In this guide
Key takeaway: The Kelly Criterion determines the optimal proportion of your capital to deploy on each wager, accounting for your edge and available odds. In prediction markets, it guards against two critical pitfalls: excessive exposure (which threatens total loss) and insufficient exposure (which squanders potential returns).
The distinction between a successful trader and financial ruin often hinges on position sizing discipline. The Kelly Criterion — a mathematical framework conceived by John Kelly, a researcher at Bell Labs in 1956 — offers the theoretically optimal approach to stake sizing for compounding wealth over time. This guide walks through its implementation within prediction market environments.
The Kelly formula
For a binary prediction market (YES/NO), the Kelly fraction is:
f* = (p * b - q) / b
Where:
- f* = proportion 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 affirmatively. 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 deploying 27.2% of your capital. If your account holds $1,000, this translates to a $272 position.
Why full Kelly is dangerous
The Kelly formula presupposes perfect knowledge of your true winning probability — a condition that never materialises in practice. Miscalculating your edge upward results in severe overexposure. Institutional and professional traders consistently employ fractional Kelly instead:
- Half Kelly (f*/2): The industry standard. Surrenders roughly 25% of theoretical maximum growth whilst cutting drawdown volatility in half
- Quarter Kelly (f*/4): A prudent strategy when edge estimates carry substantial uncertainty
- Capped Kelly: Establish a ceiling (typically 5-10% per market) that supersedes Kelly output on individual positions
Applying Kelly to multi-market portfolios
Holding stakes across numerous prediction markets concurrently requires recalibrating individual Kelly allocations. The cumulative Kelly fractions across all active positions must remain at or below 1.0 (your total capital). Practically speaking, maintain aggregate deployment below 50% to preserve dry powder for emerging opportunities and USDC settlement flexibility.
When Kelly does not apply
Kelly's framework depends on reliable probability estimation. Several contexts undermine this assumption:
- Unprecedented events lacking historical data or comparable precedents
- Interconnected markets (such as presidential election and legislative control outcomes that move together)
- Markets where your analysis yields no informational advantage relative to market consensus
Leverage PolyGram's embedded Kelly Criterion calculator to determine stake sizes ahead of execution. The analytics suite encompasses payoff visualisations and historical drawdown metrics. Start trading on PolyGram →