Pro Investor Academy
LESSON 5.4
6 min learning time

Position Sizing

Concentration happens quietly, usually through success rather than through a decision.

The short answer

Concentration risk is exposure to a single company, sector or theme large enough that its outcome dominates your own, and it usually builds up quietly as a successful holding grows rather than through any single deliberate decision. Checking position size and true underlying exposure periodically is how it stays a choice rather than an accident.

Learning objectives

  • Explain how concentration accumulates without a new purchase
  • Identify hidden concentration created by overlapping fund holdings
  • Describe why employer stock is a distinct concentration risk

How it accumulates

A holding that performs well becomes a larger share of the total. Nothing was purchased, but the risk profile changed. Employer shares and a single favourite company are the usual culprits.

Hidden concentration

Broad funds are weighted by size, so a handful of very large companies can account for a substantial share of a supposedly diversified fund. Adding those same companies individually multiplies the exposure.

The employer case

Holding a large position in your employer ties your salary, your career and your savings to the same single outcome. If the company struggles, income and savings can be affected at the same time, which is the opposite of what diversification is meant to achieve.

Position sizing as the practical fix

Position sizing means deciding, in advance, how large any single holding is allowed to become relative to the whole portfolio. It is usually more consequential to a long-term outcome than which specific company was chosen, because a large enough position size turns even a good company's normal volatility into an outsized swing in total wealth.

Key concepts

Select a term to read its definition.

  • Concentration risk

    Risk created when too much of your outcome depends on one holding, sector or theme.

    View in the glossary
  • Position sizing

    Deciding how much to hold in one investment. It limits how much a mistake can cost; it does not make the choice unimportant.

    View in the glossary
  • Weighting

    How much each holding counts in an index or portfolio — for example by market capitalization, or equally.

    View in the glossary
See the full glossary

Why this matters

Concentration you did not decide on is an easy risk to miss, because it can build up without any new purchase.

Check your understanding

No score is recorded. This is only here to test whether the lesson landed.

1. What is the most common way concentration risk builds up in a beginner portfolio?
2. True or false: holding a large amount of employer stock only concentrates investment risk, not income risk.
3. Two investors each own a company that later fails completely. One held it as 3% of their portfolio; the other held it as 40%. What does position sizing explain about their outcomes?

0 of 3 answered. Answer every question to see the answers.

Key takeaways

  • Concentration often builds through a holding's own success, not a new purchase
  • A broad fund's largest constituents can create hidden concentration if added individually as well
  • Employer stock concentrates income and savings risk in the same outcome
  • Deciding a maximum position size in advance is a practical way to keep concentration a choice

Related concepts

Written by
Pro Investor Academy Research Team
Content type
Educational article
Published
2026-09-13
Last reviewed
2026-09-13

Sources

CorrelationRebalancing

Educational content only. This lesson is not investment, tax or legal advice and does not recommend buying or selling any specific investment. All investing involves the risk of loss.