Key takeaway: Within prediction markets, a share's price directly equals the probability. When a YES share trades at $0.65, the collective market assessment is a 65% likelihood of that outcome occurring. Grasping this fundamental relationship between price and probability forms the cornerstone of successful market participation.
For those transitioning from traditional sports betting, prediction market odds operate on an entirely different framework. You won't encounter fractional odds (5/1), American-style odds (+400), or decimal odds (5.0). Instead, prediction markets employ a straightforward mechanism: share prices function as direct probability indicators.
Price = Probability
All prediction market contracts split into two outcomes: YES and NO. Their prices combine to roughly $1.00 total (with a modest spread retained by the market maker). Here's the breakdown:
- YES at $0.72 = Market consensus suggests a 72% likelihood the event materialises
- NO at $0.28 = Market consensus suggests a 28% likelihood the event fails to occur
- YES at $0.50 = Even odds — the market shows no clear bias either direction
- YES at $0.95 = Overwhelming likelihood — merely a 5% probability of non-occurrence
Calculating Your Expected Value
Expected value (EV) determines whether a position generates profit across repeated trades. The calculation follows this pattern:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Illustration: Suppose "Event X" trades at $0.40 (40% implied), yet you assess the genuine probability at 55%. Purchasing YES at $0.40 yields:
- Gain if YES wins: $1.00 - $0.40 = $0.60
- Loss 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. Across dozens or hundreds of such positions, positive EV accumulates into tangible wealth creation.
The Spread
The gap separating the highest purchase price (best bid) from the lowest sale price (best ask) constitutes the spread. Polymarket's most active contracts typically show spreads between 1–3 cents. This mirrors the "vig" in sports wagering but remains substantially tighter:
- Prediction market spread: 1-3% (equivalent to vig)
- Sports betting vig: 5-15% embedded within the quoted odds
- Implied overround: Prediction market YES and NO prices approach $1.00 combined. Sports betting implied probabilities frequently total 110-115%
Reading the Order Book
The PolyGram order book depth view displays all active buy and sell orders across price tiers. This reveals:
- Liquidity: The volume available for purchase or sale without substantially shifting the price
- Support/resistance: Price zones where substantial orders congregate, forming "walls" that impede price shifts
- Market sentiment: Whether buying or selling momentum dominates at prevailing price levels
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 an indicator of bet quality: A $0.90 share carries no inherent advantage over a $0.10 share — only whether the price accurately reflects genuine probability matters
- Overlooking the spread: Less-traded markets may display spreads of 5-10 cents, substantially eroding your potential advantage
- Excessive conviction: Before backing a contrarian position, consider why thousands of competing traders hold the opposite view
Discover current odds spanning 1,500+ prediction markets via PolyGram. Begin trading on PolyGram →