Polymarket Odds & Half-Kelly Calculator Tutorial: How to Compute +EV Edge and Size Positions Like a Quant Desk

Beginners love buying 92¢ shares because a 92% favorite 'feels safe.' Yet if the true probability is only 88%, buying at 92¢ is a Negative EV (-EV) trade that loses money over time. Conversely, if a contract trades at 35¢ (35% implied probability, 2.86x payout) while our on-chain and base-rate research shows a 55% true probability, every $100 USDC staked carries a +$57.14 USDC (+57.1%) mathematical edge.

For a binary contract paying $1.00 on victory, the exact Kelly fraction is:
Full Kelly (f*) = (P_true - P_market) / (1 - P_market)

With P_true = 0.55 and P_market = 0.35, Full Kelly equals (0.55 - 0.35) / 0.65 = 30.8%. Because real-world probability estimates always carry some uncertainty, institutional desks divide Full Kelly by two—using Half-Kelly (15.4%) to capture 75% of theoretical compounding growth with only 50% of the volatility.

1. Hard 15% Single-Event Cap: Never risk more than 12%–15% of active capital on a single resolution.
2. Group Correlated Theses: Treat correlated markets (e.g., BTC > $100k and ETH > $5k) as a single combined exposure.
3. Run Every Trade Through the Calculator: Test your numbers on the P.DAY Half-Kelly Calculator (/calculator/) before placing a limit bid.