Calculate the probability of an event. This free probability calculator handles single events (favourable ÷ total) and two independent events (AND, OR, neither) — shown as a percentage, decimal, fraction and odds.
Mode
Probability of an event = favourable outcomes ÷ total outcomes. For one roll of a die landing on a 4: 1 ÷ 6.
Enter each probability as a decimal between 0 and 1. These results assume A and B are independent events.
Probability
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Enter your values
Probability ranges from 0 (impossible) to 1 (certain), often shown as a percentage. The two-event results below assume independence — one event doesn’t affect the other.
Use this free probability calculator to find the probability of an event. Work out a single event from favourable and total outcomes, or combine two independent events with AND / OR logic.
What this probability calculator does
A probability calculator tells you how likely something is, from 0 (impossible) to 1 (certain). The single-event mode divides favourable by total outcomes; the two-event mode combines probabilities for independent events.
How probability is calculated
Single event: P = favourable ÷ total
Both (A and B): P(A) × P(B)
Either (A or B): P(A) + P(B) − P(A)×P(B)
Rolling a 4 on a die is 1 ÷ 6 ≈ 16.67%. Two independent 50% events both happening is 0.5 × 0.5 = 25%.
Ways to express probability
Form
Example (1 in 6)
Percentage
16.67%
Decimal
0.1667
Fraction
1/6
Odds (for : against)
1 : 5
Independent vs dependent events
The two-event results assume independence — one event doesn’t change the other (like two separate coin flips). If events are dependent (such as drawing cards without replacement), the probabilities change after each draw and need a different approach.
How to use the probability calculator
Pick single or two events.
For a single event, enter favourable and total outcomes.
For two events, enter each probability as a decimal (0–1).
Read the results in percent, decimal, fraction and odds.
Probability FAQ
How do I calculate probability?
Divide the number of favourable outcomes by the total number of possible outcomes. Rolling a 4 on a die is 1 ÷ 6, about 16.67%.
How do I find the probability of two events both happening?
For independent events, multiply their probabilities: P(A and B) = P(A) × P(B). Two 50% events both occurring is 0.5 × 0.5 = 25%.
How do I find the probability of either event?
Use P(A or B) = P(A) + P(B) − P(A and B). For two 50% events that's 0.5 + 0.5 − 0.25 = 75%.
What's the difference between probability and odds?
Probability is favourable ÷ total (e.g. 1/6). Odds compare favourable to unfavourable (e.g. 1 : 5). The calculator shows both.
What are independent events?
Events where one doesn't affect the other, like separate coin flips. The two-event mode assumes independence; dependent events need a different method.
Is the probability calculator free?
Yes, this probability calculator is completely free, needs no sign-up, and gives instant results in your browser.
