Probability Calculator

Calculate the probability of single events, complementary events, and combined AND/OR probabilities for two independent events.

Use the Probability Calculator

A Event A

Example: rolling a 3 on a standard die → favorable: 1, total: 6

B Event B (optional — for AND / OR)

Example: flipping heads on a coin → favorable: 1, total: 2

Enter outcomes above and click Calculate to see results.

Summary

This probability calculator lets you compute the likelihood of one or two independent events occurring. Enter the number of favorable outcomes and total outcomes for each event to get the probability as a decimal, percentage, and fraction. The tool also computes the complementary probability (the chance the event does NOT occur) and, for two events, the combined AND (both occur) and OR (at least one occurs) probabilities.

How it works

  1. Enter the number of favorable outcomes and total possible outcomes for Event A.
  2. Optionally enter values for Event B to enable combined probability calculations.
  3. The calculator instantly computes P(A), P(A's complement), P(B), P(B's complement).
  4. Combined results show P(A AND B) — both events occur — and P(A OR B) — at least one occurs.
  5. All results display as decimals, percentages, and simplified fractions.

Use cases

  • Determine the probability of rolling a specific number on a die.
  • Calculate the chance of drawing a certain card from a deck.
  • Find the joint probability of two independent coin flips.
  • Compute the likelihood that at least one of two independent events occurs.
  • Cross-check probability homework answers step by step.
  • Understand complementary events (the probability something does NOT happen).
  • Calculate the AND probability for quality control scenarios.
  • Teach or learn the addition rule and multiplication rule of probability.

Frequently Asked Questions

Last updated: 2026-06-04 · Reviewed by Nham Vu