Position Sizing & Leverage: Kelly Criterion Strategy › Position Sizing and Leverage Management

A trading strategy with a positive expected return can still lead to ruin if positions are sized incorrectly. Conversely, a mediocre strategy with excellent position sizing can outperform a superior strategy with poor sizing over time. Position sizing, determining how much capital to allocate to each trade, is one of the most underappreciated aspects of quantitative trading, yet it fundamentally determines whether a strategy compounds wealth or destroys it.

In earlier chapters, we developed strategies for mean reversion, momentum, factor investing, and other approaches. We learned to backtest these strategies and measure their performance. But we largely sidestepped one question: given a strategy with a positive expected return, how much should you bet? The answer isn't "as much as possible." Aggressive betting increases variance and can lead to catastrophic drawdowns from which recovery becomes mathematically improbable. If position sizing is too conservative, you leave substantial returns on the table, failing to adequately utilize your edge.

This chapter addresses the mathematics and practice of optimal position sizing. We begin with the Kelly Criterion, a foundational result from information theory that provides a principled answer to optimal bet sizing. We then extend to multi-strategy portfolios through risk budgeting and capital allocation frameworks. Finally, we examine the practical constraints of leverage limits, margin requirements, and the sobering lessons from funds that have blown up due to excessive leverage.