In this guide
Key takeaway: Within prediction markets, a share's market price functions as the probability estimate. When a YES share trades at $0.65, participants collectively signal a 65% likelihood of that outcome occurring. Grasping this price-to-probability relationship forms the bedrock of successful market participation.
Those transitioning from traditional sports wagering will notice prediction market odds operate on entirely different mechanics. You won't encounter fractional notation (5/1), American-style formatting (+400), or European decimal systems (5.0). Instead, prediction markets employ a more intuitive framework: the contract's market price directly mirrors the consensus probability.
Price = Probability
All prediction market contracts split into two opposing positions: YES and NO. Their prices converge toward approximately $1.00 in aggregate (accounting for modest spreads captured by liquidity providers). Interpreting them works as follows:
- YES at $0.72 = Collective assessment suggests 72% likelihood of occurrence
- NO at $0.28 = Collective assessment suggests 28% likelihood of non-occurrence
- YES at $0.50 = Genuine uncertainty — neither outcome favoured by the crowd
- YES at $0.95 = Overwhelming consensus — merely 5% probability of the opposite outcome
Calculating Your Expected Value
Expected value (EV) serves as the metric determining whether a given trade generates profits over extended timeframes. The calculation follows this straightforward approach:
EV = (Your probability x Potential profit) - ((1 - Your probability) x Potential loss)
Suppose a market quotes "Event X" at $0.40 (implying 40%), yet your analysis suggests the genuine likelihood reaches 55%. Should you acquire YES at $0.40:
- 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 share
When EV turns positive, the position generates expected profits. Across numerous such transactions, accumulating positive EV translates into tangible wealth creation.
The Spread
The gap separating the highest purchase bid from the lowest sale ask constitutes the spread. Polymarket's more active contracts typically exhibit spreads ranging 1-3 cents wide. This mirrors the "vig" concept familiar to sports bettors, 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 see YES + NO prices near $1.00. Sports betting typically inflates implied probabilities to 110-115%
Reading the Order Book
The PolyGram order book depth display reveals all outstanding bids and asks arranged by price tier. This information illuminates:
- Liquidity: Transaction volume achievable without substantially shifting market prices
- Support/resistance: Price zones where concentrated orders form barriers against directional movement
- Market sentiment: Whether accumulated orders skew toward purchases or sales relative to current valuations
Converting to Traditional Odds
Should you prefer working within conventional odds frameworks:
| 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 inherent quality metric: A $0.90 contract doesn't automatically represent worse value than a $0.10 contract — only whether the quoted price diverges from your genuine probability assessment matters
- Overlooking the spread's impact: Thinner markets can widen spreads to 5-10 cents, substantially eroding your profit margin
- Excessive conviction in contrarian views: Before betting against prevailing market consensus, carefully examine why thousands of participants hold opposing positions
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