What this calculator does
Drop rate here refers to the chance a game gives of a specific item dropping from a source, such as a loot box, monster kill or crafting attempt, each time you try. A single low drop rate can still add up to a good chance of success once it is attempted many times, and this calculator works out exactly how good using the standard drop rate formula.
The formula is P(at least one drop) = 1 minus (1 minus p) to the power of n, where p is the per-attempt drop chance and n is the number of attempts. This is the same "at least one success" logic used anywhere an independent chance is repeated: it is not simply the drop rate multiplied by the number of tries.
The formula
Convert the drop chance to a decimal, subtract it from 1 to get the chance of missing on a single attempt, raise that to the power of the number of attempts to get the chance of missing every single time, then subtract that from 1 to get the chance of at least one success.
| Term | Meaning |
|---|---|
| Drop rate (p) | The probability of the item dropping on any single attempt, assumed constant across attempts. |
| Attempts (n) | How many independent tries you plan to make. |
| P(at least one) | The probability of getting the drop at least once across all n attempts. |
The inputs explained
| Field | What to enter |
|---|---|
| Drop chance per attempt (%) (%) | The stated or estimated chance of the drop on a single attempt. |
| Number of attempts | The number of attempts, kills, chests or rolls you plan to make. |
When to use it
Planning a farming session
Before grinding a boss or activity repeatedly for a rare drop, working out the odds over a realistic number of attempts sets expectations for how long it might reasonably take.
Comparing two drop sources
A lower drop rate from a faster-to-repeat source can still beat a higher drop rate from a slow one, once both are converted to a chance over the same time budget.
Judging whether a bad run is unlucky or expected
Checking the probability of still having no drop after many attempts shows whether a frustrating streak is actually a common outcome or a genuine outlier.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Chance of at least one drop as attempts increase
A 5% per-attempt drop rate, tried a range of times.
Questions
What is the drop rate formula?
P(at least one drop) = 1 − (1 − p)^n, where p is the per-attempt drop chance as a decimal and n is the number of attempts. It gives the probability of succeeding at least once across all attempts.
Why is the answer not just the drop rate times the number of attempts?
Multiplying drop rate by attempts overstates the chance once it approaches or exceeds 100%, and ignores that some attempts could all miss. The 1 − (1 − p)^n formula correctly accounts for every attempt missing as the only way to end up with zero drops.
Does this assume every attempt is independent?
Yes. It assumes each attempt has the same drop chance regardless of previous attempts, with no pity system, bad-luck protection or rate increase built in. Games with those systems will see better real odds than this formula alone predicts.
Is this the same as an IV drip rate calculation?
No, that is a different meaning of "drop rate". For medical infusion drip rates in drops per minute, see the drops per minute calculator instead.
Looking for IV infusion drip rates instead of loot probability? See the drops per minute calculator. For basic probability of independent events generally, see the probability calculator.