How many kills until a 50%, 90% or 95% chance you'll see the drop you want? Uses the geometric-distribution formula on Terraria's official drop rates.
No chance-based drop is ever truly guaranteed — every kill is independent. For a drop with a 1-in-N rate, the chance of getting it in k kills is 1 − (1 − 1/N)^k. This calculator shows the kills needed for 50%, 90% and 95% confidence.
Drops don't have a counter or pity system — each kill re-rolls independently. Even at a 1-in-100 rate, roughly 1 in 20 players still won't have it after 300 kills. The 90%/95% columns show how far you may need to go.
The rates are the official Terraria drop table (1-in-N chances) for each monster, so the probabilities match the game's actual rolls.