Loot Drop Probability Calculator

What this calculates

Games, gacha systems, and randomized loot tables all boil down to the same math: a fixed drop rate applied independently on every attempt. This tool answers the questions that come up when you're deciding whether to keep farming — how likely you are to get what you need within a certain number of tries, and how many tries you should budget for.

How to use it

1. Enter the drop rate as a percentage (e.g. 2 for a 2% chance per kill/chest/pull).
2. Set the number of attempts you're planning (or have already made).
3. Set the target number of drops you need — leave it at 1 for a simple "did I get it at all" check, or raise it for crafting recipes that need several copies of the same item.

What you get

• Probability of at least K drops and probability of fewer than K drops across your chosen number of attempts — a full binomial calculation, not just the "at least one" case most drop-rate calculators stop at.
• Expected number of drops — the long-run average (attempts × drop rate), useful as a sanity check against a specific run's luck.
• Attempts needed for 50% / 90% / 99% confidence of landing at least one drop — plan a farming session around a real odds threshold instead of guessing.
• An attempt-by-attempt probability curve, downloadable as CSV, showing exactly how the odds climb (and where they flatten out) as you put in more attempts.

The math

Each attempt is an independent trial with success probability p (the drop rate). Across n attempts, the number of successes follows a binomial distribution, and:

P(at least K drops) = Σ (i = K to n) C(n, i) · pⁱ · (1-p)ⁿ⁻ⁱ

computed via a numerically stable PMF recurrence rather than raw binomial coefficients (which overflow long before this tool's attempts cap). The attempts-for-confidence figures solve the simpler "at least one" case for n:

n = ⌈ ln(1 - confidence) / ln(1 - p) ⌉

Common uses

• Deciding whether a rare crafting material is worth farming for, and roughly how long that'll take
• Checking whether a "guaranteed drop by attempt N" claim in a game's patch notes actually holds up
• Estimating gacha/loot-box odds before spending real money on pulls

For general probability distributions beyond a fixed per-attempt drop rate — dice, coin flips, custom weighted outcomes — try the Random Generator or Weighted Random Picker tools.

Privacy

This tool runs entirely in your browser. Your numbers are never uploaded to a server.