What is the Kelly Criterion?
The Kelly Criterion is a mathematical formula that calculates the optimal fraction of your bankroll to risk on a single trade or bet, given your strategy's expected value and edge. Betting exactly "Full Kelly" maximizes your long-term geometric compound growth rate. However, betting Full Kelly is highly volatile and carries a near-100% chance of experiencing a 50% drawdown over time, which most retail traders are unprepared for.
Because of this volatility, professional traders, quantitative funds, and syndicates often bet Half Kelly or Quarter Kelly. This approach provides a much smoother equity curve, significantly lower drawdowns, and still captures a large percentage of the optimal compound growth. The most critical rule in position sizing is: Never bet more than Full Kelly, as this mathematically guarantees long-term ruin, taking you into negative expected growth territory.
Understanding Sequence Risk and Drawdowns
Even with a 70% win rate, you will inevitably encounter losing streaks. Sequence risk refers to the danger that a cluster of losses occurs early in your trading career, reducing your capital so severely that recovering becomes mathematically improbable. A Monte Carlo simulator models this by shuffling the order of your wins and losses thousands of times.
Once you run the simulation below, look closely at the Max Drawdown distribution and the Risk Analysis card. If your strategy has a 30% chance of seeing a 75% drawdown, your position size is likely too large for your psychology to handle—even if the strategy is mathematically profitable on a spreadsheet. Adjust your "Bet Size (%)" downward and run the simulation again until the drawdowns align with your personal risk tolerance.
Case Study: The 1.5 R:R Strategy
Consider a swing trader with a win rate of 55% and a risk-to-reward ratio of 1:1.5 (winning $1.50 for every $1.00 risked). The calculator below will show that the Full Kelly fraction for this edge is approximately 25%. However, running the Monte Carlo simulation with a 25% bet size will reveal terrifying drawdowns (often exceeding 80%). By reducing the bet size to 5% (a Fractional Kelly approach), the trader maintains excellent long-term capital growth while keeping the median max drawdown under a manageable 20%.