Math
Probability Calculator
Calculate basic probability of single and combined events.
Inputs
Adjust the assumptions to match your scenario. Results update instantly.
Results
Primary outputs and comparison insights are built from the current inputs.
P(A)
30%
Probability of event A occurring.
P(B)
40%
Probability of event B occurring.
P(A and B) independent
12%
Probability of both A and B if independent.
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How this Probability Calculator works
The Probability Calculator computes the likelihood of single events and the combined probability of two independent events. Enter favorable outcomes a, total outcomes, and favorable outcomes b to estimate p(a), p(b), and p(a and b) independent. The calculator updates instantly and adds a comparison table plus chart so you can test the sensitivity of the result before you use it in a decision.
Quick guide
Jump to the section you need, then return to the probability calculator.
Inputs
- Number of favorable outcomes for event A.
- Total number of possible equally likely outcomes.
- Number of favorable outcomes for event B.
Outputs
- Probability of event A occurring.
- Probability of event B occurring.
- Probability of both A and B if independent.
Assumptions
- All outcomes are equally likely.
- Events A and B are independent.
Tips
- For dependent events, use conditional probability instead.
- Convert between odds and probability as needed.
Probability Calculator formula guide
Use these probability calculator formulas to audit the output or explain it to someone else.
2 formulas
Probability Calculator examples
Review a ready-made probability calculator scenario, copy it, then tweak inputs to match your case.
Example
Dice roll example
Inputs
- Example input Favorable outcomes A: 2
- Example input Total outcomes: 6
- Example input Favorable outcomes B: 3
Outputs
- Example result P(A): 33.33%
- Example result P(B): 50%
- Example result P(A and B) independent: 16.67%
Rolling a 1 or 2 (P=33%) and rolling odd (P=50%) on a fair die are independent events.
Probability by favorable outcomes
| Favorable outcomes A | P(A) | P(A and B) independent |
|---|---|---|
| 1 | 10% | 4% |
| 2 | 20% | 8% |
| 3 | 30% | 12% |
| 4 | 40% | 16% |
| 5 | 50% | 20% |
Combined probability of independent events multiplies individual probabilities.
P(A) sensitivity
Focus point
1
10%
Position
#1 of 5
Original order
Share of total
6.67%
Total: 150%
Combined probability of independent events multiplies individual probabilities.
References
- Basic probability theory
- Independent event probability formulas
Learn more
Guides connected to the probability calculator
Use these short guides when you want the decision framework behind the numbers, not just the raw output.
Category guide
Math Calculator Guide
Pick the right math calculator by problem type: arithmetic, ratios, coordinates, geometry, or measurement.
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FAQ
Probability Calculator FAQ
What does the Probability Calculator do?
The Probability Calculator computes the likelihood of single events and the combined probability of two independent events. Enter favorable outcomes a, total outcomes, and favorable outcomes b to estimate p(a), p(b), and p(a and b) independent. The calculator updates instantly and adds a comparison table plus chart so you can test the sensitivity of the result before you use it in a decision. It is part of our math toolkit.
What inputs do I need?
Typical inputs include Number of favorable outcomes for event A., Total number of possible equally likely outcomes., Number of favorable outcomes for event B..
How are the results calculated?
We follow the formulas and assumptions outlined in the "How this calculator works" section. You will see outputs like Probability of event A occurring., Probability of event B occurring., Probability of both A and B if independent..
Can I share or download the results?
Use the Copy link or Print buttons to share your results. If a table or chart appears, you can download the data as CSV.
Is my data stored?
No. Calculations run in your browser and we do not store your inputs.
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