Well, wonder no more! This site has come to your rescue!
Now you can know your minimum safe distance from, e.g., a 50 Megaton thermonuclear detonation. In my case, if we assume the Capitol building as Ground Zero, then out here in Reston, VA, I'm just outside the "widespread destruction" radius, but well within the "3rd degree burns" radius. Uh oh.
Or, just for fun, plug in the historical values for the Hiroshima and Nagasaki fission blasts (a quaint 0.15 and 0.22 Mtons, respectively).
You'll gain a new appreciation of Seizo Yamada's picture of the Hiroshima mushroom cloud, taken at about 7 km.
"What seems to be the trouble, soldier? You look a little bit worried."