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Probability Calculator

Find the probability and odds of an event.

How it is calculated

For equally likely outcomes, the probability of an event is P = favorable outcomes / total outcomes. The tool shows it as a percentage (P × 100) and as odds written 1 : total / favorable, meaning "one in so many". The favorable count cannot be negative or larger than the total, and the total must be above zero. With zero favorable outcomes the percentage is 0% and the odds show a dash. The formula only works when every outcome is equally likely; it does not handle weighted outcomes or combinations of several events.

Worked example

Probability of drawing an ace from a standard 52-card deck: 4 favorable cards out of 52. P = 4 / 52 = 0.0769, which is 7.69%. Odds are shown as 1 : 52 / 4 = 1 : 13.00, so about one draw in 13 gives an ace.

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Common mistakes

  • Dividing favorable by unfavorable outcomes, which gives the odds ratio rather than the probability.
  • Confusing "1 : 13" in this tool (one in 13 trials) with betting odds of 1 to 13 against.
  • Assuming outcomes are equally likely when they are not, such as a loaded die or a weighted draw.
  • Using this for repeated events and adding probabilities; several draws need multiplication or other rules.

About this calculator

This calculator applies the classical probability formula, P(event) = favorable outcomes / total outcomes, and also converts the result into odds format (e.g., 1 in 4, or 3-to-1 against), since both notations are used in different contexts.

Statistics students use it for coursework on dice, cards, and coin-flip problems; anyone evaluating a game of chance or a raffle uses it to see their real chances; and it underlies basic risk assessment questions like the probability of drawing a specific card from a deck or winning a lottery. The calculator keeps the result expressed as a clean fraction, decimal, and percentage side by side, since different fields (gambling odds vs. statistics vs. everyday conversation) tend to prefer different formats.

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