What is probability?
Probability quantifies how likely an event is to happen, expressed as a number between 0 (impossible) and 1 (certain). The basic formula is P(A) = favorable outcomes / total outcomes. Flipping a fair coin and landing heads has one favorable outcome out of two, so the probability is 0.5 or 50 %. Probability is foundational to statistics, science, finance, insurance, gaming, and everyday decision-making.
How to use this calculator
Enter the number of favorable outcomes and the total number of possible outcomes. Quick-example buttons for a coin flip, a die roll, and drawing an ace from a deck let you pre-fill the inputs instantly. Click Calculate probability to see the result displayed as a decimal, percentage, simplified fraction, odds for, odds against, and the complement P(not A). A visual progress bar makes the likelihood intuitive at a glance.
Understanding the output
The decimal and percentage are the most common formats. The simplified fraction reduces the ratio to its lowest terms—for example 4/52 becomes 1/13. Odds for compare favorable to unfavorable outcomes (e.g. 1:5), while odds againstreverse the comparison. The complement tells you the probability the event does not happen: P(not A) = 1 − P(A).
Practical examples
A lottery with 1 winning ticket among 1 000 000 has a probability of 0.000001 (0.0001 %). A bag containing 3 red and 7 blue marbles gives P(red) = 3/10 = 0.3 (30 %). A quality-control batch with 2 defective items in 500 gives P(defective) = 0.004 (0.4 %). Plug any scenario into this calculator to get all formats instantly.
Tips
Favorable outcomes must be less than or equal to total outcomes. Make sure every distinct outcome is counted exactly once. This calculator handles simple (single-event) probability; for compound events, apply multiplication or addition rules separately. The tool runs entirely in your browser—no sign-up, no data sent to a server, and instant results on any device. Bookmark this page for quick probability calculations whenever you need them.