What Is Expected Value?
Expected value (EV) is the single most important concept for profitable prediction market trading. It answers a straightforward question: if you made this exact trade thousands of times, how much would you gain or lose on average?
A trade with a positive expected value (+EV) is profitable over the long run. A trade with a negative expected value (-EV) loses money over time. Every successful trader on Polymarket builds their approach around finding and taking +EV trades consistently.
The EV Formula
EV = (Probability of Winning × Profit) - (Probability of Losing × Loss)
In prediction markets, this translates to:
EV = (p × (1 - price)) - ((1 - p) × price)
Where:
- p = your estimated true probability of the event occurring
- price = the current market price of the YES share
Worked Example
A market asks "Will the EU pass new AI regulation by Q4 2026?" YES shares are priced at $0.35.
After researching, you estimate a 50% true probability.
- EV = (0.50 × $0.65) - (0.50 × $0.35)
- EV = $0.325 - $0.175
- EV = +$0.15 per share
This is a strongly positive EV trade. For every share you buy, you expect to profit $0.15 on average.
How to Estimate True Probability
The EV calculation is only as good as your probability estimate. If you overestimate your edge, your "positive EV" trade might actually be negative.
Sources of Information
- Data and statistics — Historical base rates, polling data, economic indicators
- Expert analysis — Domain-specific research, academic papers, insider knowledge
- Market signals — Related markets on Polymarket, traditional financial markets, betting lines
- Structural factors — Incentives, deadlines, institutional constraints
Calibration
Good traders are well-calibrated, meaning when they say something has a 70% chance of happening, it actually happens about 70% of the time. You can improve your calibration by:
- Recording your probability estimates before every trade.
- Tracking actual outcomes over 100+ trades.
- Comparing your estimates against results. If your "70% confident" trades only win 55% of the time, you are overconfident and need to adjust.
EV in Action: Three Scenarios
Scenario 1: Clear Positive EV
Market price: $0.30. Your estimate: 45%.
- EV = (0.45 × $0.70) - (0.55 × $0.30) = $0.315 - $0.165 = +$0.15
- This is a strong buy. The market is significantly underpricing the event.
Scenario 2: Marginal EV
Market price: $0.60. Your estimate: 65%.
- EV = (0.65 × $0.40) - (0.35 × $0.60) = $0.26 - $0.21 = +$0.05
- Technically positive, but slim. Factor in any opportunity cost — your capital might be better deployed elsewhere.
Scenario 3: Negative EV
Market price: $0.50. Your estimate: 45%.
- EV = (0.45 × $0.50) - (0.55 × $0.50) = $0.225 - $0.275 = -$0.05
- Do not take this trade. You believe the market is slightly overpriced for YES, so if anything, consider buying NO.
EV and Position Sizing
Expected value tells you whether to trade. The Kelly Criterion tells you how much to trade. These two concepts work together:
- Calculate EV — Is the trade worth taking? (Is EV positive?)
- Calculate Kelly — How much should you bet? (Based on the size of your edge)
- Apply fractional Kelly — Reduce position size for safety.
A common mistake is finding a +EV trade and then betting too much on it. Even strong +EV trades lose sometimes. Proper bankroll management ensures one bad outcome does not derail your long-term results.
The Relationship Between EV and Market Efficiency
Prediction markets are generally efficient — meaning prices usually reflect true probabilities quite well. This is because smart traders are constantly looking for mispricings and trading them away.
However, markets are not perfectly efficient. Mispricings occur because of:
- Information delays — News has not been fully incorporated yet.
- Cognitive biases — Traders overweight recent events, anchor to round numbers, or are influenced by narratives rather than data.
- Liquidity gaps — Thin markets may not attract enough informed traders to correct the price.
- Complexity — Markets involving multiple conditional factors are harder to price correctly.
Your job as a trader is to identify these inefficiencies, calculate your EV, and trade accordingly. For strategies on spotting these opportunities, read our Polymarket trading strategies guide.
Why Most Traders Ignore EV
Despite its importance, many Polymarket traders never calculate expected value. Instead, they trade based on gut feeling, narrative conviction, or a desire to be "right" about a prediction.
This is a mistake. A trader who makes 100 +EV trades will be profitable over the long run, even if many individual trades lose. Meanwhile, a trader who chases exciting narratives without quantifying their edge will almost certainly lose money.
The discipline to only take +EV trades — and to walk away from markets where you have no edge — is what separates consistent winners from everyone else. For more on developing this mindset, see our guide to common Polymarket mistakes.
Building an EV Spreadsheet
A simple spreadsheet can transform your trading. For each potential trade, record:
| Field | Example | |---|---| | Market | EU AI Regulation by Q4 2026 | | Current Price | $0.35 | | Your Probability | 50% | | EV per Share | +$0.15 | | Kelly Fraction | 21% | | Half Kelly Bet | 10.5% of bankroll | | Outcome | Pending |
After resolution, add the actual outcome and your P&L. Over time, this spreadsheet becomes your most valuable trading tool — it shows you where your edge is real and where it is imaginary.
Key Takeaways
Expected value is not complicated, but it requires discipline. Estimate probabilities honestly, calculate EV before every trade, size your positions with Kelly, and track your results religiously. The math will do the rest.
Start applying EV analysis to live markets and see how it transforms your approach to prediction market trading.