You can easily measure the tread depth of your tyre with a one pound coin. This has a gold rim that is approximately three millimetres wide. If you insert the coin into the tread of your tyre, this edge should not be visible.
The law prescribes a tread depth of 1.6 millimetres. However, experts advise replacing the tyre at a depth of three millimetres. Winter tyres should even have a tread depth of at least four millimetres.