In this guide
Key takeaway: Within prediction markets, a share's price functions as the probability estimate. When a YES share trades at $0.65, the collective market assessment indicates a 65% likelihood of that outcome occurring. Grasping this fundamental relationship between price and probability forms the cornerstone of successful market participation.
Transitioning from conventional sports wagering, prediction market odds operate on entirely different mechanics. You will not encounter fractional odds (5/1), American-style odds (+400), or decimal odds (5.0). Instead, prediction markets employ a transparent mechanism: share prices function as direct probability indicators.
Price = Probability
All prediction market contracts feature two opposing outcomes: YES and NO. The combined prices consistently approximate $1.00 (accounting for a modest spread retained by liquidity providers). The interpretation works as follows:
- YES at $0.72 = Collective market view suggests 72% probability of occurrence
- NO at $0.28 = Collective market view suggests 28% probability of non-occurrence
- YES at $0.50 = Evenly balanced — the market expresses genuine uncertainty
- YES at $0.95 = Overwhelming consensus — merely 5% perceived chance of failure
Calculating Your Expected Value
Expected value (EV) establishes whether a position generates returns across repeated trades. The underlying calculation remains straightforward:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Illustration: Suppose "Event X" trades at $0.40 (40% implied), yet your analysis suggests genuine probability reaches 55%. Purchasing YES at $0.40 yields:
- Profit scenario if YES materialises: $1.00 - $0.40 = $0.60
- Loss scenario if NO materialises: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
Positive EV signals an advantageous trade mathematically. Accumulating numerous positive-EV positions generates measurable wealth growth.
The Spread
The gap separating the highest purchase offer from the lowest sale offer constitutes the spread. On Polymarket, active markets generally maintain spreads between 1-3 cents. This parallels the "vig" concept in sports betting, though substantially tighter:
- Prediction market spread: 1-3% (functionally equivalent to vig)
- Sports betting vig: 5-15% embedded within quoted odds
- Implied overround: Prediction markets show YES + NO summing near $1.00. Sports betting typically displays combined implied probabilities of 110-115%
Reading the Order Book
The PolyGram order book depth display reveals all outstanding buy and sell orders stacked at individual price tiers. This information communicates:
- Liquidity: Transaction volume achievable without materially shifting the price
- Support/resistance: Price zones where concentrated orders establish barriers against movement
- Market sentiment: Whether aggregate demand or supply dominates at prevailing prices
Converting to Traditional Odds
Should you prefer conventional odds representations:
| 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
- Treating price as a quality indicator: A $0.90 position carries no inherent advantage over a $0.10 position — only whether pricing accurately reflects genuine probability matters
- Overlooking the spread: Illiquid venues frequently display 5-10 cent spreads, substantially eroding your mathematical advantage
- Excessive conviction: Before assuming thousands of market participants have misjudged, rigorously examine your reasoning and potential blind spots
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