Probability Calculator
Free probability calculator for single and combined events. Get probability, complement and odds from favorable outcomes or two event probabilities.
About the Probability Calculator
This free probability calculator handles the two situations students meet most. In single-event mode, enter favorable and total outcomes and it returns the probability P(A) = favorable ÷ total, the complement P(not A) = 1 − P(A), and the odds in favor. In two-event mode, enter P(A) and P(B) as percentages and it computes P(A and B), P(A or B), the chance neither happens and the chance exactly one happens, assuming independence.
The combined-event formulas are P(A and B) = P(A) × P(B) and P(A or B) = P(A) + P(B) − P(A and B). Students preparing for matric, FSc, O/A-levels and university statistics courses across Pakistan and India use these constantly, as do quality engineers and anyone reasoning about risk. Everything computes live in your browser with no signup.
How to Use the Probability Calculator
- 1Choose single event or two independent events mode.
- 2For a single event, enter favorable outcomes and total outcomes.
- 3For two events, enter P(A) and P(B) as percentages.
- 4Read the probability, complement, odds and combined probabilities instantly.
Frequently Asked Questions
How do I calculate the probability of a single event?
Divide favorable outcomes by total outcomes. Drawing a heart from a standard 52-card deck has 13 favorable outcomes out of 52, so P = 13 ÷ 52 = 0.25 or 25%. The complement (not drawing a heart) is 1 − 0.25 = 75%, and the odds in favor are 1 : 3.
What is the probability that two independent events both happen?
Multiply them: P(A and B) = P(A) × P(B). The chance of flipping heads (50%) and rolling a six (16.67%) in the same turn is 0.5 × 0.1667 = 8.33%. This only works when the events do not influence each other.
How is P(A or B) different from P(A and B)?
P(A or B) is the chance at least one event happens: P(A) + P(B) − P(A and B). With P(A) = 50% and P(B) = 25%, P(A or B) = 0.5 + 0.25 − 0.125 = 62.5%. Subtracting the overlap prevents double-counting the case where both occur.
What is the difference between probability and odds?
Probability compares favorable outcomes to all outcomes; odds compare favorable to unfavorable. A 25% probability means 1 favorable for every 3 unfavorable, written as odds of 1 : 3 in favor. Converting: odds in favor = P ÷ (1 − P).
Can a probability be greater than 1 or negative?
No. Probability always lies between 0 (impossible) and 1 (certain), i.e. 0% to 100%. If your working produces a value outside that range, an input is wrong — a common mistake is entering more favorable outcomes than total outcomes, which this calculator blocks.