What does a probability of 0.5 mean?

It means the event has a 50% chance of occurring.

How do I find the probability of two independent events?

Multiply their individual probabilities: P(A and B) = P(A) x P(B).

What is the complement of an event?

P(not A) = 1 - P(A), the probability the event does not occur.

How to Calculate Probability

Probability measures how likely an event is to happen, expressed as a number between 0 (impossible) and 1 (certain). It is the foundation of statistics and risk analysis.

The Formula

P(A) = favorable outcomes / total outcomes

Where:

P(A)Probability — Likelihood of event A occurring

favorableFavorable — Number of desired outcomes

totalTotal — Total number of possible outcomes

Step-by-Step Example

Here's how to calculate probability step by step:

1Count favorable outcomes: Determine how many outcomes satisfy your event.
2Count total outcomes: Determine the total number of equally likely outcomes.
3Divide: Divide favorable by total to get a probability between 0 and 1.

Following these 3 steps gives you the final probability value.

Skip the Math

Rolling a 6 on a fair die has probability 1/6 or about 16.7%, since there is one favorable outcome out of six total.

Use the Free Calculator

Why You Need This Calculation

Probability guides decisions in risk assessment, gambling odds, weather forecasting, and insurance calculations.

Common Mistakes

Forgetting that probability ranges from 0 to 1.

If your result exceeds 1, recheck your counts.

Adding probabilities of non-mutually-exclusive events.

Subtract the overlap: P(A or B) = P(A)+P(B)-P(A and B).

Confusing independent and dependent events.

Drawing cards without replacement changes the probability each time.