Probability Calculator
Enter the number of favorable outcomes and total possible outcomes below to instantly get the probability as a fraction, decimal, and percent.
How Probability Is Calculated
Classical probability divides the number of outcomes you care about (favorable outcomes) by the total number of equally likely outcomes possible. The result is a fraction between 0 and 1 that can also be written as a decimal or a percent.
Worked example
A bag has 8 marbles total, 3 of them red. What's the probability of drawing a red marble?
Probability = 3 ÷ 8 = 0.375
As a percent: 37.5%
Frequently Asked Questions
What is the formula for probability?
Probability = favorable outcomes ÷ total possible outcomes. It's always a number between 0 (impossible) and 1 (certain), often written as a percent instead.
What's the difference between theoretical and experimental probability?
Theoretical probability is calculated from the total number of equally likely outcomes, like a 1/6 chance for any side of a fair die. Experimental probability is based on actually running trials and counting results, which converges toward the theoretical value as the number of trials grows.
How do you find the probability of two independent events both happening?
Multiply their individual probabilities together. If flipping heads is 1/2 and rolling a 6 is 1/6, the probability of both happening is 1/2 × 1/6 = 1/12, since the two events don't affect each other.
Can probability be negative or greater than 1?
No. A probability is always between 0 and 1 inclusive. 0 means the event never happens, 1 means it always happens, and anything outside that range signals a mistake in the setup.
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