It's really very simple.

Indoors with ISO 800 film: 1/50 sec at f/2.8

Outdoors with sunny 16 rule and ISO 800 film: 1/800 sec at f/16

1/50 sec to 1/800 sec is 4 stops (1/100, 1/200, 1/400, 1/800)

f/2.8 to f/16 is 5 stops (4, 5.6, 8, 11,16)

4 = 5 = 9 stops darker