In this guide
Key takeaway: The Kelly Criterion determines the optimal proportion of your capital to allocate to each wager, accounting for your statistical advantage and available odds. In prediction markets, this approach sidesteps two frequent pitfalls: deploying excessive capital (inviting financial ruin) and deploying insufficient capital (forgoing potential gains).
The ability to correctly size positions separates successful market participants from those who deplete their accounts. Formulated by John Kelly, a researcher at Bell Labs in 1956, the Kelly Criterion is a mathematical framework for determining the ideal stake magnitude to achieve sustainable wealth expansion. The following sections explain its application within prediction markets.
The Kelly formula
For a two-sided prediction market (YES/NO), the Kelly fraction computes as:
f* = (p * b - q) / b
Where:
- f* = proportion of 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 trading at price c, b = (1 - c) / c
Worked example
Suppose you assess a 60% probability that an event concludes YES. The prevailing market quotation 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
According to Kelly, allocate 27.2% of your capital. If your account holds $1,000, this translates to a $272 position in this opportunity.
Why full Kelly is dangerous
The Kelly formula presumes you possess exact knowledge of your true probability — a condition that never materialises in practice. Misjudging your statistical edge produces severe overallocation. Experienced market operators routinely employ fractional Kelly instead:
- Half Kelly (f*/2): The predominant choice among professionals. Surrenders roughly 25% of theoretical maximum returns whilst cutting fluctuations in half
- Quarter Kelly (f*/4): A prudent strategy when confidence in your edge remains limited
- Capped Kelly: Establish a ceiling—typically 5-10% of total capital per individual market—irrespective of what Kelly indicates
Applying Kelly to multi-market portfolios
When you maintain concurrent stakes across numerous prediction markets, individual Kelly allocations require recalibration. The aggregate of all Kelly percentages must remain at or below 1.0 (your entire capital base). Practically speaking, restrict cumulative deployment to 50% of assets, preserving dry powder for emerging opportunities.
When Kelly does not apply
The Kelly framework hinges on reliable probability assessment. Several circumstances undermine this assumption:
- Situations marked by extreme ambiguity (unprecedented scenarios lacking historical data)
- Interdependent markets (such as election winner and legislative majority, which move together)
- Markets where you possess no analytical advantage relative to the broader participant base
Leverage PolyGram's integrated Kelly Criterion calculator to establish position sizes before executing any trade. The platform's analytical suite encompasses payoff visualisations and drawdown metrics. Start trading on PolyGram →