Ratings are the header element in fanworks that indicate in a general way what audiences the fic, fanart or other work is appropriate for. Ratings are distinct from warnings because a rating is less specific -- works featuring graphic sex, violence, or gore might all receive the same rating though their content differs dramatically. Because different countries use different rating systems and different fannish sources (video games, movies, television, manga, anime, etc) use different rating systems, there is a lot of variety in how fanworks are rated by the creators. Sometimes these are quirky fandoms specific systems, like Life on Mars fandom using Ford Cortinas of different colors.
One of the most popular rating systems is based on the ratings that the Motion Picture Association of America's Rating Board gives to films. These ratings, with the official meanings assigned by the MPAA, are:
- G: General Audiences
- PG: Parental Guidance Suggested.
- PG-13: Parents Strongly Cautioned
- R: Restricted
- NC-17: No one 17 or under admitted; see Explicit
In 2005, the MPAA was sending messages to fanfic archives telling them to stop using this set of trademarked ratings  Perhaps due to civil liberties objections and bad publicity, such as a New York Times article Please Don't Call It a G-Rated Dispute, there seems to have been no legal action taken, and no messages sent since then.
Australian movie ratings
The Australian Movie Rating system uses:
- G: Suitable for all viewers. It is noted by the board that a "G" movie rating in Australia doesn't indicate the movie is intended for children, simply that nothing in the movie will be disturbing or harmful to children.
- PG: Parental Guidance recommended for children under 15 years of age
- M: Mature, recommended for audiences 15 years and over
- MA: Mature Accompanied. Under 15 must be accompanied by a parent or adult guardian
- R: Restricted. Viewers 18 and older only
- X: Restricted. This rating.applies to sexually explicit material which is restricted to viewers 18 years of age and over
In Brazil it is called Guiding Rating and has levels that are organized and differentiated between the media -- Cinema & Video, Television, Books & RPG, Games & Apps: 
- L: Free for all audiences
- 10: Not recommended for under ten years
- 12: Not recommended for under twelve years
- 14: Not recommended for under fourteen years
- 16: Not recommended for under sixteen years
- 18: Not recommended for under eighteen years
The rating system for television in Canada has six levels:
- C (Children) - intended for children under the age of 8 years
- C8 - intended for children over the age of 8 years
- G (General) - all ages, should contain very little violence, either physical, verbal or emotional
- PG (Parental Guidance) - intended for a general audience, but may not be suitable for children under the age of 8 because of controversial themes or issues, limited and moderate violence, mild profanity, brief scenes of nudity, and/or discreet sexual references
- 14+ - might not be suitable for viewers under the age of 14, could contain intense scenes of violence (if integral to the development of plot or character), strong or frequent use of profanity, scenes of nudity and/or sexual activity within the context of narrative or theme
- 18+ - intended for viewers 18 years and older because of horror, explicit violence, frequent coarse language, sexually suggestive scenes (possibly including simulated sex), sexual references, full frontal nudity
Theatrical movie ratings are a provincial and territorial responsibility. As a result, there are seven provincial film classification boards or offices, many of which use slightly different rating systems. In most anglophone provinces, the possible ratings are G, PG, 14+, 18+, R, and A (or XXX). The ratings in Quebec are distinctly different.
- R (Restricted) - depending on the province this may mean that no one under the age of 18 may view the movie in any circumstances, or it may mean that someone under 18 has to be accompanied by an adult; in either case, such a movie may contain sexually explicit scenes, brutal violence, intense horror, and/or frequent use of profanity
- A (Adult) or XXX - predominantly sexually explicit activity, but tolerable to the community
A common Internet acronym, often used when linking to sites with mature content, is NSFW or "not safe for work." This usually means that the link leads to a website with some kind of nudity or sexual imagery, although really, it could be anything you wouldn't want your boss to see over your shoulder. In fandom, NSFW is usually used for fanart, such as drawings or photomanips.
- K: Intended for general audience 5 years and older. Content should be free of any coarse language, violence, and adult themes.
- K+: Suitable for more mature childen, 9 years and older, with minor action violence without serious injury. May contain mild coarse language. Should not contain any adult themes.
- T: Suitable for teens, 13 years and older, with some violence, minor coarse language, and minor suggestive adult themes.
- M: 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.
- MA: Content is only suitable for mature adults. May contain explicit language and adult themes. (Neither FictionPress nor fanfiction.net use this rating, but it is available.)
- B: (a supplemental rating). Intended for preschool level and younger children between the ages of 1 and 4. Content should be free of any coarse language, violence, adult themes, and ideas very young children cannot comprehend. (Neither FictionPress nor fanfiction.net use this rating, but it is available.)
The Archive of Our Own
The Archive of Our Own uses a unique rating system, and includes an option for fans who do not wish to apply ratings to their works.
- General Audiences - "This content is suitable for anyone: kids, teenagers, sensitive people."
- Teen And Up Audiences - "The content may be inappropriate for audiences under 13."
- Mature - "This is for content with adult themes (sex, violence, etc.) that isn't as graphic as explicit-rated content."
- Explicit - "This is for porn, graphic violence, etc."
- Not Rated - "For searching, screening, and other Archive functions, this may get treated the same way as explicit-rated content."
The French website Harry Potter Fanfiction (hpfanfiction.org), operated by the publisher Heros de Papiers Froissés, uses types of fruits to rate their work. Inspired by the traditional usage of "lemon" to designate a sex scene, they created three categories :
- Lime: "Often a vigorous kiss or the start of a lemon."
- Lemon : "A complete erotic scene", this category being divided into two groups : Lemon Soft and Lemon Hard.
Applying film ratings to fanworks
One of the shortcomings of using movie-based, television-based, or video-game based systems (when used to rate fan fiction) is that categories such as nudity and sexual explicitness must be translated by fans to the written medium. Another shortcoming is that the system was designed to help parents protect their children; in fandom, it is most often used by adult fans to help other adult fans decide what they might want to read.
Many fans refuse to rate their fanfic for the above reasons or because they feel that ratings provide little useful information about a story. There is also controversy over what some fans perceive as the unfair application of higher ratings to slash than to het with similar levels of sexual content. Fans who argue that slash should be rated higher than het (or that there is no such thing as G-rated slash) tend to do so on the grounds that many readers may in fact be more disturbed or offended by same-sex sexual or romantic content than by heterosexual content. Fans who argue that slash should be rated the same as het tend to feel that doing otherwise is unfair and implies that there is something inherently non-family-friendly about same-sex relationships. The MPAA has also been accused of showing the same biases in their own use of the ratings for films, prompting fans to not wish to use a system they see as inherently flawed.A fan in 2003 wrote:
I would argue that for the most part, [MPAA ratings are] useless information. Or, rather, anything except "NC-17" is useless. That particular label can be useful in finding or avoiding stories, but I'd be curious to know how many people have ever chosen to read or not read a story because it was R instead of PG-13, or vice versa. 
Fanfiction Ratings, as translated from other rating systems, a practical guide
G: Suitable for all audiences. This does not mean recommended for children, but rather that it includes nothing that would likely disturb children or offend their parents for them to read. Romantic content is very innocent if it exists (kissing, holding hands), and violence is infrequent and mild (someone might throw a punch, but no one will die.) The original source text may be rated higher than the story.
PG: This rating may mean a slightly higher level of violence -- action scenes in which violence and its results are not graphically described, but people fight with serious injury or death resulting. It may also mean a slightly higher level of sexual content, including textual sexual attraction and open-mouthed kissing or "making out," or may be applied simply because a story contains swearing.
PG-13/14/15: This rating, as it is in movies, is something of a gray area. It contains slightly more intense material than a PG-rated story, but is not as intense as an R-rated story. Violence may be described in detail some readers may find disturbing. References to drug addiction, incest, and other mature themes may appear, but not be described in detail. Actual sex may be implied to happen, although sex scenes are usually still "fade to black." The content would not be inappropriate in modern young adult fiction.
R/M/MA: More graphic than PG-13 or PG-14, these ratings may be used for extreme violence and gore in a story, especially by Europeans. Americans tend to use the R rating most often for stories that include sex scenes that are not graphic in their language.  (Think 'hardness' rather than 'cock.') R is also used by some authors to warn of mature or disturbing themes such as rape, incest, or child abuse/pedophilia when not described in explicit detail.
NC-17: This is commonly used as the highest rating for stories, although fans used to the Australian or Canadian movie ratings may use "R" with a similar meaning. It almost always means that there is explicit, graphic sexual content. Stories with this rating may be mainly intended to be erotic or pornographic. They may include graphic kink and explicit or vulgar language for sexual activities, sexuality, and sex organs. Stories including graphic rape or child abuse/pedophilia are almost always rated NC-17. NC-17 is rarely used to mean anything but mature sexual content, but some authors use it for violence or other themes they consider extremely disturbing (extreme, graphic torture or mass murder, cannibalism, etc.)
Fandom responded both by arguing for their legal right to use NC-17 and the rest of the MPAA's rating system, and by creating new rating systems that would be both entirely legal and fan-friendly.
One new rating system that arose is The Fan Rated Rating System:
- FRC — Fan Rated Suitable For Children
- FRT — Fan Rated Suitable For Teenagers
- FRM — Fan Rated Suitable For Mature Persons
- FRAO — Fan Rated Suitable For Adults Only
- O Big Brother, Where Are Thou? Um, actually, right over here. In the Fanfic section (Fall 2001)
- You're Messing with My OCD, Fandom. You Wanna Fight?, Archived version by viciouswishes (January 2007)
- Warnings and categories and ratings, oh my...are you up for a little debate?, Archived version (2015)
- About Minors Using This Site..., Archived version by Drift at Pillowfort (January 2019)
- Wikipedia Category:Media content ratings systems
- Wikipedia Television Content Rating Systems
- Ford Cortina rating system explained on the lifein1973 comm
- What Do The Ratings Mean? MPAA
- MPAA Gives FanFic Site a Bad Review on chillingeffects.org, March 17, 2005, accessed 2010-5-17.
- Movie Rating System: Australia
- ClassInd -- Classificação Indicativa, Federal Government Rating Guide - Brazil
- Media Awareness Network page on the Canadian TV Classification System
- Media Awareness Network page on Film Classification in Canada
- Quoted from the Archive ratings help pop-up, accessed March 28, 2010
- elfwreck, A Plea To Stop Using Movie Ratings For Fic, posted March 10, 2008, accessed March 28, 2010
- from The Sliding Scale of Story Information by cereta
- See Highlander, the section on An International Co-Production, to see how this same push-pull worked with a canon source
- MPAA, Heidi, 14 February 2005.