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Odds Converter: American, Decimal and Fractional Odds
Convert between American, decimal and fractional odds and see the implied probability of any price, with the formulas explained.
US sportsbooks quote American odds and UK bookmakers favor fractions. Exchanges and most of the world use decimals, and prediction markets price in cents. This odds converter translates any of them in one step and shows the implied probability. You can then compare a price on DraftKings with one on Betfair or Kalshi on equal terms.
How odds conversion works
Decimal odds are the common denominator. They show your total return for every $1 staked, including the stake, so every conversion runs through them.
American to decimal
- Positive odds: decimal = (American ÷ 100) + 1
- Negative odds: decimal = (100 ÷ |American|) + 1
Decimal to fractional
Fractional = (decimal − 1), written as a fraction. 3.50 becomes 2.5/1, or 5/2.
Any format to implied probability
Implied probability = 1 ÷ decimal odds.
Worked example: converting +250 and -200
Say a sportsbook prices an NFL underdog at +250 and the favorite at -200.
| American | Decimal | Fractional | Implied probability | $100 bet returns |
|---|---|---|---|---|
| +250 | 3.50 | 5/2 | 28.57% | $350 |
| -200 | 1.50 | 1/2 | 66.67% | $150 |
For +250: 250 ÷ 100 + 1 = 3.50. Subtract 1 to get 2.5, or 5/2 as a fraction. 1 ÷ 3.50 = 28.57%.
For -200: 100 ÷ 200 + 1 = 1.50. Subtract 1 to get 0.5, or 1/2. 1 ÷ 1.50 = 66.67%.
Add the two probabilities and you get 95.24%, which is below 100%. A real two-way market would never look like that. Sportsbooks price both sides so the total runs above 100%, and the excess is their margin. Our implied probability calculator strips that margin out.
Odds format cheat sheet
| American | Decimal | Fractional | Implied |
|---|---|---|---|
| -500 | 1.20 | 1/5 | 83.3% |
| -200 | 1.50 | 1/2 | 66.7% |
| -110 | 1.91 | 10/11 | 52.4% |
| +100 | 2.00 | 1/1 (evens) | 50.0% |
| +150 | 2.50 | 3/2 | 40.0% |
| +300 | 4.00 | 3/1 | 25.0% |
| +1000 | 11.00 | 10/1 | 9.1% |
When to use an odds converter
- Line shopping across formats. Comparing a US sportsbook with a UK bookmaker or exchange means converting first.
- Prediction markets. A Kalshi or Polymarket contract at 40 cents is 2.50 decimal or +150, before fees. Our prediction markets guide explains how each platform charges.
- Building parlays. Combined odds are the product of decimal prices, and our parlay calculator does it for you.
- Checking value. Convert, then compare the implied probability with your own estimate in the expected value calculator.
Why the format changes how a price feels
The same bet can look tempting or dull depending on how it is written. A UK punter sees 10/11 and a US bettor sees -110, yet both are risking 11 to win 10. Decimal odds strip away that framing. That is why exchanges, models and most betting software use them under the hood. Learn to read all three and you will never misjudge a price because of its notation.
Common odds conversion mistakes
- Forgetting the stake. Decimal odds include your stake. Fractional and positive American odds show profit only. 2.50 decimal and 3/2 are the same price.
- Mixing up the minus sign. -150 means you risk $150 to win $100. +150 means you risk $100 to win $150. The difference in implied probability is 60% versus 40%.
- Rounding too early. -110 is 1.909, not 1.90 or 1.91. Small rounding errors grow quickly when you multiply legs in a parlay.
- Ignoring fees. Exchange and prediction market prices look sharper than sportsbook odds, but commission or trading fees come off your winnings. Convert, then subtract the fee before you compare.
See all our free betting calculators.
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FAQ
How do you convert American odds to decimal?
For positive odds, divide by 100 and add 1, so +250 becomes 3.50. For negative odds, divide 100 by the odds without the minus sign and add 1, so -200 becomes 1.50.
What does +200 mean in betting?
A $100 bet at +200 wins $200 in profit, returning $300 in total. That is 3.00 in decimal odds or 2/1 in fractional, with an implied probability of 33.3%.
Are decimal odds better than American odds?
Neither pays more; they express the same price. Decimal odds are easier for math because they show your total return per $1, which is why parlay and expected value calculations convert to decimal first.
How do I convert prediction market prices to odds?
Divide 1 by the contract price in dollars. A 25-cent contract is 4.00 in decimal, +300 in American odds and a 25% implied probability, before fees.