How to Read Prediction Market Odds
Understand probability percentages, implied odds, and how to convert between different odds formats.
Unlike traditional bookmakers, prediction markets display prices as probabilities — making them more intuitive. But it still helps to understand how different odds formats relate to each other.
Probability Format (Prediction Markets)
On platforms like Polymarket and Kalshi, contracts are priced in cents: a contract at $0.72 means the market estimates a 72% probability of that event happening.
This is the most intuitive format for prediction markets. You don't need to convert anything — the price IS the probability.
American Odds (US Sportsbooks)
American odds use a plus/minus format around $100:
- Negative odds (e.g. -200): You must bet $200 to win $100. This implies a probability above 50%. The implied probability of -200 is 200/(200+100) = 66.7%.
- Positive odds (e.g. +150): You win $150 on a $100 bet. This implies below 50% probability. The implied probability of +150 is 100/(150+100) = 40%.
Decimal Odds (Europe/Australia)
Decimal odds show your total return per $1 staked (including your stake back):
- Decimal 2.00 = 50% probability (even money)
- Decimal 1.50 = 66.7% probability
- Decimal 3.00 = 33.3% probability
Formula: Probability = 1 / Decimal Odds × 100
Fractional Odds (UK)
Fractional odds like 3/1 (read "three to one") mean you win $3 for every $1 staked:
- 4/1 = 20% probability
- 2/1 = 33.3% probability
- Evens (1/1) = 50% probability
- 1/2 = 66.7% probability
Understanding the Vig (Overround)
Traditional bookmakers build a "vig" or "overround" into their odds — the probabilities of all outcomes sum to more than 100%, ensuring the bookmaker profits. On Polymarket, a YES + NO contract always sums to exactly $1.00 (100%), with no vig built in. This is why 0-fee crypto prediction markets are so attractive to serious traders.
Finding Value
Value betting means finding markets where the implied probability is lower than your estimated true probability. If you think an event has a 70% chance of happening but the market is pricing it at 60%, that's a positive-expected-value bet.
Use our probability calculator to quickly convert between formats.