In this guide
Key takeaway: The Kelly Criterion calculates the optimal proportion of your bankroll to wager based on your edge and available odds. In prediction markets, it guards against two critical pitfalls: wagering excessively (courting financial ruin) and wagering conservatively (forgoing potential gains).
The distinction between a successful prediction-market trader and one facing bankruptcy hinges on position sizing. The Kelly Criterion — a mathematical framework devised by Bell Labs scientist John Kelly in 1956 — establishes the theoretically optimal stake magnitude for enhancing compound returns over time. This guide demonstrates its practical implementation in prediction markets.
The Kelly formula
For a binary prediction market (YES/NO), the Kelly fraction is:
f* = (p * b - q) / b
Where:
- f* = proportion of bankroll to stake
- 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 event settles 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 Kelly formula recommends allocating 27.2% of your bankroll. If your bankroll totals $1,000, you would commit $272 to this position.
Why full Kelly is dangerous
The Kelly formula presupposes that you can determine your true probability with certainty — a condition rarely satisfied in practice. Miscalculating your advantage upward triggers severe overexposure. Seasoned market participants routinely employ fractional Kelly:
- Half Kelly (f*/2): The predominant choice among professionals. Surrenders roughly 25% of theoretical maximum returns whilst cutting volatility in half
- Quarter Kelly (f*/4): Prudent strategy when confidence in your edge remains limited
- Capped Kelly: Establish an upper limit—typically 5-10% of total bankroll per individual market, irrespective of what Kelly prescribes
Applying Kelly to multi-market portfolios
When maintaining concurrent stakes across numerous prediction markets, individual Kelly percentages require recalibration. The aggregate of all Kelly allocations must stay at or below 1.0 (your full bankroll). Practically speaking, maintain cumulative exposure beneath 50% to preserve capital for emerging opportunities.
When Kelly does not apply
The Kelly Criterion relies on accurate probability estimation. Several contexts undermine this assumption:
- Situations marked by radical uncertainty (unprecedented circumstances lacking comparable historical data)
- Interconnected markets (such as a presidential election and legislative composition, which exhibit statistical dependence)
- Markets where your forecasting ability matches the aggregate market view
Leverage PolyGram's integrated Kelly Criterion calculator to determine appropriate stake sizes before executing any trade. The analytics suite encompasses payoff visualisations and maximum drawdown metrics. Start trading on PolyGram →