In this guide
Key takeaway: The Kelly Criterion determines the optimal percentage of your capital to allocate to each wager, accounting for your probability edge and available odds. Within prediction markets, this approach sidesteps two prevalent pitfalls: wagering excessively (which invites bankruptcy) and wagering conservatively (which squanders potential gains).
The distinction between a successful market participant and financial ruin often hinges on position sizing discipline. Introduced by John Kelly, a researcher at Bell Labs in 1956, the Kelly Criterion is a mathematical framework for determining the ideal stake magnitude to optimise wealth accumulation over extended periods. Below is guidance on implementing this methodology within 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 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 market is currently quoting 45 cents (reflecting 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
The formula recommends committing 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. Misjudging your informational advantage results in excessive stake sizing and potential ruin. This is why seasoned market participants favour fractional Kelly:
- Half Kelly (f*/2): The industry standard. Trades away roughly 25% of maximum growth for a 50% reduction in portfolio swings
- Quarter Kelly (f*/4): Prudent selection when your edge calculation carries substantial uncertainty
- Capped Kelly: Establish an absolute ceiling (typically 5-10% per market) irrespective of Kelly's recommendation
Applying Kelly to multi-market portfolios
Once you are simultaneously holding stakes across several prediction markets, individual Kelly allocations require recalibration. The aggregate of all Kelly fractions must remain at or below 1.0 (your entire bankroll). Practically speaking, restrict cumulative deployment to 50% or less, preserving dry powder for emerging opportunities.
When Kelly does not apply
The Kelly formula depends on reliable probability estimation. Several contexts undermine this assumption:
- Outcomes shrouded in fundamental uncertainty (unprecedented events without comparable historical data)
- Interconnected markets (such as presidential election and legislative control, which share common drivers)
- Markets where your analysis provides no advantage relative to prevailing consensus pricing
Leverage PolyGram's integrated Kelly Criterion calculator to determine position sizes before executing any trade. The analytics suite encompasses payoff visualisations and maximum drawdown metrics. Start trading on PolyGram →