Odds Calculator
Convert between probability, American odds, decimal odds, and fractional odds. Calculate parlay odds for multiple betting legs.
About the Odds Calculator
Odds represent the likelihood of an event occurring, expressed in different formats depending on context. In mathematics and probability, odds are expressed as a ratio of favorable to unfavorable outcomes (e.g. 3:7 — 3 chances for, 7 against). In sports betting, three formats are commonly used: American odds (e.g. +150 or −110), decimal odds (e.g. 2.50), and fractional odds (e.g. 3/2). Our odds calculator converts between all formats and computes the implied probability — the probability baked into a given set of odds.
American odds are the standard format in the United States. A positive number (like +200) shows how much profit you win on a $100 bet. A negative number (like −150) shows how much you must bet to win $100 profit. +200 means a $100 bet returns $200 profit ($300 total). −150 means a $150 bet returns $100 profit ($250 total). The implied probability of +200 is 100/(200+100) = 33.3%. The implied probability of −150 is 150/(150+100) = 60%.
A parlay calculator combines multiple independent bets into one bet where all selections must win. Parlay odds multiply: if Leg 1 is −110 (decimal 1.909) and Leg 2 is +120 (decimal 2.2), the combined decimal odds are 1.909 × 2.2 = 4.2 and the American odds are approximately +320. Parlays offer higher payouts because the probability of all legs winning is lower than any individual leg — the true parlay probability is the product of all individual probabilities.
Pros & Cons
- +Converts between probability, American, decimal, and fractional odds
- +Implied probability reveals the true chance baked into the odds
- +Parlay calculator supports up to 12 legs with dynamic addition
- +Helps identify value bets when your estimated probability exceeds implied probability
- +Free with no account required
- −Does not account for the sportsbook's vig (overround) — implied probabilities for a two-sided market will sum to more than 100%
- −Does not factor in different parlay rules by sportsbook (some books offer enhanced parlay payouts)
- −Parlay assumes all events are independent — correlated parlays have different actual probabilities
- −Does not place bets or connect to any sportsbook
- −Fractional odds output uses a simplified fraction — some traditional fractional odds (e.g. 11/10) may display differently
Understanding Implied Probability and Value Bets
Implied probability is the probability of an outcome that is implied by the given odds. It answers the question: 'What probability does the sportsbook believe this outcome has?' For American odds: if positive, implied prob = 100 / (odds + 100). If negative, implied prob = |odds| / (|odds| + 100). A line of −110 implies 52.38% probability. This is why sportsbooks typically charge −110 on both sides of a 50/50 bet — both sides add up to 104.76%, giving the book a 4.76% margin (the 'juice' or 'vig').
A value bet exists when your estimated probability of an event is higher than the implied probability in the odds. If you believe Team A has a 60% chance of winning but the odds imply only 50%, the odds are offering positive expected value (EV). Over many such bets, a player with accurate probability estimates will profit even if individual bets lose. This is the core of professional sports betting — it is not about picking winners, but finding odds that underestimate the true probability.
The Kelly Criterion is a mathematical formula for determining the optimal bet size when you have an edge. Kelly bet fraction = (bp − q) / b, where b is the decimal odds minus 1 (the profit per unit staked), p is your estimated probability of winning, and q = 1 − p. If the implied probability is 50% but you estimate 60%, Kelly suggests betting 20% / 1 = 20% of your bankroll — though in practice, professional bettors use fractional Kelly (typically 25–50% of Kelly) to reduce variance.