There are many costs to consider for the movie beyond the production costs.

Based on statistics and data, a safe rule of thumb is four times the budget for the movie to break even.

Take the production costs (the "budget") and double it to account for all marketing, legal, and other costs outside the main production costs. Then you double it again to account for the fact that movie theaters typically take a 50% cut of all box office ticket sales. 2×2=4

2.5 times the budget is a more generous, but less safe, number to use.