with new tyres. However, it is recommended that summer tyres should be replaced at 3 mm tread depth and winter tyres at 4 mm - then they offer their full potential and driving is safe.