About this tool
The Drop Rate Calculator turns a per-attempt drop chance into the odds that actually matter when you are grinding: your chance of seeing at least one drop over many tries, and how unlucky a dry streak really is. It runs entirely in your browser with no data leaving your device.
Enter the drop chance either as a percentage or as a 1-in-N rate using the toggle, plus the number of attempts n. The chance of at least one drop is 1 − (1 − r)ⁿ; the dry-streak chance of getting none is (1 − r)ⁿ; and the expected number of attempts to a first drop is 1/r. To reach a confidence level c it needs ceil(ln(1 − c) / ln(1 − r)) attempts, shown for 50%, 90% and 99%.
For a 1% drop over 100 attempts the chance of at least one is 1 − 0.99¹⁰⁰ ≈ 63.4%, which is why the "expected" 100 attempts is far from a sure thing. Use the confidence figures to plan how many runs give you a comfortable chance rather than relying on the average alone.
Frequently asked questions
Why isn't 100 tries at 1% a guaranteed drop?
Because each attempt is independent. The chance of missing every time is 0.99¹⁰⁰ ≈ 36.6%, so the chance of at least one drop is only about 63.4%. The 1/r 'expected' value is an average, not a guarantee.
What does the confidence figure mean?
It is the number of attempts needed for the given probability of seeing at least one drop: ceil(ln(1 − c) / ln(1 − r)). For a 1% rate you need about 69 tries for 50% confidence, 230 for 90% and 459 for 99%.
How do I enter a 1-in-N drop rate?
Switch the mode toggle to '1 in N' and type N — for a 1-in-500 drop, enter 500. Internally it uses r = 1/N. In percent mode you instead type the chance directly, such as 0.2 for a 0.2% rate.
What is the dry-streak probability?
It is (1 − r)ⁿ, the chance of going n attempts with no drop at all. It is the exact complement of the 'at least one' chance, and it shows how likely a long unlucky run really is.
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