Drop Rate Probability Calculator

About this calculator

If a single attempt has a fixed drop chance p, the probability of getting at least one drop across n independent attempts is 1 − (1 − p)^n, not p × n as intuition often suggests — this is because the calculation works out the chance of getting nothing at all across every attempt, then subtracts that from 1, which correctly accounts for the fact that at least one success and all failures are complementary outcomes.

Gamers use this formula constantly to estimate their odds of getting a rare item drop, loot box pull, or gacha character over a given number of runs, pulls, or kills, and it’s the same math game designers use in reverse when balancing drop rates to hit a target average number of attempts before a player gets an item. It also explains why pity or guaranteed-drop systems exist in many games — without them, a low enough drop rate leaves a meaningful chance of going far longer than average without a single drop.

This calculator takes a drop rate and number of attempts and returns the probability of getting at least one drop.