How to use this calculator

The addition rule avoids double-counting outcomes shared by both events.

The method

P(A or B) = P(A)+P(B)−P(A and B)

Enter your values in the labeled fields and choose Calculate. The result panel shows the answer and supporting values when useful.

WORKED EXAMPLE INPUT

P(A): 0.4, P(B): 0.3, P(A and B): 0.1

Select Calculate to evaluate these example values.

Scope and limitations

Inputs must be valid probabilities and the final result must stay between 0 and 1.

Common questions

Can I use different units?

Use consistent units unless this page is a unit converter. The result follows the units implied by the inputs.

Are results exact?

Displayed values may be rounded. Keep extra precision for important follow-up calculations.

Methods & verification

Formulas use standard mathematical definitions and fixed unit factors where applicable. See our methodology for numerical conventions, rounding, privacy, and limitations.