About the Probability Calculator
What is the Probability Calculator?
Probability measures how likely an event is, on a scale from 0 (impossible) to 1 (certain), often written as a percentage. When all outcomes are equally likely, it is simply the number of favourable outcomes divided by the total number of outcomes.
Enter the favourable and total outcomes — for example 1 and 6 for rolling a six on a die. The calculator gives the probability as a percentage and a decimal, the chance the event does not happen, the odds in favour and the chance of it happening at least once over several independent tries.
How to use the Probability Calculator
- Favourable Outcomes — the number of outcomes that count as success. (sample: 1)
- Total Possible Outcomes — all equally likely outcomes. (sample: 6)
- Number of Independent Trials — how many independent times the experiment is repeated. (sample: 3)
- Press Calculate Probability (results also refresh as you type). The result card shows the Probability of the Event, along with As a decimal, Probability it does not happen, Odds in favour and At least once in 3 trials.
Formula used
P(A) = favourable outcomes ÷ total outcomes
- P(not A) = 1 − P(A)
- P(at least once) = 1 − (1 − P(A))ⁿ over n independent trials
- Odds in favour = favourable : unfavourable
The “at least once” formula works by finding the chance of the event never happening in n trials and subtracting it from 1.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Favourable Outcomes | 1 |
| Total Possible Outcomes | 6 |
| Number of Independent Trials | 3 |
| Result | Value |
|---|---|
| Probability of the Event | 16.6667% |
| As a decimal | 0.166667 |
| Probability it does not happen | 83.3333% |
| Odds in favour | 1 : 5 |
| At least once in 3 trials | 42.1296% |
Tips for using the Probability Calculator
- Probability and odds are different: a 1-in-6 chance is odds of 1 : 5.
- Past results do not change the next independent trial — after five tails, the next coin toss is still 50% heads.
- Repeated chances add up quickly: a 5% event has about a 40% chance of happening at least once in 10 tries.