Probability Calculator

Probability is a number between 0 and 1 that measures how likely an event is. 0 means the event cannot happen, 1 means it is certain, and 0.5 means a 50% chance - like a fair coin flip. Enter any value in that range and the calculator converts it to a percentage and odds ratio for you.

Multiply the two probabilities: P(A and B) = P(A) x P(B). This formula only holds when the events are independent - meaning the outcome of one does not affect the other. For example, rolling a 6 on a die (1/6) and flipping heads (1/2) gives P(both) = 1/12, about 8.33%.

The general addition rule is P(A or B) = P(A) + P(B) - P(A and B). In this calculator's independent-events mode, P(A and B) equals P(A) x P(B). If the events are dependent, enter or calculate their actual joint probability instead of multiplying the two marginal probabilities.

P(A|B) is the probability of A given that B has already occurred. The calculator computes it as P(A and B) / P(B), which requires P(B) > 0 and P(A and B) <= P(B). For example, if P(A and B) = 0.1 and P(B) = 0.4, then P(A|B) = 0.25, or 25%.

Odds express the ratio of favorable outcomes to unfavorable ones. The calculator reduces this to the lowest integer ratio using the greatest common divisor. A probability of 0.25 (25%) becomes odds of 1:3 in favor - one favorable outcome for every three unfavorable ones. To go the other way: if odds are 1:3, probability = 1 / (1 + 3) = 0.25.