In this guide
Key takeaway: The Kelly Criterion determines the optimal percentage of your capital to allocate to each wager, accounting for your statistical advantage and available odds. Within prediction markets, this approach guards against the two most damaging errors: deploying excessive capital (risking total loss) and deploying insufficient capital (sacrificing potential returns).
The ability to correctly size positions separates consistently profitable traders from those who deplete their accounts. The Kelly Criterion — a mathematical framework created by John Kelly, a researcher at Bell Labs in 1956 — establishes the theoretically ideal stake magnitude for optimising wealth accumulation over extended periods. Below is guidance on implementing this approach within prediction markets.
The Kelly formula
For a two-sided 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 (reflecting an implied 45% 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
According to Kelly, commit 27.2% of your capital. If your account contains $1,000, this translates to a $272 position in this opportunity.
Why full Kelly is dangerous
The Kelly formula rests on the assumption that you possess precise knowledge of your true probability — an assumption that rarely holds in practice. Miscalculating your informational advantage results in severe overcommitment. Experienced market participants consistently adopt fractional Kelly instead:
- Half Kelly (f*/2): The predominant choice among professionals. Forgoes roughly 25% of maximum growth but halves volatility exposure
- Quarter Kelly (f*/4): A defensive strategy suitable when edge estimates carry substantial uncertainty
- Capped Kelly: Establish a ceiling (typically 5-10% of total capital) for any single market position, overriding Kelly calculations when necessary
Applying Kelly to multi-market portfolios
Operating across several prediction markets concurrently requires recalibrating individual Kelly percentages. The aggregate of all Kelly allocations must remain at or below 1.0 (your entire bankroll). Practically speaking, restrict cumulative market exposure to 50% of capital, preserving dry powder for emerging opportunities.
When Kelly does not apply
The Kelly framework depends on reliable probability estimation. Several circumstances undermine this requirement:
- Situations characterised by extreme uncertainty (unprecedented events lacking comparable historical data)
- Interconnected markets (such as presidential outcome and legislative control, which are statistically dependent)
- Markets where your analysis provides no meaningful advantage relative to prevailing market consensus
Leverage PolyGram's integrated Kelly Criterion calculator to establish appropriate stake sizes ahead of each transaction. The analytical suite encompasses payoff visualisations and volatility metrics. Start trading on PolyGram →