The legal answer is that your tread should't be lower than 1.6mm, however the performance of a tyre starts to decrease once you get passed 3mm, making them dangerous to use. See below results for stopping distance in the wet at different tread depths.
Tread Depth Stopping Distance
From the results we can see that the distances taken to stop in the wet increase dramatically once the tread gets below 2.5mm. This creates an increased danger to both you and other road users. Which is why we recommend changing your tyres once they get to 3mm, especially in powerful vehicles.
There are quite a few gadgets on the market that can help you check the tread depth of your tyres. However there is a very simple way to check your tyres using a 20p piece, meaning there is no excuse no to check them regularly.
How to check your tyres using a 20p
Step 1 - Make sure your cars handbrake is engaged
Step 2 - Get out a 20p
Step 3 - Place your 20p in the tread grooves
Step 4 - If you can see the edge of the 20p, your tyres maybe illegal
Step 5 - This is what a full tread looks like
You can tell the huge difference in a worn and new tyres tread depth!
For a full tutorial visit our WikiHow page