This calculator converts between probability and odds, two different ways of expressing the same chance that often confuse people. Probability states the likelihood of an event as a number from 0 to 100 percent, the share of times it would happen in the long run. Odds express the same thing as a ratio, comparing the chance of the event happening against it not happening, and they come in several forms used in different settings: odds against and odds for in everyday speech and traditional betting, and decimal odds, which are standard in sports betting and show the total return per dollar staked. Moving between these forms is genuinely useful, whether you are interpreting a weather forecast, a medical statistic, a sports betting market, or a risk in a report. This tool does the conversion. You enter a probability as a percentage, and the calculator returns the odds against the event, expressed as a ratio, the odds in favour, the decimal odds, and the implied probability as a check. The results update as you type, so you can build intuition for how, say, a 25 percent chance corresponds to odds of 3 to 1 against and decimal odds of 4.0. Use it to translate betting odds into a plain probability, to convert a probability into odds, or to understand risk and chance more clearly. A key insight the calculator makes obvious is the link to betting: decimal odds are simply one divided by the probability, so they represent the fair payout, and any margin a bookmaker adds shows up as decimal odds lower than the fair value. Probability and odds describe the same reality; this tool lets you switch freely between them.
Decimal odds = 1 / probability. Odds against = (1 - p) / p. Probability must be between 0 and 100 percent. Rounded for display.
The probability is converted to a decimal between 0 and 1. The decimal odds are one divided by that probability, the fair payout per dollar. The odds against are one minus the probability divided by the probability, written as a ratio to one, and the odds for are the reverse. The implied probability is shown as a check.
A probability of 25 percent, or 0.25, gives decimal odds of one divided by 0.25, which is 4.00. The odds against are 0.75 divided by 0.25, which is 3, written as 3 to 1 against. The odds for are the reverse, 1 to 3. All describe the same one-in-four chance.
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