In this guide
Key takeaway: The Kelly Criterion determines the optimal percentage of your capital to deploy based on your informational advantage and available odds. For prediction market traders, it solves two critical problems: excessive position sizing that threatens account survival and undersizing that sacrifices achievable returns.
Stake allocation separates consistently profitable operators from those facing depletion. The Kelly Criterion — a mathematical framework conceived by John Kelly, a researcher at Bell Labs in 1956 — establishes the theoretically ideal wager magnitude for compound wealth expansion. Below is its practical implementation in prediction markets.
The Kelly formula
For a two-outcome prediction market (YES/NO), the Kelly fraction becomes:
f* = (p * b - q) / b
Where:
- f* = percentage of total capital to allocate
- p = your assessed likelihood of success
- q = likelihood of failure (1 - p)
- b = net odds (return / investment). For a prediction market contract trading at price c, b = (1 - c) / c
Worked example
Suppose you assess a 60% probability that an outcome settles affirmatively. Current market valuation sits at 45 cents (reflecting 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 formula recommends deploying 27.2% of available funds. If your account holds $1,000, allocate $272 to this position.
Why full Kelly is dangerous
The Kelly formula presumes certainty about your true edge — a condition rarely satisfied in practice. Miscalculating your actual advantage creates severe overexposure. Institutional market participants consistently adopt fractional Kelly approaches:
- Half Kelly (f*/2): Industry standard. Surrenders roughly 25% of theoretical growth whilst halving portfolio swings
- Quarter Kelly (f*/4): Prudent methodology when edge assessment carries substantial uncertainty
- Capped Kelly: Establish a ceiling—typically 5-10% per market—irrespective of Kelly calculations
Applying Kelly to multi-market portfolios
Operating across numerous prediction markets concurrently demands recalibration of individual Kelly percentages. Aggregate exposure from all positions must remain at or below 1.0 (representing 100% of capital). Practically, maintain cumulative risk beneath 50% to preserve dry powder for emerging opportunities.
When Kelly does not apply
Kelly presupposes reliable estimation of your genuine edge. Multiple contexts undermine this assumption:
- Unprecedented or highly novel events lacking comparable historical data
- Interconnected markets (such as primary election and legislative control outcomes)
- Situations where your analysis provides no meaningful advantage relative to market consensus
PolyGram furnishes an integrated Kelly Criterion calculator for position sizing prior to execution. The analytics suite encompasses scenario payoff charts and volatility tracking. Start trading on PolyGram →