Position Sizing: How Much to Invest in Each Stock (and Why It Matters)

Position sizing is deciding how much of your portfolio to put into each stock. Done well, it lets your best research make a real difference to your returns while making sure that no single mistake can do lasting damage. For most stock pickers, that means starting positions of roughly 3% to 10%, larger for the businesses you understand best and buy most cheaply, and never so large that being completely wrong would knock you out of the game.

Where this fits: position sizing is part of the risk and portfolio section of Gingernomics, which starts with what is investment risk. It is the portfolio step that follows the Five Criteria: the criteria decide what to buy, sizing decides how much.

What Is Position Sizing?

Position sizing is the percentage of your portfolio you allocate to a single holding. If your portfolio is worth $50,000 and you invest $4,000 in one company, that position is 8%.

Most investors spend nearly all their effort deciding what to buy and almost none deciding how much. That is a mistake. Two investors can own exactly the same stocks and get very different results because they sized them differently.

Why Position Sizing Is the Ultimate Edge

Position sizing is the one lever that turns the quality of your research into portfolio results, in both directions.

It decides how much your best ideas matter

Suppose one of your stocks rises tenfold. If it was a 2% position, it adds 18% to your portfolio. If it was an 8% position, it adds 72%. The insight was the same. The sizing decided how much it was worth to you.

If your best ideas are sized the same as your weakest, your research barely shows up in your results.

It decides how much your mistakes cost

Every investor is wrong sometimes, including the very best. Sizing decides whether a mistake is a bruise or a disaster.

Position size	Stock falls 50%	Stock goes to zero
3%	-1.5% of portfolio	-3% of portfolio
8%	-4% of portfolio	-8% of portfolio
20%	-10% of portfolio	-20% of portfolio
40%	-20% of portfolio	-40% of portfolio

The maths of recovery

Losses and gains are not symmetrical. A 20% portfolio loss needs a 25% gain to recover. A 40% loss needs about 67%. A 50% loss needs 100%. Large losses are disproportionately hard to recover from, which is why avoiding them matters more than squeezing out extra return. Our article on survival first explores this in detail.

What the Great Investors Teach About Sizing

Buffett and concentration

In his early partnership years, Warren Buffett was willing to make very large bets when he was confident. His investment in American Express after the 1963 “salad oil” scandal is widely reported to have become a very large share of the partnership’s assets, often cited as around 40%. He understood the business deeply and judged the damage to be temporary.

That is a lesson in conviction, not a template. Buffett had exceptional skill, deep knowledge of the business and full control of his partnership. Very few individual investors should size a single position anywhere near that level.

Pabrai: heads I win, tails I don’t lose much

Mohnish Pabrai’s book The Dhandho Investor (2007) popularised the phrase “heads I win; tails I don’t lose much.” The idea is to look for bets where the downside is limited and the upside is large. Those are the situations that deserve larger positions, because the price of being wrong is small.

The Kelly Criterion

In 1956, John Kelly published a formula for the bet size that maximises long-run growth, given your edge and the odds. The more favourable the odds and the more certain the outcome, the larger the bet.

In investing, you never know the true odds, and overestimating your edge with full Kelly sizing can be ruinous. That is why practitioners who use it typically bet only a fraction of what the formula suggests. The useful lesson is directional: better odds and more certainty justify bigger positions; uncertainty demands smaller ones.

A Practical Position Sizing Framework

Here is a simple approach that fits a portfolio of 10 to 20 stocks. The percentages are starting positions at the time you buy.

Conviction tier	What qualifies	Starting position
High	Passes all five criteria clearly; well inside your circle of competence; margin of safety of 30% or more	8% to 10%
Standard	Passes all five criteria; good understanding; margin of safety around 25%	5% to 7%
Starter	Passes the criteria but you are still learning the business or industry	2% to 4%

Three rules sit on top of the tiers:

1. Only businesses that pass all five criteria get a position at all. Sizing is not a way to take small bets on weak ideas.
2. No new position above 10%. It keeps any single mistake survivable.
3. Review any holding that grows above roughly 20% of the portfolio. You do not have to sell a winner, but you should re-check the thesis and valuation, and consider trimming if the price has moved far above intrinsic value.

The sleep test

Before sizing a position, ask: if this stock fell 50% next month for reasons I did not see coming, could I hold calmly and reread my thesis? If the honest answer is no, the position is too big. Cut it until the answer is yes.

Worked Example: Sizing Three Ideas

A hypothetical investor has a $100,000 stock portfolio and three new ideas that all pass the Five Criteria.

• Company A is a business the investor has followed for five years. It trades 35% below a conservative intrinsic value, with a free cash flow yield of 8%. It goes in the high tier at 9%, or $9,000.
• Company B is a solid business in an industry the investor knows well, trading 25% below intrinsic value. It gets a standard position of 6%, or $6,000.
• Company C clears the criteria, but the investor has only studied the industry for a few months. It gets a starter position of 3%, or $3,000.

Now suppose the investor is badly wrong about all three and each falls 50%. The total damage is 4.5% plus 3% plus 1.5%, or 9% of the portfolio. That hurts, but it is recoverable. Now suppose Company A is instead the big winner and doubles. It adds 9% to the whole portfolio on its own.

The sizing lets the best-understood idea matter most, while making even a run of mistakes survivable.

Adding to, Trimming and Averaging Down

Position sizes change after you buy, as prices move and you learn more.

• Adding. It is reasonable to build a position over time as your understanding deepens, moving a starter position up a tier.
• Averaging down. Buying more after a fall is only sensible if the business is unchanged and the new price offers a larger margin of safety. Always reread your thesis first. Never average down just to lower your average cost.
• Trimming. Consider it when a position has grown far beyond your comfort level, or when the price has run well above a conservative value. Remember that great businesses often keep compounding, and that selling triggers taxes and costs.

How We Use This in the Five Criteria

The Gingernomics philosophy has two portfolio rules: concentrate only in businesses that pass all five criteria, and size positions to survive being wrong. Position sizing puts both into practice.

• The Five Criteria decide eligibility. A stock that fails any criterion gets no position, however attractive the story.
• The margin of safety informs size. A business trading 25% below a conservative intrinsic value, or with a free cash flow yield of 5% or more, passes. A larger discount on a business you understand deeply can justify a larger position. See margin of safety explained.
• Portfolio breadth sets the scale. For most stock pickers, 10 to 20 positions with starting sizes of about 3% to 10% keeps concentration meaningful without being dangerous. Our guide to how many stocks to own explains the reasoning.

Having worked in investor relations, I know how complete a company’s story can sound from the inside. Even well-run businesses face surprises that nobody predicted. Sensible sizing is how you respect that uncertainty.

Common Position Sizing Mistakes

• Equal weighting everything by default. Your best idea and your most uncertain idea should rarely get the same weight.
• Sizing by excitement. Conviction should come from evidence and margin of safety, not from how thrilling the story feels.
• Letting one stock dominate by accident. A winner can quietly grow into a dangerous share of your portfolio. Review it.
• Copying famous investors’ sizing. Their skill, information and circumstances are not yours.
• Averaging down without re-checking the thesis. A falling price can mean the market has seen something you have missed.
• Ignoring correlated positions. Three 8% positions exposed to the same risk behave like one 24% position.

Your Next Step

• Decide how many holdings to aim for in how many stocks should I own in my portfolio.
• Understand what you are protecting against in what is investment risk.
• Track your ideas and target prices on a stock watchlist.

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About the author: Cameron Hayes is a senior investor relations and finance professional who has worked in IR at Nasdaq Copenhagen-listed companies including Nilfisk and Maersk Drilling. He holds an MSc from Copenhagen Business School and a BCom from the University of Ottawa, and founded Gingernomics to teach individual investors the Five Criteria framework. More about Gingernomics.

The content on Gingernomics is for educational and informational purposes only and does not constitute financial advice. Always do your own research and consult a licensed financial advisor before making any investment decisions. Past performance is not indicative of future results.