Tools
Odds to probability converter
Have a chance as a percentage, a line in American or fractional odds, or a price on Kalshi or Polymarket? Type it in the form you have, and see the implied probability and the same chance in every other format.
Odds and probability converter
- Probability
- 25%
- Probability as a decimal
- 0.25
- Chance it does not happen
- 75%
- About one in
- 4
- American odds
- +300
- Decimal odds
- 4.00
- Fractional odds (odds against)
- 3/1
- Odds for
- 1 : 3
- Prediction market price
- 25¢
The formula
p = 100 ÷ (American + 100) for +odds · p = −American ÷ (−American + 100) for −odds · p = 1 ÷ decimal · p = b ÷ (a + b) for fractional a/b
Percent chance to probability, and back
A percent chance is a probability written out of 100: a 25% chance is a probability of 0.25, or one in four. To turn a percentage into a probability, divide by 100; to go back, multiply by 100. The chance it does not happen is what is left: 100% − 25% = 75%.
Odds are another way of writing the same thing, and the converter above does each step for you.
Odds for and odds against
Odds for compare the chances of something happening with the chances of it not happening. A 25% chance is 25 to 75, which reduces to 1 : 3 — one way it happens for every three ways it does not.
Odds against turn that round: 3 : 1, or 3/1. Fractional odds in betting are odds against: at 3/1 you win 3 for every 1 staked, and the implied probability is 1 ÷ (3 + 1) = 25%. In general, fractional odds a/b imply a probability of b ÷ (a + b).
Implied probability from American and decimal odds
American odds show either what you win on a $100 stake (+150) or what you stake to win $100 (−150). +150 implies 100 ÷ 250 = 40%; −150 implies 150 ÷ 250 = 60%. Decimal odds show the total return per unit staked, stake included, so 2.50 implies 1 ÷ 2.50 = 40%.
A bookmaker’s odds include its margin, so the probabilities of all outcomes add up to more than 100%. To see the fair probability with the margin removed, use the no-vig calculator.
| Probability | American | Decimal | Fractional (against) | Odds for |
|---|---|---|---|---|
| 10% | +900 | 10.00 | 9/1 | 1 : 9 |
| 20% | +400 | 5.00 | 4/1 | 1 : 4 |
| 25% | +300 | 4.00 | 3/1 | 1 : 3 |
| 33.33% | +200 | 3.00 | 2/1 | 1 : 2 |
| 40% | +150 | 2.50 | 3/2 | 2 : 3 |
| 50% | +100 | 2.00 | 1/1 | 1 : 1 |
| 60% | −150 | 1.67 | 2/3 | 3 : 2 |
| 66.67% | −200 | 1.50 | 1/2 | 2 : 1 |
| 75% | −300 | 1.33 | 1/3 | 3 : 1 |
| 80% | −400 | 1.25 | 1/4 | 4 : 1 |
| 90% | −900 | 1.11 | 1/9 | 9 : 1 |
Prediction market prices are probabilities
A contract on Kalshi or Polymarket pays $1 if the event happens and nothing if it does not, so its price in cents reads directly as the market’s probability in percent: YES at 62¢ is a 62% chance. As with bookmaker odds, the YES and NO prices usually add to a little over $1, and the trading fee comes on top.
Frequently asked questions
- How do you convert a percentage to a probability?
Divide by 100. A 25% chance is a probability of 0.25. As odds, that is 3/1 against, 1 : 3 for, +300 in American odds or 4.00 in decimal odds.
- What is the difference between odds for and odds against?
Odds for compare the chances of an event happening with it not happening (1 : 3 for a 25% chance); odds against are the reverse (3 : 1). Fractional betting odds such as 3/1 are odds against.
- How do you calculate implied probability from odds?
For positive American odds, 100 ÷ (odds + 100); for negative American odds, −odds ÷ (−odds + 100); for decimal odds, 1 ÷ odds; for fractional odds a/b, b ÷ (a + b). +150, 2.50 and 3/2 all imply 40%.
- What does a 1 in 4 chance mean as a percentage?
A 1 in 4 chance is 25%, or odds of 3/1 against.
Not financial advice. These calculators do arithmetic on the numbers you enter; they do not know a platform’s current prices or whether it is open to you. Check both in the platform reviews.