Math / Browser calculator

Probability Calculator

Probability Calculator works from the inputs you provide and returns both (%), at least one (%), neither (%). Use the calculation panel to explore a scenario, then check the formula and limitations before applying the result.

Your inputs

Minimum: 0 · Maximum: 100
Minimum: 0 · Maximum: 100

Your results

Enter your values and calculate to see the result.

Formula and calculation method

For independent events: P(A and B)=P(A)P(B); P(A or B)=P(A)+P(B)−P(A)P(B).

Assumes independent events. Do not apply these formulas to dependent events without a conditional probability model.

How to use probability calculator

  1. Set Probability of A (%). The example below uses 40.
  2. Set Probability of B (%). The example below uses 30.
  3. Select Calculate to update the result. Reset restores the illustrated inputs. Copy, print or download your own result if you need a record.

Worked example

Example inputs
Probability of A (%)
40
Probability of B (%)
30
Both (%)
12
At least one (%)
58
Neither (%)
42

Factors affecting your result

Assumes independent events. Do not apply these formulas to dependent events without a conditional probability model.

Changing probability of a (%) changes the scenario being evaluated. Adjust one input at a time when comparing results, keep a copy of the assumptions, and compare values using the same unit and period. A precise arithmetic result does not make an uncertain assumption precise.

Frequently asked questions

How does the probability calculator work?

For independent events: P(A and B)=P(A)P(B); P(A or B)=P(A)+P(B)−P(A)P(B).

What inputs does Probability Calculator need?

Enter probability of a (%), probability of b (%). Read the unit labels; percentages are entered as ordinary percentages, not decimal fractions.

How should I interpret the probability calculator result?

Assumes independent events. Do not apply these formulas to dependent events without a conditional probability model.

Are my probability calculator inputs uploaded?

No. The calculation runs in this browser. Favorites, recent tool names, theme and formatting preferences may be saved locally, but calculation inputs are not sent to a calculation server or stored by this website. Shared links do not include entered values.

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