Probability Calculator
Find the odds over repeated tries. Enter the chance of a single event and how many times you try to see the chance of at least one success.
About the Probability Calculator
This calculator answers a question that trips up a lot of people: if something has a fixed chance of happening on each try, how likely is it to happen at least once across many independent tries? It uses the complement rule, so the chance of at least one success equals 1 minus the chance of failing every single time. It also reports the expected number of successes.
How to use it
- Enter the probability of one event as a percent, for example
10for a one-in-ten chance. - Enter the number of trials, the times you repeat the attempt.
- See the chance of at least one success and the expected count.
The key assumption is that the trials are independent and the per-event probability stays the same each time, which fits dice rolls, loot drops and coin flips but not situations where earlier outcomes change later odds. Notice that even a small per-try chance becomes very likely over many tries: a 10 percent event has roughly a 65 percent chance of occurring at least once in 10 attempts. The maths runs in your browser and nothing is uploaded.
Frequently asked questions
How do I calculate the chance of something happening at least once?
Use the complement rule: 1 minus (1 minus p) raised to the power of the number of trials, where p is the single-event probability. For a 10 percent event over 5 tries, that is 1 - 0.9^5, about 41 percent.
What does the expected number of successes mean?
It is the average number of times the event would occur if you repeated the whole experiment many times. It equals the per-event probability multiplied by the number of trials, so 10 percent over 5 trials gives an expected 0.5 successes.
Does this assume the trials are independent?
Yes. It assumes each trial has the same probability and does not affect the others, which suits dice, coin flips and random drops but not draws without replacement.
Why is at least one success so likely over many tries?
Each extra trial gives another chance to succeed, and the probability of failing every time shrinks fast. A rare per-try event can become more likely than not after enough independent attempts.