Probability Calculator

Work out the probability of two independent events, with and, or, and not.

Runs in your browser — nothing is sent or saved.

How to use the probability calculator

  1. Enter the probability of event A, as a decimal between 0 and 1.
  2. Enter the probability of event B for two-event results.
  3. Read the combined probabilities.
  4. Copy the link to save or share it.

Single and two-event probability

Probability measures how likely something is, from 0 (impossible) to 1 (certain). Enter one value for a single event probability and its complement, the chance it does not happen. Enter a second for the two-event results: the chance both happen, the chance at least one happens, the chance neither does, and the chance of exactly one. Written as odds, a probability of 0.25 is the same as 1 in 4.

And, or, and not

The core rules are simple for independent events. The probability of A and B both happening is P(A) times P(B). The probability of A or B, meaning at least one, is P(A) plus P(B) minus the overlap P(A and B). The probability of not A is 1 minus P(A). These are the building blocks behind most everyday probability questions.

Independent events only

The and/or results here assume the two events are independent, meaning one does not affect the other, like two separate coin flips. If the events are linked, for example drawing cards without replacement, the simple multiplication rule does not hold and you need conditional probability instead. The tool notes this so you do not misapply it.

A worked example

Suppose P(A) is 0.5 and P(B) is 0.3, and they are independent. The chance both happen is 0.5 times 0.3, which is 0.15. The chance at least one happens is 0.5 plus 0.3 minus 0.15, which is 0.65. The chance neither happens is 0.5 times 0.7, which is 0.35.

FAQ

How do I calculate the probability of two events?

For independent events, multiply the two probabilities for both to happen, and add them and subtract the overlap for at least one. The tool shows P(A and B), P(A or B), and more.

What does independent mean?

Independent events do not affect each other, like separate coin tosses. The multiplication rule for and only works when events are independent.

How do I write a probability as odds?

Odds compare the chance it happens to the chance it does not. A probability of 0.2 is odds of 1 to 4, since it is one favorable to four unfavorable.

What is the complement of an event?

The complement is the chance the event does not happen, equal to 1 minus its probability. The tool shows P(not A) and P(not B).

Is my information private?

Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.