Probability Calculator
Calculate the probability of single and combined events
Single Event Probability
out of
Two Independent Events (A and B)
P(A) =
%
P(B) =
%
What is the Probability Calculator?
Probability measures how likely an event is to happen, on a scale from 0 (impossible) to 1 (certain). This calculator covers two common cases: the probability of a single event based on favorable vs. total outcomes, and the combined probability of two independent events happening together (AND) or at least one happening (OR).
How to Use the Probability Calculator
- For a single event, enter the number of favorable outcomes and the total number of possible outcomes (e.g. rolling a 3 on a six-sided die: 1 favorable out of 6 total).
- For two independent events, enter each event's individual probability as a percentage to see the combined AND/OR probability.
Probability Formulas
Single event: P(A) = Favorable Outcomes ÷ Total Outcomes
Both happen (independent): P(A and B) = P(A) × P(B)
At least one happens: P(A or B) = P(A) + P(B) − P(A and B)
Both happen (independent): P(A and B) = P(A) × P(B)
At least one happens: P(A or B) = P(A) + P(B) − P(A and B)
Example
The probability of drawing a heart from a standard deck of 52 cards is 13 ÷ 52 = 0.25 (25%). For two independent coin flips, the probability of both landing heads is 0.5 × 0.5 = 0.25 (25%), and the probability of at least one heads is 0.5 + 0.5 − 0.25 = 0.75 (75%).
Frequently Asked Questions
Two events are independent when the outcome of one doesn't affect the probability of the other — like two separate coin flips or dice rolls. The AND/OR formulas here assume independence; dependent events require conditional probability instead.
0 means an event can never happen, and 1 (or 100%) means it's certain to happen. Every other probability falls somewhere in between, reflecting how likely the event is relative to all possible outcomes.
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