Statistical calculator for "drop rates" in video games and similar scenarios. The outcome of each roll (monster kill) is assumed to be independent of every other and has an equal probability of giving the desired outcome.
% chance of desired outcome per roll.
After rolls, the probability of getting at least one desired outcome is 5%.
p n = 1 - ( 1 - p ) n
p = probability of one roll success pn = probability of at least one success in n rolls n = number of rolls
To achieve % probability of at least one desired outcome happening, 58 rolls are needed.
n = ⌈ log ( p n ) log ( p ) ⌉
p = probability of one roll success pn = confidence of a success after n rolls n = number of rolls needed