Probability Calculator
Work out basic probabilities — a single event, its complement, and two events happening together or at least one happening — with the formula and assumptions spelled out.
Enter a decimal (0.25), a percentage (25%) or a fraction (1/4).
For independent events (one doesn't affect the other, like two separate coin flips), P(A and B) = P(A) × P(B). If your events affect each other (drawing cards without replacement, for example), neither assumption holds and you'd need the conditional probability instead.
0.083333
P(A and B) as a decimal
8.3333%
As a percentage
1 in 12
Odds of it happening
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How to use probability calculator
- 1Choose a calculation: P(A) from outcomes, Not A, A and B, or A or B.
- 2Enter probabilities as decimals (0.25), percentages (25%) or fractions (1/4).
- 3For two events, say whether they're independent or mutually exclusive.
- 4Read the probability as a decimal, a percentage and as odds.
About this tool
A probability is a number from 0 (impossible) to 1 (certain). With equally likely outcomes, P(A) = favorable outcomes ÷ total outcomes, so rolling a 6 on a fair die is 1/6. The complement — the chance that A doesn't happen — is 1 − P(A).
For two independent events, where one doesn't affect the other (two separate coin flips), the chance of both is P(A) × P(B), and the chance of at least one is P(A) + P(B) − P(A) × P(B). For mutually exclusive events, which can't both happen (rolling a 1 or a 6 on a single roll), P(A and B) is 0 and P(A or B) is simply P(A) + P(B).
Many real situations are neither: drawing two cards without putting the first back changes the odds for the second. Those dependent events need conditional probability, which these basic modes don't cover — so check which assumption fits before relying on the result.
Frequently asked questions
Two events are independent if the outcome of one has no effect on the other — like flipping a coin twice. The probability of both happening is then the product of their individual probabilities.
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