Probability Calculator
Calculate probability for single events, two independent events, and conditional probability. Includes visual bars, presets, and step-by-step work.
About Probability Calculator
Understanding probability is essential for statistics, data science, and everyday decision making. This calculator covers the three most common probability calculations with visual feedback and clear explanations.
Single Event Probability
Enter the number of favorable outcomes and total possible outcomes to get P(A). The calculator shows the probability as a decimal, percentage, and simplified fraction. It also shows the complement P(not A). Quick presets let you test common scenarios like coin flips, dice rolls, and card draws.
Two Independent Events
Enter P(A) and P(B) as decimals to see P(A and B), P(A or B), and their complements. Color-coded probability bars make it easy to compare the different outcomes at a glance. This assumes the events are independent (one does not affect the other).
Conditional Probability
For dependent events, the conditional probability tab calculates P(A|B) from P(A intersect B) and P(B). A step-by-step breakdown walks through the formula so you can follow along. If you need to compute standard deviations alongside your probability work, the Standard Deviation Calculator is a useful companion.
Privacy
Everything runs in your browser with nothing sent to any server. For related fraction work, check the Fraction Calculator.
Frequently Asked Questions
How do I calculate basic probability?
Divide the number of favorable outcomes by the total number of possible outcomes. For example, rolling a 6 on a standard die has probability 1/6 because there is 1 favorable outcome out of 6 total.
What is the difference between P(A and B) and P(A or B)?
P(A and B) is the probability that both events happen. For independent events, it equals P(A) multiplied by P(B). P(A or B) is the probability that at least one event happens, which equals P(A) + P(B) minus P(A and B).
What is conditional probability?
Conditional probability P(A|B) is the probability of event A occurring given that event B has already occurred. It is calculated as P(A and B) divided by P(B). The condition (event B happening) narrows down the sample space.
What are independent events?
Two events are independent when the outcome of one does not affect the probability of the other. For example, two coin flips are independent since the first flip result has no influence on the second.
Can I enter fractions instead of decimals?
The single event calculator accepts favorable and total outcomes as whole numbers and displays the result as a simplified fraction. For the two-event and conditional calculators, enter probabilities as decimals between 0 and 1.