Result
Result reflects the current submitted inputs.
- Risk B
- Reviewed 2026-05-26
- 2 sources
Breakdown
- Input scale
- decimal
- Normalized P(A)
- 0.4
- Normalized P(B)
- 0.25
- Assumption
- A and B are independent events
- A and B are assumed to be independent events.
- Decimal inputs use 0 to 1; percent inputs use 0 to 100 and are normalized to decimal probabilities.
- Outputs are decimal probabilities from 0 to 1.
- Dependent events, conditional probabilities, and user-entered overlap are out of scope for this MVP.
- Intermediate values are not rounded; final numeric outputs are rounded for stable display.
Accuracy notes
- Risk level
- B
- Reviewed
- 2026-05-26
- Sources
- 2
- Primary result
- Probability of A or B
Formula logic is kept in a pure calculator module with fixtures, source notes, and page-visible assumptions.
What the result means
Probability of A or B is the number to carry forward from this probability calculation. Union probability under the independent-events assumption. Read the center or spread metric first, then compare count, minimum, maximum, and sample/population notes. Use probability of A and B, probability of not A, and probability of not B to explain why probability of A or B moved when an input changed. Review the raw values and decide whether outliers or missing data should be handled before reporting the result.
Use the result this way
- Start with Probability of A or B, then use supporting outputs only to explain the primary answer.
- Verify probability of A, probability of B, and input scale before copying the result.
- Choose the mode or method first because it can change which formula is applied and stay within the documented minimum and maximum ranges.
- Review the raw values and decide whether outliers or missing data should be handled before reporting the result.
User job
How to use this calculator
Use Probability Calculator when you need probability of a or b, then use probability of a and b and probability of not a to check the context for data review, classwork, quality checks, and quick descriptive summaries.
Best for
- Summarizing a list of values
- Checking spread, center, or sample assumptions
- Reviewing a default example before entering your own probability of a and probability of b.
Check before relying
- Confirm whether the data is a sample or population and whether outliers should stay in the list.
- A and B are assumed to be independent events.
- Decimal inputs use 0 to 1; percent inputs use 0 to 100 and are normalized to decimal probabilities.
- Source context: OpenStax, Rice University, reviewed 2026-05-26.
Next useful step
- Confidence Interval CalculatorUse next when the probability task needs lower bound instead of probability of A or B.
- P-Value CalculatorUse next when the probability task needs p-value instead of probability of A or B.
- Permutation and Combination CalculatorUse next when the probability task needs count instead of probability of A or B.
Formula
For independent events, P(A and B) = P(A) * P(B), P(A or B) = P(A) + P(B) - P(A and B), and complements are 1 - P(event). Key assumptions: A and B are assumed to be independent events. Decimal inputs use 0 to 1; percent inputs use 0 to 100 and are normalized to decimal probabilities. Outputs are decimal probabilities from 0 to 1.
- For independent events, P(A and B) = P(A) * P(B), P(A or B) = P(A) + P(B) - P(A and B), and complements are 1 - P(event).
- A and B are assumed to be independent events.
- Decimal inputs use 0 to 1; percent inputs use 0 to 100 and are normalized to decimal probabilities.
- Primary source context: OpenStax, Rice University.
Inputs
Enter probability of A, probability of B, and input scale for data review, summaries, quality checks, and classwork. Before calculating, choose the mode or method first because it can change which formula is applied and stay within the documented minimum and maximum ranges. Probability of A: Enter 0 to 1 in decimal mode, or 0 to 100 in percent mode. Probability of B: Enter 0 to 1 in decimal mode, or 0 to 100 in percent mode. Input scale: Choose decimal for inputs like 0.4, or percent for inputs like 40.
Example
Using the default inputs, Probability Calculator returns probability of A or B of 0.55. Adjust probability of A, probability of B, and input scale to match your own scenario.
FAQ
How is probability of A or B calculated here?
For independent events, P(A and B) = P(A) * P(B), P(A or B) = P(A) + P(B) - P(A and B), and complements are 1 - P(event). The first assumption to check is: A and B are assumed to be independent events.
What does Probability of A or B mean for probability?
Read the center or spread metric first, then compare count, minimum, maximum, and sample/population notes. Secondary values such as probability of A and B, probability of not A, and probability of not B are there to explain the primary answer, not to replace it.
What should I enter for Probability of A?
Enter 0 to 1 in decimal mode, or 0 to 100 in percent mode. Choose the mode or method first because it can change which formula is applied and stay within the documented minimum and maximum ranges.
How does Probability of B change probability of A or B?
Enter 0 to 1 in decimal mode, or 0 to 100 in percent mode. Changing it can alter probability of A or B because the formula uses the submitted inputs together. Also compare sample versus population mode, separators, missing values, outliers, and rounding precision.
Why does the probability example show 0.55 for probability of A or B?
The default inputs produce 0.55 for probability of A or B. Treat that as a format and scale check, then replace every default value with your own inputs.
What should I check before reporting probability of A or B?
Confirm how the values were parsed, whether the data is a sample or population, and whether outliers or missing values should stay in the set.
Sources
Last reviewed: 2026-05-26
- Reviewed 2026-05-26Introductory Statistics 2e, Section 3.2: Independent and Mutually Exclusive EventsOpenStax, Rice University. Independent intersection formula P(A and B) = P(A) * P(B).
- Scope
- English-language introductory statistics coverage of independent events.
- Supports
- Independent intersection formula P(A and B) = P(A) * P(B).
- Limits
- This calculator uses only the independent-events MVP; dependent and conditional probability are out of scope.
- Reviewed 2026-05-26Introductory Statistics 2e, Section 3.3: Two Basic Rules of ProbabilityOpenStax, Rice University. Complement rule P(not A) = 1 - P(A) and union rule P(A or B) = P(A) + P(B) - P(A and B).
- Scope
- English-language introductory statistics coverage of complement and addition rules.
- Supports
- Complement rule P(not A) = 1 - P(A) and union rule P(A or B) = P(A) + P(B) - P(A and B).
- Limits
- The general addition rule needs P(A and B); this calculator supplies it from the independent-events assumption.
Disclaimer
This calculator is an educational estimate based on the inputs and assumptions shown on the page.