In this guide
Key takeaway: Within prediction markets, a share's price functions as the market's probability estimate. When a YES share trades at $0.65, participants are collectively pricing in a 65% likelihood of that outcome occurring. Grasping this relationship between market price and probability underpins all successful trading strategies.
Coming from traditional sports wagering, prediction market odds operate on entirely different mechanics. You won't encounter fractional odds (5/1), American-style odds (+400), or decimal odds (5.0). Instead, prediction markets employ a transparent framework where the quoted share price directly translates to the underlying probability assessment.
Price = Probability
All prediction market contracts split into two opposing positions: YES and NO. The combined prices converge to roughly $1.00 (accounting for a modest spread retained by the market maker). Here's what the numbers mean:
- YES at $0.72 = Collective market view: 72% probability of occurrence
- NO at $0.28 = Collective market view: 28% probability of non-occurrence
- YES at $0.50 = Equiprobable outcome — neutral market positioning
- YES at $0.95 = Overwhelming consensus — merely 5% perceived chance of failure
Calculating Your Expected Value
Expected value (EV) serves as the arbiter of long-term profitability across your trading portfolio. The calculation follows this straightforward formula:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Suppose "Event X" trades at $0.40 (40%), yet your analysis suggests the genuine probability sits at 55%. Purchasing YES at $0.40 yields:
- Upside if YES materialises: $1.00 - $0.40 = $0.60
- Downside 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 expectancy-based edge. Across numerous trades, this edge accumulates into measurable wealth creation.
The Spread
The gap separating the highest bid (best purchase price) from the lowest ask (best sale price) constitutes the spread. On Polymarket, actively traded markets typically display spreads between 1–3 cents. This mirrors 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 the quoted odds
- Implied overround: Prediction markets see YES + NO sum near $1.00. Sports books typically engineer implied probabilities totalling 110-115%
Reading the Order Book
The PolyGram order book depth display illustrates all unexecuted buy and sell orders stacked at each price tier. This visibility reveals:
- Liquidity: The volume available for execution before price slippage becomes material
- Support/resistance: Price zones where substantial order clusters form "walls" that dampen directional movement
- Market sentiment: Whether aggregate demand or supply dominates at prevailing price levels
Converting to Traditional Odds
For traders preferring conventional odds notation:
| 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 contract carries no inherent superiority over a $0.10 contract — only whether the quoted price aligns with reality matters
- Overlooking the spread: Thin markets can exhibit spreads of 5-10 cents, substantially eroding your mathematical edge
- Excessive conviction: Before betting against consensus, ensure you've genuinely identified something thousands of competing traders have overlooked
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