Free Odds Converter
Convert between decimal, American, fractional, and implied probability odds formats instantly. Works for sportsbooks, prediction markets, and exchanges.
An odds converter turns any odds format into the others so you can compare prices across sportsbooks and prediction markets without doing the math by hand. Decimal odds (2.50) are standard in Europe and Australia. American odds (+150 or -200) are the US default. Fractional odds (3/2) appear on UK sportsbooks and racing markets. Implied probability (40%) is what Kalshi and Polymarket display.
The free odds converter below works in single mode (one odds value at a time) and bulk mode (paste a list and convert all at once). Every conversion includes the implied probability, which is the only fair way to compare two prices side by side. To learn how to spot pricing gaps once you have everything in the same units, read our guide on how to find arbitrage bets.
TL;DR
This free odds converter instantly translates between decimal, American (moneyline), fractional, and implied probability formats. Understanding odds conversion is essential for comparing lines across sportsbooks and prediction markets, spotting value, and identifying arbitrage opportunities where the same event is priced differently on different platforms.
What Are Betting Odds?
Betting odds represent the payout ratio of a wager and, by extension, the implied likelihood of an outcome. Every odds format encodes the same two pieces of information: how much you stand to win relative to your stake, and the probability the bookmaker assigns to that outcome. The reason multiple formats exist is purely historical. Different regions developed their own conventions long before online betting made cross-border wagering routine.
If you are comparing lines across US sportsbooks, UK bookmakers, European exchanges, and prediction markets like Kalshi or Polymarket, understanding how to convert between formats is not optional. It is the foundation of sports arbitrage betting and value betting. Our arbitrage calculator and EV calculator both depend on accurate odds conversion under the hood.
Decimal Odds
Decimal odds are the most straightforward format. The number represents your total return per unit staked, including your original stake. Decimal odds of 2.50 mean that a $10 bet returns $25 ($15 profit + $10 stake). This format is standard across Europe, Australia, and most online exchanges.
To calculate implied probability from decimal odds: probability = 1 / decimal. For example, 2.50 implies a 40% chance (1 / 2.50 = 0.40).
Decimal odds are always greater than 1.00. If a bookmaker shows 1.01, they consider the outcome near-certain. Decimal 2.00 is a coin flip (50% implied), and anything above 2.00 is an underdog.
American (Moneyline) Odds
American odds use a plus/minus system centered around $100. Positive odds like +150 tell you the profit on a $100 bet ($150 profit). Negative odds like -200 tell you how much you need to bet to profit $100 (bet $200 to win $100). This format dominates US sportsbooks and is the default on platforms like DraftKings, FanDuel, and BetMGM.
The conversion to decimal is simple. For positive American odds: decimal = (american / 100) + 1. So +150 becomes 2.50. For negative odds: decimal = (100 / |american|) + 1. So -200 becomes 1.50.
Fractional Odds
Fractional odds, written as 5/2 or 1/4, show your profit relative to your stake. Odds of 5/2 mean you profit $5 for every $2 staked. This format is traditional in UK horse racing and still widely used across British and Irish bookmakers.
To convert fractional to decimal: decimal = (numerator / denominator) + 1. So 5/2 becomes (5/2) + 1 = 3.50. Odds of 1/1 (known as "evens") become 2.00 in decimal.
Implied Probability
Implied probability expresses odds as a percentage chance. This is the native format for prediction markets. Kalshi prices contracts in cents from 1 to 99 (a 65-cent contract implies 65% probability), while Polymarket uses a 0.00–1.00 scale. Neither platform uses traditional bookmaker odds.
Converting probability to decimal: decimal = 1 / probability. A 40% implied probability gives decimal odds of 2.50. When comparing a sportsbook line against a prediction market price, converting both to implied probability is the fastest way to spot discrepancies.
Keep in mind that sportsbook implied probabilities for all outcomes in a market typically sum to more than 100%. The excess is the bookmaker's margin (vig or juice). True probability stripping is a separate step. See our Kelly Criterion calculator for optimal staking based on true edge.
Quick Reference Table
| Decimal | American | Fractional | Prob |
|---|---|---|---|
| 1.50 | -200 | 1/2 | 66.67% |
| 2.00 | +100 | 1/1 | 50.00% |
| 2.50 | +150 | 3/2 | 40.00% |
| 3.00 | +200 | 2/1 | 33.33% |
| 5.00 | +400 | 4/1 | 20.00% |
| 10.00 | +900 | 9/1 | 10.00% |
Why Odds Conversion Matters for Arbitrage
Arbitrage betting works by exploiting price differences between bookmakers. When one sportsbook lists an outcome at +150 (American) and another prices it at 2.60 (decimal), you cannot immediately tell which is the better price without converting to a common format. Converting both to implied probability (40.0% vs 38.5%) makes the comparison instant.
In cross-venue arbitrage between sportsbooks and prediction markets, odds conversion is even more critical. A Kalshi contract at 62 cents and a Pinnacle line at 1.55 decimal represent the same 62% vs 64.5% implied probability, and that 2.5% gap could be a profitable edge after fees.
Claw Arbs handles all of this automatically. The system normalizes every price from every venue (Kalshi cents, Polymarket probabilities, PS3838 decimal odds, BetInAsia lines) into a unified probability space and scans for edges in real time. But understanding the math yourself is still valuable for validating opportunities and building intuition. Use the converter above alongside our arbitrage calculator and expected value calculator to run the numbers manually when you want to double-check.
Frequently asked questions
How do I convert American odds to decimal odds?
For positive American odds, divide by 100 and add 1. For example, +150 becomes (150 / 100) + 1 = 2.50. For negative American odds, divide 100 by the absolute value and add 1. For example, -200 becomes (100 / 200) + 1 = 1.50.
What is implied probability in betting?
Implied probability is the likelihood of an outcome as suggested by the odds. It is calculated as 1 divided by the decimal odds, expressed as a percentage. For example, decimal odds of 2.00 imply a 50% probability. Sportsbook implied probabilities typically sum to more than 100% due to the built-in margin (vig).
How do I convert fractional odds to decimal?
Divide the numerator by the denominator, then add 1. For example, fractional odds of 5/2 become (5 / 2) + 1 = 3.50 in decimal format. Fractional odds of 1/4 become (1 / 4) + 1 = 1.25.
What odds format do Kalshi and Polymarket use?
Kalshi and Polymarket both use probability-based pricing. Kalshi displays prices in cents from 1 to 99 (a 65-cent contract represents a 65% implied probability). Polymarket uses a 0.00 to 1.00 scale. Neither platform uses traditional betting odds formats like decimal or American. See our prediction market calculator for the full workflow when working with Kalshi and Polymarket prices.
Why do different odds formats exist?
Different odds formats evolved in different regions. Decimal odds are standard in Europe and Australia for their simplicity. Fractional odds are traditional in UK horse racing and sports betting. American odds (moneyline) are the standard in US sportsbooks. Probability is used by prediction markets. They all represent the same information, just expressed differently.
How can I quickly compare odds across different formats?
The fastest way is to convert everything to implied probability. This gives you a single percentage that is easy to compare regardless of the original format. For example, decimal 2.00, American +100, and fractional 1/1 all represent the same 50% implied probability. Use the converter above to instantly see all formats side by side, or the no-vig calculator when you also want to strip the bookmaker margin from the numbers.
Related calculators
Odds conversion is usually the first step in a wider workflow. Once you have a common format across venues, these tools cover the next steps:
- Arbitrage calculator: verify sure bets and calculate stake splits after converting all odds to a common format.
- No-vig calculator: strip the bookmaker margin from converted odds to get true fair-value probabilities.
- Expected value calculator: once you have converted odds and a fair probability, compute the actual edge on any single bet.
- Kelly criterion calculator: size your bets optimally against any converted odds and probability estimate.
- Prediction market calculator: purpose-built for Kalshi and Polymarket cents-based pricing without needing to convert manually.