Comment by patall

Obviously depends on the weight / load per area. Here in Stockholm, our skating club starts organizing trips from 5 cm (=2inch), but black ice should hold you from 3.2 or so cm. And you can hear from the sound of the ice approximately which thickness you are on. But this is done with security equipment and in a group, so more general rules apply to anyone without experience and acting on their own. Ice is dangerous, like the outdoors is always dangerous when taken without necessary precaution.