Whether you need to get your film rated before you can screen it depends entirely on which country you are planning to conduct your screening in. The approach to regulating film screenings (and video/DVD distribution) varies dramatically across the world.
In the USA, the movie rating system is administered by the Motion Picture Association of America (MPAA) and is voluntary. Because all of the Studios and major production companies are members of the organisation most will tow the line and respect the MPAA’s ratings decisions.
When a film moves into the video/DVD market, it will normally retain its theatrical rating, however the MPAA does not force distributors to do this, hence the fact you will occasionally see a film listed as being “unrated”. You may run into problems with local governments if your film has content which might be controversial and it hasn’t been rated by the MPAA, but as far as we’re aware, there isn’t a legal requirement to have it rated.
If you live in another country, the rules can be completely different. This is often because film ratings are administered by the government, rather than through an industry self-regulation model like in the US. For example, in the UK film ratings are managed by the British Board of Film Classification (BBFC). All films which are shown in cinemas must have a BBFC certificate if you intend to charge people to view them. The only exception to this is where you have received permission from the local council to screen the film in your borough. Interestingly, local councils have the power to change a BBFC rating or ignore it altogether, although instances of this happening are extremely rare.
In the case of video/DVD releases in the UK, under the Video Recordings Act, virtually all films must have received a video certificate before they can be sold in the UK (including being sold over the Internet from a UK-based store). There are a few types of film which are exempt from this rule, such as some music, documentaries, and educational works, but you will need to get proper advice in order to determine whether your film falls into an exempt category (most will not). Trading Standards imposes stiff penalties on distributors and retailers who sell unrated films on video/DVD.
In Australia, ratings are administered at a federal government level by the Office of Film & Literature Classification (OFLC). Under the Commonwealth Classification Act, all films which are sold or publicly exhibited in Australia must have a certificate from the OFLC. There is however an exemption for films which are shown solely in a festival environment. In this case, an unrated film can be shown provided that the festival is restricted to people 18 years or older. Although the OFLC is responsible for assigning classifications, the federal and state attorney generals can overrule the OFLC’s decision and ban a film from being shown in their state (or refuse classification in the case of the federal attorney general). A very famous case of this happening occurred at the 2003 Sydney Film Festival where the federal attorney general stepped in to ban the festival’s screening of Larry Clarke’s Ken Park.
In Canada, theatrical movie ratings are a provincial and territorial responsibility. There are a total of seven provincial film classification boards or offices, many of which use different rating systems and descriptors.
Negotiations have begun to create a standard film classification system for Canada (with the exception of Quebec).
British Columbia classifies films for Saskatchewan and the Yukon. Alberta classifies films for the Northwest Territories and Nunavut. Nova Scotia classifies films for the Maritimes system which includes New Brunswick and Prince Edward Island. Newfoundland and Labrador does not follow a film classification system (although many cinemas and video stores follow the classifications laid down in Nova Scotia).
In other countries, the rules can vary dramatically so it’s always best to contact the local ratings authority before screening your film.