LOADING
Position sizing determines how much capital to allocate to each individual trade or holding. It is arguably the most important risk management tool available to an investor and is often the difference between a strategy that compounds wealth and one that experiences catastrophic losses. The core principle is to size positions in proportion to the confidence in the trade and inversely proportional to the risk of the position.
Fixed fractional sizing allocates a constant percentage of the portfolio to each position. If you have a 20-position portfolio, each position receives 5% of capital. This approach is simple and ensures no single position can devastate the portfolio. However, it ignores differences in the risk profiles of individual holdings. A 5% position in a low-volatility utility stock represents much less portfolio risk than a 5% position in a high-volatility biotech stock.
Volatility-based position sizing addresses this limitation by scaling position sizes inversely to each stock's volatility. If Stock A has an annualized volatility of 20% and Stock B has a volatility of 40%, Stock A receives twice the allocation. This equalizes the risk contribution of each position. The formula is: Position Size = Target Risk per Position / Stock Volatility. This approach, popularized by trend followers, ensures that every position has roughly the same impact on portfolio returns and that no single volatile holding dominates performance.
The Kelly Criterion, developed by John Kelly at Bell Labs in 1956, provides a mathematically optimal sizing formula for maximizing long-term geometric growth. The Kelly fraction is: f = (bp - q) / b, where b is the odds ratio, p is the win probability, and q is the loss probability. While full Kelly sizing maximizes long-term growth, it results in very large positions and severe drawdowns. Most practitioners use half-Kelly or quarter-Kelly to reduce the volatility of the portfolio's equity curve at the cost of slightly lower expected growth.
Risk budgeting allocates a total portfolio risk budget (say, 10% annualized volatility) across positions, strategies, or asset classes. Equal risk contribution (ERC) portfolios ensure that each holding contributes equally to total portfolio risk, accounting for both individual volatilities and correlations. Risk parity, popularized by Bridgewater's All Weather fund, applies this concept across asset classes, typically resulting in higher allocations to bonds (which are less volatile) and lower allocations to equities than traditional portfolios.
Regardless of the method chosen, position limits are a critical safeguard. Maximum position sizes (typically 2-10% of portfolio) prevent catastrophic losses from any single holding. Sector and factor exposure limits prevent unintended concentration. These constraints should be set in advance and enforced mechanically, not adjusted in response to emotions or market conditions.