What Is the Kelly Criterion and Should I Use It for Position Sizing?

1. The Kelly formula explained conceptually

The Kelly Criterion, originally developed for gambling and investment contexts, calculates the theoretically optimal fraction of capital to risk on a given bet or trade, based specifically on your genuine probability of winning and the payout odds involved, aiming to maximise long-term capital growth mathematically given these specific inputs.

It's worth understanding this as a mathematically optimal formula under specific, idealised conditions, discussed further below, the theoretical elegance of Kelly doesn't automatically translate into practical suitability for genuine, real-world trading with imperfect information.

South African traders should approach this aspect of trading with the same systematic discipline they apply to their entry and exit rules. Maintaining written records, reviewing outcomes periodically, and adjusting approach based on evidence rather than gut feeling produces better long-term results than relying on informal methods. The structured approach that separates consistently profitable traders from the majority is not about exceptional market insight but about consistently applying a sound framework to every decision.

2. Why full Kelly is rarely used in practice

While mathematically optimal for long-term growth under its specific assumptions, full Kelly position sizing tends to produce extremely volatile, large swings in account value, including potentially severe short-term drawdowns, that most traders find genuinely uncomfortable and difficult to sustain psychologically, even if mathematically justified.

It's worth understanding why this theoretical optimum proves genuinely impractical, full Kelly sizing produces such significant volatility and drawdown, discussed elsewhere on this site regarding drawdown generally, that few traders could genuinely tolerate living through it even if their inputs were perfectly accurate.

3. The fractional Kelly approach most traders prefer

Many traders and investors who do incorporate Kelly thinking use a fraction of the full calculated figure, commonly a quarter or half of the mathematically optimal amount, deliberately sacrificing some theoretical long-term growth in exchange for meaningfully reduced volatilityVolatility measures how much and how quickly an instrument's price fluctuates.Click to read more → and drawdown, a trade-off most find considerably more sustainable in practice.

It's worth testing a few different fractional levels through backtesting if you're genuinely interested in this approach, discussed elsewhere on this site regarding backtesting generally, finding your own appropriate fraction requires understanding your personal tolerance for the resulting volatility.

4. The genuine input accuracy problem

The Kelly formula's output is only as reliable as your input win probability and odds figures, which themselves require a sufficiently large, representative sample to verify genuine edge. Applying Kelly sizing based on inaccurate or insufficiently sampled inputs can produce a genuinely poor, miscalibrated result despite the formula's mathematical elegance.

It's worth appreciating why this is Kelly's most significant practical limitation, the formula requires precisely accurate win rate and payoff estimates, discussed elsewhere on this site regarding sample size requirements, inputs that are genuinely difficult to know with confidence for most retail trading strategies.

5. How this compares to the simpler percentage rules

The simpler fixed-percentage approach most beginners and many experienced traders use, like the 2% rule, avoids Kelly's input-accuracy sensitivity entirely, providing a more sound, if theoretically less precisely optimised, approach that doesn't depend on confidently knowing your exact win probability and odds in advance.

It's worth appreciating why the simpler fixed-percentage approach, discussed elsewhere on this site regarding the 2% rule, remains more practical for most retail traders, it doesn't require the precise statistical inputs Kelly demands, making it more sound to genuine estimation uncertainty.

6. A practical recommendation for most retail traders

For most retail traders, particularly those still developing genuinely validated, large-sample strategy statistics, the simpler percentage-based approach generally provides a more practical, reliable starting point than Kelly sizing, which becomes more genuinely useful once you have a longer, more confidently validated track record supporting accurate input figures.

The mathematics of recovery from drawdown is fundamental knowledge for any trader managing risk. A 10% drawdown requires an 11% gain to recover. A 25% drawdown requires a 33% gain. A 50% drawdown requires a 100% gain. A 75% drawdown requires a 300% gain to return to the starting equity level. This asymmetric relationship between losses and recovery is why controlling drawdown is mathematically more valuable than maximising returns. A trader who generates consistent 20% annual returns without a drawdown exceeding 15% will outperform a trader generating 40% returns but periodically experiencing 50% drawdowns, not just on a risk-adjusted basis but in absolute capital terms over a multi-year compounding period. Building a trading system around drawdown control as the primary objective, with returns as the secondary outcome, reflects the true mathematics of capital growth correctly.

The mathematics of recovery from drawdown is fundamental knowledge for any trader managing risk. A 10% drawdown requires an 11% gain to recover. A 25% drawdown requires a 33% gain. A 50% drawdown requires a 100% gain. A 75% drawdown requires a 300% gain to return to the starting equity level. This asymmetric relationship between losses and recovery is why controlling drawdown is mathematically more valuable than maximising returns. A trader who generates consistent 20% annual returns without a drawdown exceeding 15% will outperform a trader generating 40% returns but periodically experiencing 50% drawdowns, not just on a risk-adjusted basis but in absolute capital terms over a multi-year compounding period. Building a trading system around drawdown control as the primary objective, with returns as the secondary outcome, reflects the true mathematics of capital growth correctly.