I use scales based on
Fiction Ratings.com, using the ratings used by
the Motion Picture Association of America (because those're the scales that were used on FFNet when I first got into fanfiction and the scales most people in my audiences recognize). Simply put, the definitions, as stolen from FR.com (for ratings for written work) and Wikipedia (movie-style ratings/more detailed views because FR.com sugarcoats the definitions):
K / G
FR.com's definition:
Intended for general audience 5 years and older. Content should be free of any coarse language, violence, and adult themes.
Wikipedia's definition:
No nudity, no sex, no drugs, minimal violence, and very limited use of language that goes beyond polite conversation, such as an occasional damn or hell.
K+ / PG
FR.com:
Suitable for more mature children, 9 years and older, with minor action violence without serious injury. May contain mild coarse language. Should not contain any adult themes.
Wikipedia:
May contain some profanity and nudity, mild violence, or very mild drug references.
Xanthine's note: By "mild violence," that usually means punches and kicks at most, not knifing or shooting someone in the shoulder. Nudity is never complete and frontal, and drug references are implicit. As in, kids wouldn't get them, but the adults might.
T / PG-13
FR.com:
Suitable for teens, 13 years and older, with some violence, minor coarse language, and minor suggestive adult themes.
Wikipedia:
May contain strong language, some explicit nudity, mildly bloody violence or gore, or mild drug content.
M / R
FR.com:
Not suitable for children or teens below the age of 16 with possible strong but non-explicit adult themes, references to violence, and strong coarse language.
Wikipedia:
May contain very strong or sexual language, strong explicit nudity, strong violence and gore, or strong drug content.
Xanthine's note: But still no sex.
MA / NC-17
FR.com:
Content is only suitable for mature adults. May contain explicit language and adult themes.
Wikipedia:
May contain very strong sexual or offensive language, strong explicit nudity, very strong gore or disturbing violence, or strong drug abuse.
(There's also the B rating for fiction, which is the rating that represents anything for preschool-age children, but there's really no corresponding rating in the MPAA except G.)
So, really, it depends on what your fic is about. If it's pretty much kid-friendly, it's probably G, and you actually don't have to list a rating. PG doesn't need a rating either because a lot of people are actually okay with that kind of content. It's when you start hitting blood, violence, sex, and adult language that you start wanting the PG-13 or R ratings.
Long story short, look at the scales above. If it's PG or lower, you don't have to rate it. If it's got the criteria for a PG-13 fic, you'll want to consider adding a warning. R fics need a warning either way because not everyone's cool with reading the kind of stuff that would warrant that high of a rating.
