About the Odds Converter
Betting odds show up in American, decimal, and fractional formats depending on the sportsbook or region, and converting between them by hand is easy to get wrong, especially with negative American odds. Our Odds Converter translates American odds into decimal and fractional form, plus the implied probability behind them.
How It Works
You enter a single American odds value, positive for an underdog or negative for a favorite. The calculator converts it to decimal odds and an approximate fractional form, then calculates the implied probability of that outcome.
Formula & Methodology
American odds work differently depending on their sign: positive odds (like +150) show how much profit a $100 bet would win, while negative odds (like −150) show how much you'd need to bet to win $100 profit. Converting to decimal odds folds the original stake back into the total payout, so a positive value adds the profit fraction to 1, while a negative value adds the reciprocal fraction of the stake required. Implied probability comes directly from that same relationship, expressing how likely the outcome would need to be for the bet to break even in the long run, purely a mathematical inverse of the odds, with no adjustment for the bookmaker's built-in margin (the 'vig' or 'juice'), which is why implied probabilities from both sides of a bet typically add up to slightly more than 100%.
Step-by-Step: Calculating It By Hand
- 1Check whether the American odds are positive (underdog) or negative (favorite).
- 2For positive odds, add the odds divided by 100 to 1 to get decimal odds.
- 3For negative odds, add 100 divided by the absolute value of the odds to 1 to get decimal odds.
- 4For implied probability, use the corresponding formula for positive or negative odds.
Examples
Underdog odds
+150 American odds convert to 2.50 decimal odds, with an implied probability of 40%.
Favorite odds
−150 American odds convert to 1.67 decimal odds, with an implied probability of 60%.
Advantages
- Converts American odds to decimal and fractional formats in one step
- Calculates implied probability alongside the converted odds
- Handles both positive (underdog) and negative (favorite) American odds correctly
- Removes the guesswork from comparing odds formats across different sportsbooks
Common Mistakes
- Treating the plus or minus sign in American odds as a literal positive or negative number rather than an underdog/favorite indicator
- Forgetting that implied probability includes the bookmaker's margin and isn't the 'true' probability
- Mixing up which formula applies to positive versus negative American odds
- Assuming decimal odds of 2.0 mean something other than an even-money (50/50) bet
Edge Cases to Watch For
- American odds of exactly 0 are invalid and undefined in this system.
- Implied probability always includes the bookmaker's margin, so it reads higher than the true statistical probability of the outcome.
- +100 and −100 both convert to the same decimal odds (2.0) and the same 50% implied probability, the breakeven point.
- Fractional odds shown here are an approximation of the decimal value, not always the simplest whole-number fraction a sportsbook might display.
Common Use Cases
- Comparing odds listed in different formats across sportsbooks
- Understanding the implied probability behind a given betting line
- Sports betting analysis and bankroll management
- Quick conversions when American odds are given but decimal odds are needed for calculations