Kelly Criterion
Overview
The Kelly Criterion is a mathematical formula for determining the optimal bet size to maximise long-term growth rate. Kelly = (bp - q) / b, where b = odds received, p = probability of winning, q = probability of losing. While theoretically optimal, full Kelly is too aggressive for most traders — half-Kelly or quarter-Kelly is more practical.
Key Concepts
Kelly % = (Win% × Average Win/Average Loss - Loss%) / (Average Win/Average Loss). Full Kelly maximises geometric growth but with extreme variance. Half-Kelly: 75% of full Kelly's growth with significantly less variance. Requires accurate estimation of win rate and payoff ratio. Overestimating edge leads to ruin with Kelly.
Entry Signals
Calculate the Kelly percentage from your historical trade data. Apply half-Kelly for practical use. Example: 55% win rate, 1:1.5 payoff ratio → Kelly = (0.55 × 1.5 - 0.45) / 1.5 = 25%. Half-Kelly = 12.5%.
Exit Signals
If your measured edge (win rate, payoff) decreases, Kelly size decreases. During drawdowns, reduce to quarter-Kelly. Review and recalculate monthly with updated statistics.
Best Timeframes
Applies to overall position sizing strategy, not individual trade management
Pro Tips
Kelly Criterion is theoretically elegant but dangerous with imperfect data. Most traders don't have enough trade history for reliable win rate and payoff ratio estimates. Use Kelly as a ceiling, not a floor.
More Topics in This Category
Hedging Fundamentals
Hedging is the practice of taking offsetting positions to reduce exposure to adverse price movements in your primary holdings. Rather than closing a profitable position or accepting full downside risk, hedging allows traders to protect capital during uncertain periods while maintaining their core exposure. Effective hedging balances protection cost against the risk being mitigated.
Risk-Reward Ratios
The risk-reward ratio (R:R or RRR) compares the potential loss (distance to stop loss) to the potential gain (distance to target) for each trade. A 1:2 R:R means you risk $1 to potentially make $2. By maintaining favourable risk-reward ratios, a trader can be profitable even with a win rate below 50%.
Trailing Stop Strategies
Trailing stop strategies dynamically adjust your stop-loss level as a trade moves in your favour, locking in progressively more profit while still giving the trade room to develop. Unlike fixed stops, trailing stops adapt to market volatility and price action, allowing traders to capture the majority of a trend move without exiting prematurely on normal pullbacks.
Risk of Ruin Modeling
Risk of ruin calculates the probability that a trader will lose a specified percentage of their account — typically enough to end their trading career — given their win rate, average reward-to-risk ratio, and percentage risked per trade. This mathematical framework quantifies whether a trading strategy is survivable over the long run and helps traders set appropriate risk limits to ensure longevity.