Every registered user can rate any game in the BGG database. To do this, find the game and go to that game's Main Page; this is most easily done using the Search option near the top of every page. In the topmost section is a series of stars labeled "My Rating", which you can click on to set your rating on a scale from 1 to 10 (once set, you can click on edit to use decimals). Although these ratings are entirely subjective, here are the suggested guidelines:
- 10 - Outstanding - will always enjoy playing and expect this will never change.
- 9 - Excellent - always enjoy playing it
- 8 - Very good - enjoy playing and would suggest it.
- 7 - Good - usually willing to play.
- 6 - Ok - will play if in the mood.
- 5 - Mediocre - take it or leave it.
- 4 - Not so good - but could play again.
- 3 - Bad - likely won't play this again.
- 2 - Very bad - won't play ever again.
- 1 - Awful - defies game description.
A further rating you can give to a game is the Game Play Weight. This is another personal opinion expressing how difficult the game is to play - either because it has long and/or complex rules, requires deep thought to play well or some other factor. "Weight" is not actually defined at BGG so different people have different ideas of what it means. The choices for Game Play Weight Are:
- Medium Light
- Medium Heavy
The Game Play Weight is selected from a Drop Down List. Your Game Play Weight rating is automatically saved when you make a selection.
All recorded Game Play Weights are then averaged to produce a single Game Play Weight that is shown in the "Statistics" section for each game as Avg. Game Weight.
All recorded User Ratings are averaged to produce a single Average Rating shown in a game's "Statistics" section.
If a user has and rates several copies of a game in their collection, then only their highest rating for the game is used.
BGG does not allow people to give multiple ratings to a game through the use of separate accounts.
The User Ratings are also used to determine the Rank of a game in the BGG database. Only games that have at least 30 User Ratings are eligible for Ranking and to the User Ratings are added a number of "dummy" ratings, which are then used to produce a new average Rating. (E.g. see this thread.) This is the rating that shows up in BGG searches and the number can, and does, vary from the Average Rating. In effect the "dummy" ratings move a game's average towards the norm of all games on the database - making games with few votes but very high ratings lower ranked than games with many more ratings but a lower Average Rating. (If you want to know more about this process, search on "Bayesian" within BGG.) Additionally, secret undocumented stuff is done to try to filter out obviously bogus "shill" or "hate" ratings. (There are many threads from people asking about or trying to figure out the details, but they are intentionally undocumented.)
Note that the number of "dummy" ratings apparently depends on the total number of ratings. This explains the apparent "paradox" (often asked in the forums, e.g. here) of why game X is higher ranked than game Y overall, but game Y is higher ranked than game X in subdomain Z.