Probability Calculator

Calculate the probability of events occurring. Find the likelihood of outcomes with our easy-to-use probability calculator.

Probability Facts

Probability Range
0 to 1
Or 0% to 100%
Coin Flip
50% (0.5)
Heads or tails
Rolling a 6
16.67% (1/6)
On a standard die
Certain Event
100% (1.0)
Will definitely happen

Your Results

Calculated
Probability
0
As a decimal
Percentage
0%
Chance of occurring
Odds
0 : 0
Favorable to unfavorable

Key Takeaways

  • Probability = Favorable Outcomes / Total Possible Outcomes
  • Values range from 0 (impossible) to 1 (certain)
  • Multiply by 100 to convert to percentage
  • The sum of all probabilities in a sample space equals 1
  • Independent events can be multiplied together

What Is Probability?

Probability is a measure of the likelihood that an event will occur. It is quantified as a number between 0 and 1, where 0 indicates impossibility and 1 indicates certainty. The higher the probability, the more likely the event is to happen.

In everyday language, probabilities are often expressed as percentages. For example, a probability of 0.25 is equivalent to 25%, meaning there's a 1 in 4 chance of the event occurring.

The Probability Formula

P(A) = Number of Favorable Outcomes / Total Number of Possible Outcomes
P(A) = Probability of event A
Favorable Outcomes = Ways the event can occur
Total Outcomes = All possible outcomes

Example: Rolling a Die

Rolling a 4 1/6 = 16.67%
Rolling Even Number 3/6 = 50%
Rolling 1, 2, or 3 3/6 = 50%

How to Use This Calculator

Using our probability calculator is simple:

  • Step 1: Enter the number of favorable outcomes (the events you want to happen)
  • Step 2: Enter the total number of possible outcomes
  • Step 3: Click "Calculate" to see the probability as a decimal, percentage, and odds

Understanding the Results

The calculator provides three formats for your probability:

  • Decimal: A number between 0 and 1 (e.g., 0.25)
  • Percentage: The decimal multiplied by 100 (e.g., 25%)
  • Odds: The ratio of favorable to unfavorable outcomes (e.g., 1:3)

Types of Probability

Theoretical Probability

Based on the possible outcomes in theory. For example, the theoretical probability of flipping heads on a fair coin is exactly 0.5 or 50%.

Experimental Probability

Based on actual experiments or observations. If you flip a coin 100 times and get heads 48 times, the experimental probability is 48/100 = 0.48 or 48%.

Subjective Probability

Based on personal judgment or experience. For example, estimating the probability that your favorite team will win their next game.

Probability Rules

Addition Rule (Or)

For mutually exclusive events: P(A or B) = P(A) + P(B)

For non-mutually exclusive events: P(A or B) = P(A) + P(B) - P(A and B)

Multiplication Rule (And)

For independent events: P(A and B) = P(A) x P(B)

For dependent events: P(A and B) = P(A) x P(B|A)

Complement Rule

P(not A) = 1 - P(A)

If the probability of rain is 30%, the probability of no rain is 70%.