Quantitative Methods & RiskPosition Sizing & Capital Allocation

Fractional Kelly & Volatility Targeting

12 min read

How professionals actually size

The two ideas that tame Kelly in practice are fractional Kelly and volatility targeting. Fractional Kelly simply bets a constant fraction — a quarter or a half — of the full Kelly amount, trading a little long-run growth for a large reduction in drawdown and a large margin of safety against an over-estimated edge. Volatility targeting scales position size so the portfolio's risk, not its notional, stays constant: you hold less when markets are turbulent and more when they are calm.

Formula

Volatility targeting

Leverage_t = σ_target / σ_estimate,t

FIXED-FRACTIONAL

Constant % risk per trade

Risk budget is a fixed slice of equity; size falls in drawdowns, rises as you grow. Simple, robust, edge-agnostic. The everyday workhorse for discretionary trading.

0.5%–2% per trade

FRACTIONAL KELLY

A fraction of growth-optimal

Bets ¼–½ of full Kelly. Near-maximal long-run growth with far less drawdown and a buffer against over-estimating the edge. Needs a reliable edge estimate.

¼ to ½ Kelly

VOLATILITY TARGETING

Constant portfolio risk

Scales exposure inversely to current volatility to hold risk steady. De-risks into turbulence without forecasting direction. Standard for systematic and multi-asset books.

e.g. 10% target vol

How three sizing rules behave across regimesfactor exposures

Growth (calm regime)+0.90

Drawdown control+0.80

Edge-estimate sensitivity-0.70

Turbulence de-risking+0.85

Implementation simplicity+0.50

Illustrative profile of volatility targeting (bars) versus simple fixed-fractional sizing (benchmark). Vol targeting de-risks hard into turbulence and is less sensitive to edge mis-estimation, at the cost of more machinery. None is universally best — the right choice depends on how well you trust your edge estimate.

Tip

Combine, don't choose

These rules are not mutually exclusive. A common professional stack is: size each position fixed-fractional, cap total portfolio risk with a volatility target, and never let any single-bet sizing exceed half Kelly even if the edge looks large. Layered constraints catch each other's failure modes — exactly the spirit of the whole course.

Heads up

Vol targeting can lever you into a calm-before-the-storm

Because volatility targeting raises exposure when realized vol is low, it can load you up right before a regime break — the calm that precedes a crash. Always cap the maximum leverage the rule can take, and pair it with the stress tests from Module 3. No single sizing rule removes the need for a hard exposure ceiling.

Knowledge check

Your volatility target is 12% annualized and the current estimated volatility is 24%. What leverage does the vol-targeting rule prescribe?