Position Sizing: How Much to Put Into One Investment

Position sizing is the decision about how much of your portfolio goes into one investment, one theme or one asset class. It matters as much as the choice of what to own, because the size of a position decides what a mistake can cost. A great idea held too large can wreck a portfolio. A bad idea held small is only a lesson.

Why the "how much" question matters

Most investors spend hours researching what to buy and a few seconds deciding how much. That order is backwards. The quality of an idea is uncertain. The size is the one thing that is fully under your control, and it sets the upper limit on the damage any single decision can do.

Two investors can own exactly the same stock and have completely different experiences. For one it is a small line in a broad portfolio. For the other it is half of everything they own. When that stock falls, only the second investor faces a crisis.

Even the biggest winners fell almost to zero

It surprises many people that some of the best long-term investments of the last thirty years went through brutal falls along the way. Measured peak to trough on weekly closes (total return, past performance is not a guide to the future):

• Amazon, 1999–2001: −94%, and afterwards one of the best stocks in history.
• Nvidia, 2002: −88%, then −83% again in 2007–08.
• Bitcoin, 2017–18: −82%, and −74% again in 2021–22.

A drawdown of this kind is hard to sit through at any size. In a large position it becomes almost impossible. An investor who had a big share of their wealth in Amazon in 2001 would most likely have sold long before the recovery. The investors who benefited were often those whose position was small enough to ignore the pain.

The arithmetic of one position falling 90%

Now take one holding that falls 90%. The idea is the same in every row; only the size changes:

At 2% the loss is annoying but not dangerous. At 25% more than a fifth of the portfolio is gone. At 50% almost half of everything disappears because of one decision. On a $100,000 portfolio that is the difference between losing $1,800 and losing $45,000.

Position sizing on three levels

Good sizing does not stop at single holdings. It works on three levels, and each one guards against a different kind of mistake:

• The single position: one stock, one fund, one coin.
• The theme: a sector, an industry or one country. Ten different tech stocks are not ten bets. They are one bet on technology, and they tend to fall together.
• The asset class: how much sits in stocks, bonds, gold, cash or crypto.

A limit on each level stops one mistake, or one fashionable trend, from taking over the whole portfolio. Many people who believe they are diversified own a dozen holdings that all depend on the same story. Real diversification shows up only when you look at themes and asset classes too.

Size by risk, not by conviction

The second principle is to size by risk rather than by how much you like an idea. Some assets swing far more than others. A 10% position in an asset with 70% annual volatility, a level Bitcoin has shown, carries more risk than a 30% position in a broad stock index with 15% volatility. As a rough comparison, 10% × 70% is 7 points of swing, while 30% × 15% is 4.5 points.

The idea is that each holding should contribute a fair share of the portfolio's risk. Assets that swing more get less money.

Conviction is a poor guide here. Every experienced investor has had a "sure thing" that failed. Conviction feels like information, but it is a feeling. The investors who last are not the ones who are always right. They are the ones whose mistakes are small enough to survive, and whose winners get time to grow.

Among systematic investors, the limits tend to follow a consistent pattern (examples, not advice):

• Single stocks: a few percent of the portfolio at most.
• One theme or sector: a clearly capped share.
• Highly volatile assets such as crypto: small.
• Broad, diversified index funds: allowed to be large.

Just as important is when these limits are set. They are written down in calm times, so they are not renegotiated in the middle of a boom or a panic.

What to do with this

• Write down three numbers: a maximum share of your portfolio for one single position, for one theme or sector, and for each asset class.
• Compare your current portfolio against these limits and note every holding or cluster that is above them.
• Group holdings by theme: list which positions would likely fall together and add up their combined weight.
• Note the most volatile holdings and consider whether their size reflects their swings or only your enthusiasm.
• Keep the list somewhere visible and review it in calm markets, not during sharp moves.

Nothing has to change today. The goal is to know where the biggest risk in your portfolio sits.

This article is based on Lesson 3.4 of our course Mastering Investments, where Module 1 is free.

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