OSRS Drop Rate Calculator
Use exact binomial probability to understand OSRS drop odds. Enter a 1-in-X drop rate, attempts, observed drops, and target confidence.
- Chance of Zero Drops
- 36.751989%
- Chance of Exactly Selected Drops
- 36.823911%
- Expected Number of Drops
- 1 drops
- Attempts for 50%
- 355 attempts
- Attempts for 90%
- 1,178 attempts
- Attempts for 95%
- 1,533 attempts
- Attempts for 99%
- 2,356 attempts
- Attempts for Custom Confidence
- 1,533 attempts
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Results show the chance of no drop, at least one drop, exactly the selected number of drops, expected drops, attempts at 50%, 90%, 95%, 99%, and attempts for a custom confidence.
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How to Use This Calculator
Enter the denominator for a 1-in-X rate, total independent attempts, the exact number of drops you want to model, and a target confidence percentage. Treat each attempt as an independent roll with the same probability.
Formula & Methodology
Per-attempt probability is 1 divided by the drop-rate denominator. Zero-drop probability is (1-p)^n. At-least-one probability is 1-(1-p)^n. Exactly k drops uses the binomial formula C(n,k)p^k(1-p)^(n-k). Attempts for a target confidence use logarithms.
One-in-512 drop example
For a 1-in-512 item after 512 independent attempts, the chance of at least one drop is about 63.25%, not 100%.
The model assumes independent identical rolls. It is not valid without adjustment for pity systems, changing rates, tertiary tables with special conditions, multiple linked rolls, team allocation, thresholds, or mechanics where one outcome changes another.
Probability describes long-run likelihood, not certainty. A player can receive a drop early or remain dry far beyond the listed milestones.
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Related Guides
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Frequently Asked Questions
- What is the chance of at least one drop?
- It is 1 minus the probability of missing the drop on every independent attempt.
- Am I guaranteed the item at the listed drop rate?
- No. At n attempts for a 1-in-n item, the chance of at least one drop is only about 63.2% for large n.
- What does exactly-k probability mean?
- It is the probability of receiving exactly the selected number of drops across all attempts.
- Can I use combined drop rates?
- Only when the combined per-attempt probability is known and the events are represented correctly.
- Does previous bad luck improve the next roll?
- Not for independent rolls without bad-luck protection. The next attempt keeps the same probability.
Last updated 7/17/2026