# Reading Odds as Probabilities > Learn to turn prediction market prices into probabilities, adjust for spreads and long shot bias, compare with your own estimate and find value with expected value. Source: https://learn.tradelabsai.com/prediction-markets/reading-odds-as-probabilities/ Track: Prediction Markets · Level: Beginner · Updated: 2026-10-03 Publisher: TradeLabs AI (https://tradelabsai.com). Education, not financial advice. Cite as: TradeLabs Learn, "Reading Odds as Probabilities", https://learn.tradelabsai.com/prediction-markets/reading-odds-as-probabilities/ A prediction market price is a probability in disguise. A share that costs 70¢ and pays $1 if correct implies the market thinks the outcome has about a 70% chance. Reading prices this way is the foundation of prediction market trading, but the conversion is not perfect. Spreads, fees, thin liquidity and human biases all distort prices. This lesson shows how to read odds correctly, compare them with your own estimates and decide when a price offers value. ## The basic conversion ``` implied probability = share price / payout ``` On a $1 payout, a 70¢ share implies 70%. Yes and No should add up to about 100%: if Yes is 70¢, No should be near 30¢. | Yes price | Implied chance | Profit per share if right | Loss per share if wrong | |---|---|---|---| | 10¢ | 10% | 90¢ | 10¢ | | 50¢ | 50% | 50¢ | 50¢ | | 70¢ | 70% | 30¢ | 70¢ | | 95¢ | 95% | 5¢ | 95¢ | ## Converting from other odds formats | Format | Example | Implied probability | |---|---|---| | Decimal odds | 2.50 | 1 / 2.50 = 40% | | Fractional odds | 3/1 | 1 / (3 + 1) = 25% | | American odds (positive) | +150 | 100 / (150 + 100) = 40% | | American odds (negative) | minus 200 | 200 / (200 + 100) = 66.7% | ## Adjusting for the spread The midpoint between bid and ask is usually the best single estimate of the market's probability, but you pay the ask when buying and receive the bid when selling. **Example: Spread adjusted odds** Yes shows a bid of 62¢ and an ask of 66¢. The midpoint implies 64%. If you buy at 66¢, you need the true chance to be above 66% just to break even. With a 4¢ spread, buying and immediately selling would lose 4¢ a share. In thin markets, the spread can be the biggest cost. See [Bid-Ask Spread](https://learn.tradelabsai.com/markets/bid-ask-spread/). ## Biases in prediction market prices - **Long shot bias:** people tend to overpay for unlikely outcomes. Studies of betting markets have long found that long shots win less often than their prices imply, while favourites win slightly more often. A 3¢ outcome may really have a 1% to 2% chance. - **Thin markets:** a single large trader can push the price away from fair value. - **Partisan or fan bias:** people bet on what they want to happen. - **Capital lock up:** holding a 95¢ share for months to earn 5¢ ties up money, so near certain outcomes can trade below their true probability. - **Recency and hype:** news can cause overreaction. See [Recency Bias](https://learn.tradelabsai.com/psychology/recency-bias/). ## Comparing with your estimate The market price is only useful if you compare it with your own estimate. ``` expected value per share = (your probability × payout) - price ``` **Example: Finding value** A market prices Yes at 40¢. After research, you estimate the true chance at 50%. Expected value is 0.50 × $1 minus $0.40, or 10¢ a share. If you are right about the probability over many such bets, you profit. If your estimates are no better than the market's, you lose the spread and fees over time. See [Expected Value](https://learn.tradelabsai.com/math/expected-value/). ## Updating your probability Good forecasters update estimates as information arrives, rather than holding fixed views. Bayes' theorem formalises this: start with a prior probability and adjust it based on how strongly new evidence points one way. See [Bayes' Theorem](https://learn.tradelabsai.com/math/bayes-theorem/) and [Conditional Probability](https://learn.tradelabsai.com/math/conditional-probability/). ## Calibration: checking your skill Keep a record of your probability estimates and outcomes. If you are well calibrated, things you call 70% happen about 70% of the time. Group your forecasts into buckets (for example 60% to 70%) and compare with actual hit rates. This is the only honest way to know whether you can beat market odds. See [Trading Journal](https://learn.tradelabsai.com/start-here/trading-journal/). ## Sizing bets by edge Even with a real edge, betting too much can ruin you. The Kelly criterion suggests a stake based on your edge and the odds, and many traders use a fraction of it to reduce swings. See [Kelly Criterion](https://learn.tradelabsai.com/risk/kelly-criterion/) and [Fractional Kelly](https://learn.tradelabsai.com/risk/fractional-kelly/). ## Common mistakes - **Treating price as truth** without considering spread and liquidity. - **Buying long shots** because the payout looks big. - **Ignoring time value** on long dated markets. - **Never checking calibration.** ## Frequently asked questions ### How do you convert prediction market prices to probability? Divide the share price by the payout. On a $1 payout, a price of 65¢ implies about a 65% chance. ### Are prediction market odds accurate? They are often reasonably accurate, but spreads, thin liquidity and biases such as overpaying for long shots can distort them. ### What is long shot bias? The tendency for bettors to overpay for unlikely outcomes, so long shots win less often than their prices imply. Next, see how the fastest Polymarket markets work in [Up or Down Markets Explained](https://learn.tradelabsai.com/prediction-markets/up-or-down-markets-explained/). ## Sources - Wikipedia, [Favourite longshot bias](https://en.wikipedia.org/wiki/Favourite-longshot_bias) ## Continue learning - Next lesson: [Up or Down Markets Explained](https://learn.tradelabsai.com/prediction-markets/up-or-down-markets-explained/) - Previous lesson: [How Polymarket Works](https://learn.tradelabsai.com/prediction-markets/how-polymarket-works/) - Related: [How Polymarket Works](https://learn.tradelabsai.com/prediction-markets/how-polymarket-works/): A clear guide to how Polymarket works: Yes and No shares, the order book, USDC, fees, how markets resolve through UMA and Chainlink, and the risks to know. - Related: [Expected Value](https://learn.tradelabsai.com/math/expected-value/): Expected value is the average result of a bet over many repetitions. Learn the formula, trading and prediction market examples, and why EV alone is not enough. - Related: [Probability for Traders](https://learn.tradelabsai.com/math/probability-for-traders/): Probability is the language of uncertainty in trading. Learn the core rules, independent vs dependent events, odds, and how probability shapes sizing and edge. - Related: [Bayes' Theorem](https://learn.tradelabsai.com/math/bayes-theorem/): Bayes' theorem shows how to update a probability when new evidence arrives. Learn the formula, trading and prediction market examples, and base rate errors. - Related: [Kelly Criterion](https://learn.tradelabsai.com/risk/kelly-criterion/): The Kelly criterion finds the bet size that maximises long term growth given your edge. Learn the formula, worked examples and why most traders use less. - Related: [Prediction Market Strategies and Risks](https://learn.tradelabsai.com/prediction-markets/prediction-market-strategies/): Learn practical prediction market strategies: research based value betting, news trading, arbitrage, market making and short round tactics, with risk rules for each.