Probability Calculator
Calculate the probability of an event — P(A) = favorable / total outcomes.
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Quick Tip
Enter the number of favourable outcomes — the results you are interested in.
About This Tool
Why use Probability Calculator?
Probability is the number of favourable outcomes divided by the total number of equally likely outcomes. Enter both and this calculator returns the probability as a percentage, a decimal and a fraction, plus the probability of the event not happening and the odds.
Rolling a 6 on a fair die is 1 ÷ 6 = 0.1667, or 16.67%, with odds of 1:5 against. The complement rule always applies: P(not A) = 1 − P(A), so the chance of not rolling a 6 is 83.33%. Probabilities always sit between 0 (impossible) and 1 (certain).
Key Benefits
Percentage, decimal, fraction and odds
The same probability in every notation, including the odds format used in betting and risk work.
Complement calculated for you
P(not A) is often the quicker route to an answer, especially for "at least one" questions.
Kept within range
Results are capped at 1, so an input mistake cannot produce a probability above 100%.
How to use Probability Calculator
- 1
Enter the number of favourable outcomes — the results you are interested in.
- 2
Enter the total number of possible outcomes.
- 3
Read the probability as a percentage, decimal and fraction.
- 4
Use the complement (probability of not happening) when the question is phrased the other way round.
Best use cases
Work out the chance of drawing a particular card or rolling a given number.
Convert a probability into odds for a report or a bet.
Check probability homework answers.
Estimate simple risk, such as the chance a randomly picked item is defective.
Probability Calculator FAQs
What is the basic probability formula?
P(A) = favourable outcomes ÷ total outcomes, assuming every outcome is equally likely. Drawing a heart from a full deck is 13 ÷ 52 = 0.25.
What is the difference between probability and odds?
Probability compares favourable outcomes with all outcomes (1 in 6). Odds compare favourable with unfavourable (1:5). A probability of 0.5 is odds of 1:1.
How do I find the probability of two events both happening?
For independent events, multiply them: two heads in a row is 0.5 × 0.5 = 0.25. If the first event changes the second, use the conditional probability for the second step.
How do I calculate the chance of at least one success?
Use the complement: 1 − P(no successes). For four coin flips, the chance of at least one head is 1 − 0.5⁴ = 93.75%.
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