In this guide
Key takeaway: The Kelly Criterion determines the optimal proportion of your capital to allocate to each wager, accounting for your competitive advantage and available odds. Within prediction markets, this methodology guards against two prevalent pitfalls: excessive wagering (which threatens total capital loss) and insufficient wagering (which forgoes potential returns).
The distinction between sustained profitability and financial collapse hinges on position sizing discipline. The Kelly Criterion — a mathematical framework conceived by John Kelly, a researcher at Bell Labs, during 1956 — establishes the theoretically ideal stake magnitude for optimising wealth accumulation over extended periods. The following sections demonstrate its practical implementation in prediction market contexts.
The Kelly formula
For a binary prediction market (YES/NO), the Kelly fraction is:
f* = (p * b - q) / b
Where:
- f* = proportion of available capital to allocate
- p = your assessed likelihood of a successful outcome
- q = likelihood of an unsuccessful outcome (1 - p)
- b = net odds (payout / stake). For a prediction market share at price c, b = (1 - c) / c
Worked example
Suppose you assess a 60% probability that an event concludes with a YES resolution. The current market valuation stands at 45 cents (suggesting a 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 committing 27.2% of your capital. If your total capital is $1,000, this corresponds to a $272 position in this particular trade.
Why full Kelly is dangerous
The Kelly formula presupposes certainty regarding your true edge — an assumption that rarely holds in practice. Miscalculating your competitive advantage produces severe overexposure. Industry practitioners consistently favour fractional Kelly approaches:
- Half Kelly (f*/2): The predominant choice among traders. Surrenders roughly 25% of theoretical maximum growth whilst cutting volatility in half
- Quarter Kelly (f*/4): A prudent strategy when edge confidence remains limited
- Capped Kelly: Establish an absolute ceiling—typically 5-10% of total capital—for any single market position, overriding Kelly calculations when necessary
Applying Kelly to multi-market portfolios
Simultaneous participation across numerous prediction markets requires recalibration of individual Kelly allocations. The aggregate of all Kelly fractions must remain at or below 1.0 (representing 100% of available capital). Practically speaking, maintain cumulative positioning below 50% to preserve liquidity for emerging opportunities and unforeseen market developments.
When Kelly does not apply
The Kelly framework depends on reliable probability estimation. Several circumstances undermine this foundational requirement:
- Scenarios characterised by radical uncertainty (unprecedented circumstances lacking empirical data)
- Interdependent markets (such as presidential election outcomes and legislative chamber control, which exhibit statistical dependence)
- Markets offering no informational superiority relative to prevailing market consensus
Leverage PolyGram's integrated Kelly Criterion calculator to establish appropriate position magnitudes preceding each transaction. The comprehensive risk management suite encompasses payoff visualisations and maximum drawdown metrics. Start trading on PolyGram →