Probability calculator.
Use this free probability calculator to get fast estimates, compare scenarios, and understand the numbers with clear step-by-step context.
Use the Probability Calculator.
Results update automatically as you type. No account is required.
Enter your values
About the Probability Calculator
This probability calculator works with two independent events and finds the chance of both happening, either happening, and neither happening. Probabilities are entered as percentages from 0 to 100.
How it works
For independent events: P(A and B) = P(A) × P(B); P(A or B) = P(A) + P(B) − P(A and B); P(not A) = 1 − P(A). Independent means one event does not affect the other.
Reading the result
The headline is the chance of both events occurring, with the "or", "not A", and "neither" probabilities shown alongside.
Note: Calculations run instantly in your browser and use exact mathematical formulas. For very large numbers, results are shown to standard floating-point precision.
FAQ
What does "independent events" mean?
Two events where the outcome of one has no effect on the other — like two separate coin flips. The formulas here assume independence.
Why subtract P(A and B) for "or"?
Because adding the two probabilities counts the overlap (both happening) twice, so you subtract it once to correct.
Can probabilities exceed 100%?
No. A single probability is always between 0% and 100%. If your inputs are larger, they are not valid probabilities.