Measure your film's physical length with a ruler or tape measure. Next, multiply your film's physical length in whole feet by 16 (if you're using 35mm film) or by 40 (if you're using 16mm film) to determine the number of frames in that portion of your film.
Count the number of frames remaining in the film that fell outside the whole feet measurement (that is, in the extra inches left over after your whole feet calculation). Add this number of remaining frames to your whole feet frame number to discover the total number of film frames in your film.
Divide the total number of frames in your film by 1,440 to determine the total number of minutes your film will run (running length).