In this guide
Key takeaway: The Kelly Criterion determines the optimal percentage of your capital to deploy based on your informational advantage and available odds. In prediction markets, it solves two critical problems: risking too much capital (leading to total loss) and risking too little (forfeiting potential returns).
Stake allocation separates successful market participants from those who deplete their funds. The Kelly Criterion — a mathematical framework created by Bell Labs scientist John Kelly in 1956 — calculates the ideal stake magnitude for achieving sustainable wealth accumulation. This guide shows how to implement it within prediction markets.
The Kelly formula
For a binary prediction market (YES/NO), the Kelly fraction is:
f* = (p * b - q) / b
Where:
- f* = percentage of capital to allocate
- p = your assessed likelihood of success
- q = likelihood of failure (1 - p)
- b = net odds (payout / stake). For a prediction market share trading at price c, b = (1 - c) / c
Worked example
Suppose you assess a 60% probability that an outcome resolves YES. The current market quotation stands at 45 cents (suggesting 45% implied probability).
- p = 0.60, q = 0.40
- b = (1 - 0.45) / 0.45 = 1.222
- f* = (0.60 * 1.222 - 0.40) / 1.222 = (0.733 - 0.40) / 1.222 = 0.272
The formula recommends committing 27.2% of your capital. If your account holds $1,000, you would place $272 into this position.
Why full Kelly is dangerous
The Kelly formula presumes perfect knowledge of your true probability — a condition that never materialises in practice. Misjudging your informational edge creates severe overbetting risk. Seasoned market operators adopt fractional Kelly instead:
- Half Kelly (f*/2): The industry standard. Surrenders roughly 25% of maximum growth but cuts fluctuations in half
- Quarter Kelly (f*/4): Prudent method when edge calculations carry substantial uncertainty
- Capped Kelly: Establish a ceiling — never exceed 5-10% of total capital on any single market, irrespective of Kelly calculations
Applying Kelly to multi-market portfolios
Operating across numerous prediction markets at once requires recalibrating individual Kelly percentages. The aggregate of all Kelly percentages must remain at or below 1.0 (100% of capital). Practically speaking, maintain combined positions below 50% to preserve dry powder for emerging opportunities.
When Kelly does not apply
Kelly hinges on reliable probability assessment. Several circumstances undermine this assumption:
- Situations involving extreme uncertainty (unprecedented events lacking historical data)
- Linked markets (presidential election and legislative control share dependencies)
- Markets where your analysis offers no advantage relative to existing pricing
Deploy PolyGram's integrated Kelly Criterion calculator before executing any position. The analytics suite encompasses payoff visualisations and volatility tracking. Start trading on PolyGram →