Probability Calculator
Calculate single event probability, complementary probability, and odds for/against an event.
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How to use this calculator
Probability ranges from 0 (impossible) to 1 (certain). Expressed as fraction, decimal, or percentage.
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Enter the number of favourable outcomes (the event you are interested in).
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Enter the total number of possible outcomes.
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Optionally enter the number of trials for the "at least one success" calculation.
Frequently asked questions
What is probability?
Probability measures how likely an event is to occur, expressed as a number between 0 and 1. P=0 means impossible; P=1 means certain. P=0.5 means equally likely to happen or not.
What is the difference between probability and odds?
Probability = favourable / total. Odds (for) = favourable : unfavourable. A probability of 1/4 = odds of 1:3. Odds are common in gambling; probability is used in statistics.
What is complementary probability?
The probability that an event does NOT happen = 1 − P(event). If P(rain) = 0.3, then P(no rain) = 0.7.
How do I find the probability of at least one success?
P(at least one) = 1 − P(none). P(none in n trials) = (1-p)^n. So P(at least one) = 1 − (1-p)^n. Example: rolling a 6 at least once in 4 rolls = 1 − (5/6)^4 ≈ 51.8%.
Probability fundamentals
How to use the probability
Use this probability to ngle event probability, complementary probability, and odds for/against an event. Enter your values above and get your result in seconds. The tool is free, works on all devices, and keeps your data private — nothing is stored or shared.
How the probability works
The probability calculator uses standard formulas used in mathematics, science, and engineering. Enter your inputs, and the tool calculates the result instantly in your browser. No server-side processing means your data stays on your device. Results update in real time as you change inputs.
Classical probability
Classical probability assumes all outcomes are equally likely. P(A) = favourable outcomes / total outcomes. Rolling a fair die: P(rolling a 3) = 1/6. Flipping a fair coin: P(heads) = 1/2. Drawing a heart from a deck: P = 13/52 = 1/4.
Probability vs. statistics
Probability works forward: given a known model, predict outcomes. Statistics works backward: given observed outcomes, estimate the model. If you know a die is fair (model), probability predicts each face has a 1/6 chance. If you observe 1,000 rolls, statistics estimates whether the die is fair.
Real-world probability examples
Medical tests: P(disease | positive test). Insurance: P(accident in a year). Finance: P(market down >10% in a year). Weather: P(rain tomorrow). Quality control: P(defective part). Probability underpins decision-making in medicine, engineering, business, and science.
Probability: how it works
Built on well-established mathematical principles, this tool delivers accurate results for students, researchers, and professionals. Enter your values and get instant clarity without specialist software or manual arithmetic.
Who uses this tool?
Teachers, students, engineers, and data analysts use it to verify calculations, check homework, and solve problems faster. It is intuitive enough for beginners while comprehensive enough for professional use.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →