Question

How does the percentage-of-equity model for position sizing prevent a single trade from depleting an entire portfolio&#x27;s capital?

Accepted Answer

The percentage-of-equity model prevents total portfolio depletion by mathematically linking the size of a trade to a fixed portion of the total available capital, ensuring that no single loss can mathematically exhaust the account. In this model, an investor defines a specific percentage of their total equity—the current value of all cash and holdings in their account—that they are willing to risk on a single trade. Before entering a position, the investor determines a stop-loss level, which is a pre-set price point where they will exit the trade to prevent further losses. The position size is then calculated by dividing the dollar amount to be risked by the difference between the entry price and the stop-loss price. For example, if an investor has a 100,000 dollar account and decides to risk 2 percent of that equity, they will risk exactly 2,000 dollars on the trade. If the trade hits the stop-loss, the investor loses only those 2,000 dollars, which represents only 2 percent of the total account. Because the dollar amount at risk is a small, constant fraction of the total equity, even a consecutive string of losing trades will result in a diminishing dollar amount risked per trade rather than a total loss of capital. This creates a mathematical barrier where the portfolio value theoretically approaches zero but never reaches it, as each subsequent loss is calculated against a smaller remaining balance. This approach mandates that the position size decreases as the total equity decreases, providing an automatic mechanism for capital preservation during unfavorable market conditions.

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How does the percentage-of-equity model for position sizing prevent a single trade from depleting an entire portfolio's capital?