how much tread depth is minimum amount
This means fitting the right tyres and for safety ensuring that they are inflated to the manufacturer's recommended pressure. The legal limit for minimum depth of the tread on your tyres is 1.6 millimetres, across the central  of the tread around the complete circumference of the tyre. For safety reasons it is recommended that you replace your tyres before the legal limit is reached. Many vehicle manufacturers recommend replacing at 3 millimetres. At 1.6 millimetres in wet weather it takes an extra two car lengths (8 metres) to stop at 50 mph than if your tread was 3 millimetres. A regular check of your tyres can help you to avoid 3 penalty points and 2,500 in fines for having tyres worn beyond the legal minimum limit on your vehicle.