Probability Calculator
Find single-event probability plus the odds, complement and repeated-trial chances.
What the Probability Calculator does
Probability is the count of outcomes you want divided by the count of outcomes possible, assuming each is equally likely. The "at least once" figure uses the complement rule: it is far easier to compute the chance of never happening and subtract from one.
Formula
P(A) = favourable ÷ totalP(not A) = 1 − P(A)P(at least one in n) = 1 − (1 − p)ⁿ
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Favourable outcomes | number | Yes | Accepts 0 or more. |
| Total possible outcomes | number | Yes | Accepts 1 or more. |
| Number of trials | number | Optional | For "at least once in N tries". Accepts 1 or more. |
How to use it
- Enter Favourable outcomes and Total possible outcomes.
- Optionally add Number of trials.
- Select Calculate.
Worked example
Rolling a six on a die, across 10 rolls.
- Favourable
- 1
- Total
- 6
- Trials
- 10
Single roll 16.67%; at least one six in ten rolls is 83.85%.
Frequently asked questions
What is the difference between odds and probability?
Probability compares favourable outcomes to all outcomes (1 in 6). Odds compare favourable to unfavourable (1 to 5).
Does a run of losses make a win more likely?
No, for independent events. That belief is the gambler's fallacy — a fair die has no memory of previous rolls.