Understanding Prediction Market Odds and Probability
Key takeaway: In prediction markets, the price of a share IS the probability. A YES share priced at $0.65 means the market believes there is a 65% chance the event will happen. Understanding this connection between price and probability is the foundation of profitable trading.
If you are coming from sports betting, prediction market odds look different. There are no fractional odds (5/1), no American odds (+400), and no decimal odds (5.0). Instead, prediction markets use a simpler system: the share price directly represents the implied probability.
Price = Probability
Every prediction market contract has two sides: YES and NO. The prices always add up to approximately $1.00 (with a small spread for the market maker). Here is how to read them:
- YES at $0.72 = Market thinks 72% chance the event happens
- NO at $0.28 = Market thinks 28% chance the event does not happen
- YES at $0.50 = Coin flip — the market has no directional conviction
- YES at $0.95 = Near-certainty — only a 5% chance of failure
Calculating Your Expected Value
Expected value (EV) determines whether a trade is profitable in the long run. The formula is simple:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Example: A market prices "Event X" at $0.40 (40%), but you believe the true probability is 55%. If you buy YES at $0.40:
- Potential profit if YES: $1.00 - $0.40 = $0.60
- Potential loss if NO: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
A positive EV means the trade is profitable in expectation. Over hundreds of trades, positive EV compounds into real returns.
The Spread
The difference between the best bid (highest buy price) and best ask (lowest sell price) is the spread. On Polymarket, liquid markets typically have spreads of 1-3 cents. This is analogous to the "vig" in sports betting but dramatically smaller:
- Prediction market spread: 1-3% (equivalent to vig)
- Sports betting vig: 5-15% built into odds
- Implied overround: In prediction markets, YES + NO prices sum close to $1.00. In sports betting, implied probabilities often sum to 110-115%
Reading the Order Book
The PolyGram order book depth chart shows all pending buy and sell orders at each price level. This tells you:
- Liquidity: How much you can buy or sell without moving the price
- Support/resistance: Price levels where large orders are stacked, creating "walls" that resist price movement
- Market sentiment: Whether there is more buying or selling interest at the current price
Converting to Traditional Odds
If you are more comfortable with traditional odds formats:
| Market Price | Implied Prob. | Decimal Odds | American Odds |
| $0.80 | 80% | 1.25 | -400 |
| $0.65 | 65% | 1.54 | -186 |
| $0.50 | 50% | 2.00 | +100 |
| $0.25 | 25% | 4.00 | +300 |
| $0.10 | 10% | 10.00 | +900 |
Common Mistakes
- Confusing price with "goodness" of a bet: A $0.90 share is not inherently worse than a $0.10 share — what matters is whether the price reflects the true probability
- Ignoring the spread: On illiquid markets, the spread can be 5-10 cents, eating into your edge
- Overconfidence: If you think the market is wrong, make sure you understand why thousands of other traders disagree with you
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