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Position Sizing: The Unsung Hero of Portfolio Management

Updated: Mar 22

When embarking on the journey of investment, a lot of emphasis is usually placed on picking the right assets, the perfect timing, and the most promising market trends. However, one crucial aspect often overlooked by novice investors is position sizing. This tool can significantly improve the risk management strategy of your investment portfolio. Understanding and applying this concept is integral for successful investing.

What is Position Sizing?

Position sizing is the process of deciding the amount of capital you're willing to risk in a single trade or investment. It's about determining how many shares, lots, or units of a given investment you should buy given the size of your portfolio, your risk tolerance, and the particular risk parameters of the investment itself. The goal of position sizing is to limit your risk on any single trade to a small percentage of your total investment capital. This allows you to absorb potential losses without severely damaging your portfolio, which is especially important in volatile and uncertain markets.

Why is Position Sizing Important?

Position sizing can help in the following ways:

  • Preventing significant losses: No matter how confident you are about a particular investment, there's always a risk of loss. Position sizing ensures that even if the investment doesn't pan out as expected, you won't lose a significant portion of your portfolio.

  • Portfolio longevity: By risking a small portion of your portfolio on any single investment, you ensure that you'll be able to continue investing even after inevitable setbacks.

  • Emotional control: By mitigating the risk of major losses, position sizing helps keep emotional decision-making in check. Fear and greed can be damaging in investment, and knowing you've limited your downside can keep these emotions at bay.

Position Sizing Strategies

Various strategies can be used to determine position sizing:

  • Percentage of Portfolio: This straightforward method involves deciding to risk a fixed percentage of your total portfolio on each trade. For example, you might decide to risk 1% of your portfolio on each trade. Consider you have a portfolio of $100,000 and you decide to follow the 1% rule. This means that you can risk $1,000 on each trade. If you're looking at a stock that's trading at $50 per share and you decide to place your stop loss at $45 (10% below your purchase price), you can buy 200 shares. If the stock falls to your stop loss, you'd lose $1,000, which is 1% of your portfolio.

  • Dollar Amount: Similar to the percentage method, this strategy involves risking a predetermined dollar amount on each trade. If you choose to risk $1,000 on each trade, the amount of your portfolio this represents will fluctuate based on the total value of your portfolio.

  • The Kelly Criterion: The Kelly Criterion is a mathematical formula that calculates the optimal position size based on the probability of winning and the potential payoff. The formula is: Kelly % = W – [(1 – W) / R] Where: W is the probability of a winning trade, R is the win/loss ratio. This formula might be best used by advanced traders with a strong understanding of probabilities and a clear history of their trading performance.

  • Volatility-Based: This position sizing strategy uses the volatility of the asset to determine the investment size. For example, an investor could use the Average True Range (ATR), a common measure of market volatility. If an asset has high volatility, the position size would be smaller compared to an asset with low volatility, assuming equal risk tolerance for both.

Position Sizing in Practice

Let's illustrate with an example: Suppose you have a portfolio of $200,000. You have identified a promising stock to invest in, and after thorough analysis, you're confident in the potential upside. For risk management, you decide to employ the Percentage of Portfolio strategy, with a maximum risk per trade of 2% of your total portfolio. This means you're willing to risk $4,000 (2% of $200,000) on this trade. You decide to set your stop loss at 10% below the purchase price. To determine how many shares you can purchase without exceeding your risk tolerance, you would divide your risk dollar amount by the per share risk. Let's say the stock is currently trading at $50 per share. With a stop loss set 10% below the purchase price, your risk per share is $5 ($50 * 10%). Therefore, you can buy 800 shares ($4,000 / $5 per share). If the stock's price falls to your stop loss, you would lose $4,000 (800 shares * $5 loss per share), which is exactly 2% of your portfolio. This ensures that even if the trade doesn't work out as expected, your portfolio won't be severely damaged. However, if the stock's price rises, as you anticipated, you stand to make a significant gain with your 800 shares.

Position sizing is a crucial, yet often overlooked aspect of successful investing. By determining beforehand how much of your portfolio to risk on each trade, you can manage potential losses, increase portfolio longevity, and reduce emotional decision-making. Whether you're a novice investor just starting out or a seasoned trader looking to refine your strategies, consider employing position sizing techniques in your investment strategy. Remember, successful investing isn't just about the gains—it's also about managing and mitigating the inevitable losses. Position sizing can be a powerful tool in your investment toolkit to help achieve this balance.


Interestingly, the concept of position sizing is rooted in the ancient art of money management, dating back to 16th-century Italy. Back then, a mathematician named Luca Pacioli introduced the concept of the 'Divine Proportion' or the 'Golden Ratio', which later became a foundational principle of risk management in gambling and trading. Pacioli's principle, essentially, emphasized not placing all your bets on a single outcome, a concept we now understand as diversification in investment terms. He suggested dividing one's resources in a way that not all would be lost in a single unfavorable event. This rudimentary concept evolved over the centuries and forms the bedrock of position sizing strategies in modern portfolio management. Even though the term 'position sizing' might not have been used explicitly in Pacioli's time, the basic premise remains the same – don't risk so much on one outcome that it threatens your ability to continue playing the game. It's an old concept but one that remains as relevant today in the world of investing as it was in the 16th-century games of chance. Position sizing, thus, serves as a powerful reminder of the adage "Don't put all your eggs in one basket". It ensures that a negative outcome in one investment does not jeopardize the entire portfolio. Remember, the tortoise won the race not by speed but by maintaining a steady pace without risking it all in a reckless sprint. As an investor, it might be helpful to embrace your inner tortoise, using position sizing as your protective shell.

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