The film was released in the United States on July 25, 2008. In an interview with Entertainment Weekly, Carter said that if I Want to Believe proved successful, he would propose a third movie that would return to the television series mythology and focus on the alien invasion foretold within the series, due to occur in December 2012. The film grossed $4 million on its opening day in the United States. It opened fourth on the U.S. weekend box office chart, with a gross of $10.2 million. By the end of its theatrical run, it had grossed $20,982,478 domestically and an additional $47,373,805 internationally, for a total worldwide gross of $68,369,434. Among 2008 domestic releases, it finished in 114th place. The films stars both claimed that the timing of the movies release, a week after the highly popular Batman film The Dark Knight (film), negatively affected its success. The film received mixed to negative reviews. Metacritic, which assigns a standard score rating out of 100 reviews from mainstream film critics, reported "mixed or average" reviews, with an weighted mean score of 47 based on 33 reviews. Rotten Tomatoes reported that 32% of 160 listed film critics gave the film a positive review, with an average rating of 4.9 out of 10. The website wrote of the critics consensus stating; "The chemistry between leads David Duchovny and Gillian Anderson do live up to The X-Files televised legacy, but the roving plot and droning routines make it hard to identify just what were meant to believe in."

How much of the film's gross was not done domestically?
47,373,805