Probability from a count
Divide the outcomes you want by the total number of equally likely outcomes.
Drawing a heart from a pack gives 13 / 52 = 0.25, so the chance is 25%.
Worked example
A pack has 52 cards and 13 are hearts, so the probability is 13 / 52 = 0.25.
The odds in favour are 13 to 39, and the chance of not drawing a heart is 0.75.
Two events
For independent events, multiply for and: P(A and B) = P(A) × P(B). Add and subtract for or: P(A or B) = P(A) + P(B) − P(A and B).
For mutually exclusive events, both cannot happen, so P(A and B) = 0 and the probabilities simply add.
The complement
The chance of an event not happening is 1 minus the chance of it happening, because something has to happen.
That is often the quickest route: the chance of at least one six in four rolls is 1 minus the chance of no six at all.
Inputs for the probability calculator
What are you working out?
One event from a count of outcomes, or two events from their probabilities.
Outcomes
How many outcomes count as a success, and how many there are altogether.
Probabilities of A and B
Each as a percentage. For mutually exclusive events they cannot add up to more than 100%.
CALCULATOR QUESTIONS
Probability calculator questions
What is the probability of drawing a heart?
13 of the 52 cards are hearts, so 13 / 52 = 0.25, or 25%.
How do I combine two independent events?
Multiply for both happening. For either, add the two probabilities and subtract the chance of both.
What does mutually exclusive mean?
The two events cannot both happen, so the chance of both is zero and their probabilities add directly.
What are odds, and how are they different from probability?
Odds compare winning outcomes with losing ones, 13 to 39 here, while probability compares winning outcomes with all outcomes.