Tonight I watched a TV show in which a character warned others about a possible explosion that will "destroy everything within a one-hundred-square kilometer area."
Silly V scriptwriters, bomb damage is round, not square! (But in all fairness, saying "area of 100 square kilometers" is much more dramatic than using the phrase "blast radius of almost six kilometers" in a TV sci-fi script.)
It's actually a radius of 5.642km, as calculated by feeding the WolframAlpha Computational Knowledge Engine the query "calculate circle radius area 100km^2". No need for pencil/paper/calculator or remembering formulas, it took only 10 seconds on the internet:
Very useful, WolframAlpha is! The back of my envelopes remain mostly blank these days.