Key takeaway: Within prediction markets, a share's price functions as the probability itself. When a YES share trades at $0.65, the collective market assessment reflects a 65% likelihood of that outcome occurring. Grasping this fundamental relationship between price and probability forms the cornerstone of successful market participation.
Should you transition from traditional sports wagering, prediction market odds operate quite differently. You will not encounter fractional odds (5/1), American odds (+400), or decimal odds (5.0). Instead, prediction markets employ a more transparent mechanism: share prices function as direct probability indicators.
Price = Probability
Each prediction market contract splits into two opposing positions: YES and NO. These prices consistently total roughly $1.00 (accounting for a modest spread retained by the market operator). Here is the interpretation framework:
- YES at $0.72 = Market consensus suggests 72% probability the event materialises
- NO at $0.28 = Market consensus suggests 28% probability the event fails to occur
- YES at $0.50 = Evenly balanced — the market shows neutral positioning
- YES at $0.95 = Overwhelming likelihood — merely 5% downside risk
Calculating Your Expected Value
Expected value (EV) establishes whether a position generates profit across repeated trades. The calculation follows this straightforward approach:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Scenario: "Event X" trades at $0.40 (40% implied), yet your assessment places true probability at 55%. Purchasing YES at $0.40:
- Upside if YES wins: $1.00 - $0.40 = $0.60
- Downside if NO wins: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
Positive EV signals a mathematically sound trade. Accumulating numerous positive-EV positions compounds into tangible wealth growth.
The Spread
The gap separating the highest purchase offer (bid) from the lowest sale offer (ask) constitutes the spread. Polymarket's most active contracts typically display spreads ranging 1-3 cents. This resembles sports betting's "vig" though substantially tighter:
- Prediction market spread: 1-3% (comparable to vig)
- Sports betting vig: 5-15% embedded in quoted prices
- Implied overround: Prediction markets see YES + NO sum near $1.00. Sports betting typically produces 110-115% combined probability
Reading the Order Book
The PolyGram order book depth visualization displays all outstanding purchase and sale orders across price tiers. This reveals:
- Liquidity: Volume available for execution without material price slippage
- Support/resistance: Price zones hosting concentrated orders, forming barriers against movement
- Market sentiment: Whether demand outweighs supply (or vice versa) at prevailing levels
Converting to Traditional Odds
Should conventional odds formats feel more intuitive:
| 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
- Equating price with bet quality: A $0.90 contract carries no inherent disadvantage versus a $0.10 contract — only whether the quoted price aligns with genuine probability matters
- Overlooking the spread: Thinner markets sometimes feature 5-10 cent spreads, which substantially erode your mathematical advantage
- Excessive conviction: Before dismissing the market consensus, consider why thousands of participants hold opposing views
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