The legal tread depth limit for motorcycles, mopeds and scootersover 50cc in the UK is 1mm across three quarters of the width of the treadpattern, and with visible tread still remaining on the other quarter. At thispoint though, the ability of your tyre to disperse water will be limited, andit will be performing far from its best in the dry.
For anything under 50cc, the law simply states that you must beable to see the original tread pattern across the whole tyre.