Fixed Fractional vs. Kelly Sizing: Which Position Sizing Method Wins?

Two Approaches to the Same Problem

Every trader faces the same question before each trade: how many shares, contracts, or lots? Fixed fractional and Kelly criterion are the two most common answers, and they come from very different philosophies. Fixed fractional sizing was popularized by Alexander Elder in *Trading for a Living* (Wiley, 1993), while Kelly sizing was adapted for markets by Ed Thorp after its original publication by John L. Kelly Jr. (*Bell System Technical Journal*, 1956).

Fixed Fractional Sizing

Risk the same percentage of your current account on every trade, regardless of the setup quality or confidence level.

Position Size = (Account Balance × Risk%) / Dollar Risk Per Share

Example: $50,000 account, 2% risk, $5 stop distance = 200 shares.

Strengths:

• Simple to calculate and execute
• Automatically reduces size after losses (self-correcting)
• No edge estimation required
• Psychologically consistent

Weaknesses:

• Doesn't adapt to the quality of individual setups
• May be too conservative if your edge is large
• May be too aggressive if your edge is small
• The chosen percentage (1%, 2%, 3%) is often arbitrary

Kelly Criterion Sizing

Risk the mathematically optimal fraction to maximize long-term geometric growth.

f* = (bp - q) / b

Example: 55% win rate, 1.5:1 reward-to-risk. f* = (1.5 × 0.55 - 0.45) / 1.5 = 25%.

Strengths:

• Maximizes long-term account growth (proven by Thorp, 2017)
• Adjusts to your actual edge
• Backed by rigorous mathematical theory

Weaknesses:

• Requires accurate estimates of win rate and payoff ratio
• Full Kelly produces extreme drawdowns (50%+ is common)
• Overestimates optimal size if your edge estimates are wrong by even 5%
• Psychologically difficult to execute

Head-to-Head Comparison

| Factor | Fixed Fractional | Kelly |

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

| Simplicity | High | Medium |

| Growth rate | Sub-optimal | Theoretically optimal |

| Drawdown risk | Controlled by chosen % | High at full Kelly |

| Edge sensitivity | None | Critical — errors amplified |

| Psychological ease | High | Low |

| Error tolerance | Forgiving | Punishing |

The Practical Middle Ground: Fractional Kelly

Most professionals don't use full Kelly. According to Ed Thorp's empirical data, fractional Kelly offers the best risk-adjusted returns:

• Half Kelly (f*/2): Captures ~75% of the growth rate with significantly smaller drawdowns
• Quarter Kelly (f*/4): Much smoother equity curve, approximately 50% of optimal growth

This combines Kelly's intelligence with fixed fractional's stability.

Which Should You Use?

Use fixed fractional if:

• You're early in your trading career
• Your edge estimates are uncertain (fewer than 200 trades of data)
• You value consistency and simplicity
• You trade many uncorrelated setups

Use fractional Kelly if:

• You have 200+ trades of data to estimate your edge
• You're comfortable with the math
• You want to maximize growth without excessive drawdown risk
• You can handle adjusting position size as your stats change

Test Your Sizing Method

Run your trade statistics through the Risk of Ruin Calculator at different risk levels to see the survival probability of each approach. RangeSight lets you model both methods with real market data.

Bottom Line

Fixed fractional is the safer default. Kelly is the smarter target. Fractional Kelly is the practical compromise most traders should aim for once they have reliable trade statistics.