In this guide
Key takeaway: Within prediction markets, a share's market price directly equals the implied probability. 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 valuation and probability forms the cornerstone of successful market participation.
Those transitioning from traditional sports betting environments will notice that prediction market odds operate on entirely different mechanics. Fractional odds (5/1), American odds (+400), and decimal odds (5.0) do not apply here. Instead, prediction markets employ a straightforward mechanism: share prices function as direct probability indicators.
Price = Probability
Each prediction market contract bifurcates into two opposing positions: YES and NO. The combined prices consistently approximate $1.00 (accounting for modest spreads maintained by liquidity providers). The interpretation works as follows:
- YES at $0.72 = Participants collectively assign a 72% likelihood to occurrence
- NO at $0.28 = Participants collectively assign a 28% likelihood to non-occurrence
- YES at $0.50 = Perfect equilibrium — market participants show no consensus bias
- YES at $0.95 = Overwhelming consensus — merely a 5% residual doubt remains
Calculating Your Expected Value
Expected value (EV) serves as the metric determining long-term trade profitability. The calculation follows this framework:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Scenario: A market quotes "Event X" at $0.40 (implying 40%), yet your assessment suggests 55% represents the genuine likelihood. Suppose you acquire YES at $0.40:
- Profit upon YES resolution: $1.00 - $0.40 = $0.60
- Loss upon NO resolution: $0.40
- EV = (0.55 x $0.60) - (0.45 x $0.40) = $0.33 - $0.18 = +$0.15 per share
Positive EV indicates a mathematically sound trade. Across numerous transactions, this positive expectation accumulates into tangible wealth generation.
The Spread
The gap separating the highest purchase bid from the lowest sale ask constitutes the spread. On Polymarket, well-traded markets typically display spreads ranging from 1–3 cents. This mechanism parallels the "vig" concept in traditional sports betting, though substantially narrower:
- Prediction market spread: 1-3% (functionally equivalent to vig)
- Sports betting vig: 5-15% embedded within quoted odds
- Implied overround: Prediction markets see YES + NO sum near $1.00. Sports betting typically produces implied probability totals of 110-115%
Reading the Order Book
The PolyGram order book depth display reveals all outstanding purchase and sale orders across price tiers. This visibility furnishes:
- Liquidity: Transaction volume achievable without triggering substantial price slippage
- Support/resistance: Price zones containing concentrated order clusters, functioning as barriers against directional movement
- Market sentiment: Whether aggregate demand or supply dominates at prevailing valuations
Converting to Traditional Odds
Should conventional odds notation 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
- Treating price as a quality indicator: A $0.90 position carries no inherent advantage over a $0.10 position — only whether the valuation aligns with actual probability matters
- Neglecting spread costs: Illiquid venues frequently feature 5–10 cent spreads, substantially eroding your mathematical edge
- Excessive conviction: Before assuming the market misprices an outcome, consider whether your reasoning truly surpasses the aggregate intelligence of thousands of participants
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