Position Sizing
How much to put into any one holding. Three common approaches, equal weights, caps, and sizing by the loss you could accept, and why being right about a company matters less than how much you bet on it.
The short version
Position sizing is deciding how much of your portfolio goes into each holding. A good idea in an oversized position can still do serious damage if you're wrong. Common approaches include equal weights, a cap on any one position, and working backward from the most you'd accept losing if things go badly. Broad funds can safely be large positions because they're already diversified. Single stocks, and especially speculative ones, generally deserve smaller ones.
By the end of this lesson, you'll be able to
- Define a position and explain why its size matters as much as the choice of investment
- Size a position by working backward from an acceptable loss
- Explain why broad funds and single stocks deserve different limits
Why this matters
Part 4 was about deciding whether an investment is worth owning. Position sizing is the next question: how much? It's easy to overlook, and it often matters more. Two investors can buy the same stock for the same reasons and have completely different outcomes, because one put in 3% of their money and the other put in 40%.
What a position is
A Position. How much you own of a single investment, often described as a share of your whole portfolio. Glossary is how much you own of a single investment, usually described as a share of your whole portfolio. If your portfolio is worth $50,000 and you own $5,000 of STU Corp, that's a 10% position.
The size sets the stakes. If a 10% position halves, your portfolio falls 5%. If a 40% position halves, your portfolio falls 20%, and as Part 5's Drawdowns and the Math of Recovery shows, a 20% loss needs a 25% gain just to get back to even.
Three common approaches
- Equal weights. Split the money evenly: with 20 stocks, each gets 5%. It's simple, and it stops any single idea from dominating, even the one you're most excited about.
- A cap. Set a maximum for any one position, such as 5% or 10% for a single stock, and never buy beyond it. Positions can be smaller, but never bigger.
- Sizing by acceptable loss. Decide the most you'd accept losing from one position if things go badly, then work backward to the size. This links size directly to risk, as the example below shows.
The approaches can be combined. Many investors use a cap as an outer limit and size each position by its risk within it.
Jordan has a $50,000 portfolio. They decide that no single stock should be able to cost the portfolio more than 2%, or $1,000, in a bad outcome. For each stock, they estimate a realistic bad case from their research.
| Holding | Bad-case fall | Largest position | Share of portfolio |
|---|---|---|---|
| A steady, well-understood business | 25% | $1,000 ÷ 0.25 = $4,000 | 8% |
| STU Corp, with its patent about to expire | 40% | $1,000 ÷ 0.40 = $2,500 | 5% |
| A speculative company with no profits | 80% | $1,000 ÷ 0.80 = $1,250 | 2.5% |
The riskier the holding, the smaller the position, so each one puts about the same $1,000 at stake. The bad-case estimates are judgments, and a real loss can be worse than the estimate: a company can fail completely. That's why the result is a maximum, not a target. All figures are hypothetical.
Funds and single stocks are different
A broad index fund already spreads its money across hundreds or thousands of companies, so it can sensibly be a large share of a portfolio, even most of it. A single company can fail. Sizing limits like the ones above are mainly about individual stocks and narrow funds, where one bad outcome can do real damage. Part 5's Concentration vs. Diversification explained why.
Size and confidence
It's natural to want bigger positions in your best ideas, and there's some logic to that: a larger margin of safety, as in Part 4's Margin of Safety and Valuation Ranges, means more room for error. But confidence and accuracy aren't the same thing. People tend to be more certain than their track record justifies. A sensible rule is to let conviction move a position around within your limits, never past them.
Key takeaways
- A position's size sets how much one investment can help or hurt the whole portfolio.
- Common approaches: equal weights, a cap per position, and sizing backward from an acceptable loss.
- Largest position = the most you'd accept losing ÷ the realistic bad-case fall.
- Broad funds can be large positions; single stocks, especially speculative ones, usually shouldn't be.
Check your understanding
Your portfolio is $40,000. You'll accept losing at most $800 from one stock, and you think a realistic bad case is a 40% fall. What's the largest position?
$800 ÷ 0.40 = $2,000, which is 5% of the portfolio. A 40% fall in a $2,000 position costs $800.
Why can a broad index fund reasonably be a much larger share of a portfolio than any single stock?
The fund's diversification removes most single-company risk. It still falls when the whole market falls, which is what asset allocation addresses.
What's next
Once you know how big a position should be, you can decide how to build it: all at once or in steps. Scaling Into a Position shows the step-by-step approach with STU Corp.
Go deeper
- Scaling Into a PositionPart 5: Building a Portfolio
- When One Position Gets Too BigPart 5: Building a Portfolio
- Probability and Expected ValueMath and Statistics for Investors (depth track)