Risk of Ruin vs. Kelly Criterion: Which Should Guide Your Position Sizing?

Two Sides of Position Sizing

Risk of ruin and the Kelly criterion both address how much capital to risk per trade, but they answer fundamentally different questions. Risk of ruin calculates the probability that a trader's account will decline below a usable threshold, while Kelly calculates the bet size that maximizes long-term geometric growth rate. As Ed Thorp demonstrated in *A Man for All Markets* (Random House, 2017), the practical solution is to use both: Kelly sets the ceiling, and risk of ruin sets the floor.

Risk of ruin asks: "What is the probability that I lose enough capital to stop trading?"

Kelly criterion asks: "What bet size maximizes my long-term growth rate?"

These are not competing frameworks. They're complementary lenses on the same problem.

Risk of Ruin: The Survival Question

Risk of ruin focuses on the downside. Given your win rate, risk-reward ratio, and position size, it calculates the probability of hitting a drawdown threshold that ends your trading.

Key characteristics:

• Outputs a probability (e.g., 8% chance of ruin)
• Conservative by nature
• Accounts for worst-case sequences
• Tells you the maximum position size you can use without unacceptable ruin risk

Kelly Criterion: The Growth Question

Kelly focuses on the upside. First published by John L. Kelly Jr. in the *Bell System Technical Journal* (1956), it calculates the position size that maximizes geometric growth.

f* = (bp - q) / b

Key characteristics:

• Outputs an exact fraction of capital to risk
• Aggressive by nature — full Kelly produces drawdowns of 50%+ regularly
• Assumes perfect knowledge of your edge
• Tells you the theoretical optimum, not necessarily the practical one

Where They Diverge

| Factor | Risk of Ruin | Kelly Criterion |

|--------|-------------|-----------------|

| Primary concern | Survival | Growth |

| Output | Probability | Position size |

| Risk tolerance | You define the threshold | Implicit (accepts large drawdowns) |

| Practical use | "Am I safe at this size?" | "What size grows fastest?" |

| Drawdown consideration | Central | Secondary |

| Edge estimate sensitivity | Moderate | Critical — errors are amplified |

The Practical Approach

Most professional traders combine both:

1. Use Kelly (or fractional Kelly) to determine a starting position size

2. Run Monte Carlo simulation at that size to check the ruin probability

3. If ruin probability exceeds 5–10%, reduce to Half Kelly or Quarter Kelly

4. Verify that the adjusted size still preserves a meaningful growth rate

Half Kelly captures approximately 75% of optimal growth while reducing maximum drawdown by roughly 50%, according to Thorp's empirical data from managing Princeton Newport Partners.

The Risk of Ruin Calculator lets you test survival probability at different risk levels. RangeSight runs full Monte Carlo simulation so you can see both the growth and ruin distributions.

Bottom Line

Use Kelly to find where growth is maximized. Use risk of ruin to make sure you survive long enough to get there. Neither tool alone gives you the full picture.