How is the at-least-once probability calculated?

The chance of failing every attempt is (1 - p) raised to the power N, where p is the per-attempt drop rate and N is the number of attempts. The at-least-once probability is 1 minus that value. This is the standard formula for independent repeated trials.

Does a low drop rate guarantee the item after enough attempts?

No. Each attempt is independent, so the probability only approaches 100% but never reaches it. Even after the expected number of attempts you have roughly a 63% chance, not a guarantee.

What is the expected number of attempts?

For a per-attempt chance p, the average number of attempts before the first success is 1 divided by p. A 5% drop averages 20 attempts, but individual streaks vary widely.

How many attempts give me a 90% chance?

The tool solves for N using ln(1 - 0.90) divided by ln(1 - p). For a 5% drop that is about 45 attempts to reach 90% confidence.

Are drop rates in Stardew Valley always fixed?

Many drops have fixed base rates, but some scale with luck, profession, or daily luck modifiers. Enter your effective rate after those bonuses for the most accurate odds.

Stardew Valley Drop Rate & Probability Calculator

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Calculate your real Stardew Valley drop odds

When an item drops at a small percentage per attempt, it is easy to feel unlucky after a long farming session. This calculator turns a single drop rate into the real cumulative probability over many attempts, so you know whether to keep grinding or expect a longer haul.

How it works

Each attempt is an independent trial with success probability p (your drop rate). The probability of getting the item at least once in N attempts is:

P(at least one) = 1 - (1 - p)^N

The expected (average) number of attempts before the first success is 1 / p. To find how many attempts you need for a target confidence C (for example 0.90), solve:

N = ln(1 - C) / ln(1 - p)

rounded up to the next whole attempt. These are the standard formulas for repeated independent RNG rolls and apply to kills, harvests, fishing casts, and chest opens alike.

Tips and example

A 1.5% drop over 50 attempts gives 1 - (0.985)^50 ≈ 53% — barely better than a coin flip. To hit 90% confidence you would need about 152 attempts. Remember the expected value is an average: streaks of bad luck well beyond 1 / p are common and statistically normal. If your effective rate changes (luck buffs, professions), recalculate with the adjusted percentage.