Probability Calculator

Probability is the likelihood of an event occurring, expressed between 0 (impossible) and 1 (certain). P(A) = favourable outcomes ÷ total possible outcomes.

Single event probability = number of favourable outcomes ÷ total possible outcomes. P(rolling a 3 on a die) = 1 ÷ 6 ≈ 0.167 = 16.7%.

Key Features

• Calculates single event probability from favourable and total outcomes.
• Calculates P(A AND B) for two independent events.
• Calculates P(A OR B) using the addition rule.
• Calculates complement probability P(not A).
• Results shown as fraction, decimal, and percentage.
• Runs entirely in your browser.

How to Use the Tool

1. Select the calculation type: single event, A AND B, or A OR B.
2. Enter the favourable outcomes and total possible outcomes for each event.
3. The probability as a percentage and fraction are shown instantly.

Core Benefits

• Avoid manual calculation errors in statistics coursework.
• Quickly verify probability answers in textbook problems.
• Understand basic chance mechanics for games, risk analysis, and research.

Common Use Cases

• Calculating the chance of rolling a specific number on a die.
• Finding the probability of drawing two specific cards in succession.
• Statistics homework involving independent and combined events.
• Risk analysis: probability that two independent systems both fail.
• Game theory: chance calculations for board games or card games.

Pro Tips

• P(A AND B) = P(A) × P(B) only for INDEPENDENT events. If events are not independent, you need conditional probability P(A|B).
• P(A OR B) = P(A) + P(B) − P(A AND B). The last term prevents double-counting.
• Probability is always between 0 and 1. If you get a number outside this range, check your inputs.