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 market odds. Within prediction markets, this approach guards against two critical pitfalls: deploying excessive capital (which threatens total loss) and deploying insufficient capital (which forgoes attainable returns).
Determining how much to stake on each opportunity separates consistently profitable traders from those who exhaust their funds. Introduced by John Kelly, a Bell Labs mathematician, in 1956, the Kelly Criterion is a mathematical framework designed to identify the ideal stake magnitude for achieving sustainable wealth expansion. The following sections explain 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 calculated likelihood of success
- q = likelihood of failure (1 - p)
- b = net odds (payout / stake). For a prediction market share at price c, b = (1 - c) / c
Worked example
Suppose you estimate a 60% probability that an outcome resolves affirmatively. Current market quotation stands at 45 cents (reflecting 45% implied likelihood).
- 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 available funds. If your account holds $1,000, commit $272 to this position.
Why full Kelly is dangerous
The Kelly formula presumes you possess perfect knowledge of your true probability — a condition never satisfied in practice. Miscalculating your informational advantage produces severe overallocation. Experienced market participants consistently adopt fractional Kelly instead:
- Half Kelly (f*/2): Industry standard approach. Surrenders roughly 25% of peak expansion but cuts fluctuation in half
- Quarter Kelly (f*/4): Prudent method when edge estimates carry substantial uncertainty
- Capped Kelly: Establish an upper limit of 5-10% per market regardless of formula recommendations
Applying Kelly to multi-market portfolios
Holding simultaneous stakes across numerous prediction markets demands recalibration of individual Kelly percentages. The aggregate of all Kelly allocations must remain at or beneath 1.0 (your complete capital). Practically speaking, maintain combined positions below 50% to preserve dry powder for emerging opportunities.
When Kelly does not apply
Kelly's framework relies on reliable probability assessment. Several contexts undermine this assumption:
- Situations marked by profound ambiguity (unprecedented circumstances lacking historical data)
- Interdependent markets (a presidential election and legislative composition are not autonomous)
- Markets where consensus pricing already reflects all available information
PolyGram provides an integrated Kelly Criterion calculator to determine position magnitude prior to execution. The analytics suite encompasses scenario analysis and maximum drawdown metrics. Start trading on PolyGram →