In this guide
Key takeaway: The Kelly Criterion determines the optimal percentage of your capital to allocate to each wager, accounting for your probability advantage and available odds. Within prediction markets, it guards against the twin pitfalls of excessive exposure (potential account depletion) and excessive caution (forgoing achievable returns).
The gap between sustained profitability and financial collapse hinges on bet sizing discipline. Formulated by John Kelly, a Bell Labs scientist, in 1956, the Kelly Criterion supplies the theoretically sound stake magnitude for achieving maximum compound returns over time. This guide demonstrates its practical implementation in prediction market contexts.
The Kelly formula
For a two-sided prediction market (YES/NO), the Kelly percentage is:
f* = (p * b - q) / b
Where:
- f* = percentage of total capital to allocate
- p = your assessed likelihood of success
- q = likelihood of failure (1 - p)
- b = decimal odds (return / investment). For a prediction market contract trading at price c, b = (1 - c) / c
Worked example
Suppose you assess a 60% probability that an outcome materialises as YES. The current market quotation stands at 45 cents (reflecting 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 staking 27.2% of your capital. If you have $1,000 available, this translates to a $272 position in this opportunity.
Why full Kelly is dangerous
The Kelly formula presumes you possess perfect knowledge of your true edge — a condition never met in practice. Miscalculating your advantage upward triggers severe overexposure. Seasoned market participants routinely employ fractional Kelly instead:
- Half Kelly (f*/2): Industry standard across professional shops. Surrenders roughly 25% of theoretical maximum gains whilst cutting portfolio swings in half
- Quarter Kelly (f*/4): Prudent selection when your edge estimate carries substantial uncertainty
- Capped Kelly: Establish an absolute ceiling—typically 5-10% per market—irrespective of what the formula suggests
Applying Kelly to multi-market portfolios
Holding concurrent stakes across numerous prediction markets requires recalibrating individual Kelly percentages. The aggregate of all positions must remain at or below 1.0 (your entire bankroll). Most practitioners maintain cumulative exposure beneath 50% to preserve dry powder for emerging opportunities.
When Kelly does not apply
The formula relies on your capacity to estimate your true edge with reasonable accuracy. Several circumstances undermine this assumption:
- Unprecedented or highly ambiguous events (situations lacking comparable historical data)
- Interconnected markets (a presidential election and congressional composition are statistically dependent)
- Situations where your insight offers no advantage relative to market consensus
Leverage PolyGram's integrated Kelly Criterion calculator to determine position sizing ahead of each transaction. The analytics suite encompasses payoff visualisation and historical drawdown metrics. Start trading on PolyGram →