Probability Calculator

Favorable outcomes

Total outcomes

Probability —

As a percentage —

Complement P(not E) —

Odds in favor —

How to Calculate Probability

Enter favorable and total outcomes, or the probabilities of two events.

Count the outcomes

For a single event, enter the number of favorable outcomes and the total number of equally likely outcomes.

Read probability, complement and odds

The probability is favorable ÷ total. The complement is the chance it does not happen, and the odds express the same thing as a ratio.

Combine independent events

Switch tabs and enter two probabilities to get the chance that both happen (AND) and that at least one happens (OR).

Frequently Asked Questions

How do you calculate the probability of an event?

To calculate the probability of an event, divide the number of favorable outcomes by the total number of equally likely outcomes. Drawing a card of one particular suit from a deck gives 13 ÷ 52 = 0.25, or 25%. The result always lies between 0 (impossible) and 1 (certain), and can be written as a decimal, a fraction or a percentage. The formula only applies when the outcomes really are equally likely, which is why loaded dice need a different treatment.

What is the complement of a probability?

The complement of an event is the probability that it does not happen, equal to 1 minus the event probability. An event with probability 0.25 has a complement of 0.75. Complements are often the shortest route to an answer: the chance of at least one head in three coin tosses is easier as 1 − 0.5³ = 0.875 than as a sum of separate cases. An event and its complement always add to exactly 1.

How do I combine two independent events?

For independent events the probability that both happen is P(A) × P(B), so two fair coins both landing heads is 0.5 × 0.5 = 0.25. The probability that at least one happens is 1 − (1 − P(A))(1 − P(B)), which for the same two coins is 1 − 0.25 = 0.75. Independence is the key assumption, meaning the outcome of one event does not change the probability of the other. For dependent events you need the conditional form instead.

Is my data sent anywhere?

Nothing you type is sent anywhere. The probability calculator runs entirely in your browser with JavaScript, so your outcome counts and probabilities never leave your device. There is no account, no upload step and nothing is logged. The page keeps working offline once it has loaded.

How to Calculate Probability

Count the outcomes

Read probability, complement and odds

Combine independent events

Frequently Asked Questions

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