In this guide
Key takeaway: Within prediction markets, a contract's price functions as the market's probability estimate. When a YES contract trades at $0.65, participants collectively assess a 65% likelihood of that outcome occurring. Grasping this price-to-probability relationship forms the cornerstone of successful market participation.
If your background includes sports wagering, prediction market odds operate on unfamiliar principles. Fractional notation (5/1), American formats (+400), and decimal systems (5.0) do not apply here. Prediction markets employ a transparent mechanism: contract prices directly encode the market's probabilistic assessment.
Price = Probability
Each prediction market contract splits into two opposing positions: YES and NO. Their prices converge toward $1.00 in aggregate (minus a modest spread retained by liquidity providers). Interpretation follows this pattern:
- YES at $0.72 = Collective assessment: 72% probability of occurrence
- NO at $0.28 = Collective assessment: 28% probability of non-occurrence
- YES at $0.50 = Maximum uncertainty — the market holds no lean either direction
- YES at $0.95 = Overwhelming consensus — merely 5% downside risk perceived
Calculating Your Expected Value
Expected value (EV) underpins whether a position generates profits across repeated transactions. The calculation follows this framework:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Illustration: "Event X" trades at $0.40 (40% implied), yet your analysis suggests 55% genuine likelihood. Purchasing YES at $0.40 yields:
- Upside if YES resolves: $1.00 - $0.40 = $0.60
- Downside if NO resolves: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per contract
Positive EV signals an advantageous position in expectation. Across numerous transactions, positive EV accumulates into tangible wealth creation.
The Spread
The gap separating the maximum bid (highest purchase offer) from the minimum ask (lowest sale offer) constitutes the spread. On Polymarket, well-traded contracts typically exhibit spreads of 1-3 cents. This mirrors the "vig" in sports betting yet proves substantially tighter:
- Prediction market spread: 1-3% (functionally equivalent to vig)
- Sports betting vig: 5-15% embedded within quoted odds
- Probability overround: Prediction markets see YES + NO sum near $1.00. Sports books typically show implied totals of 110-115%
Reading the Order Book
The PolyGram order book depth display reveals all unexecuted buy and sell orders across price tiers. This information communicates:
- Liquidity: Transaction volume achievable without material price movement
- Support/resistance: Price zones containing concentrated orders that impede directional shifts
- Market sentiment: Whether buying or selling dominates at present valuations
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 trade quality: A $0.90 contract carries no inherent disadvantage versus a $0.10 contract — only whether the quoted price deviates from genuine likelihood matters
- Neglecting spread costs: Thinly-traded markets may exhibit spreads of 5-10 cents, substantially diminishing your mathematical advantage
- Excessive conviction: Before betting against market consensus, consider why thousands of participants hold the opposite view
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