You've heard it before:
"No ice is safe".
But that's not practical advice. In fact I believe it tends to DE-sensitize people to the actual dangers as opposed to the real dangers.
North Dakota Game and Fish has a wonderful idea, and
I like it:
Here are a few links to some nice "Ice Fishing Safety" pages that tell it like it is:
Minnesota DNR ice fishing thickness recommendations
Old Farmer's Almanac ice thickness ideas