Day 17 of 30 · 14 min

Position sizing, diversification and managing risk

By the end of this lesson you can

Size positions so that no single loss is devastating
Set simple portfolio rules
Understand stop-losses and their trade-offs

Size before you buy

Position sizing decides how much of your portfolio goes into one idea. A useful question is: if this goes badly wrong, how much of my total portfolio do I lose?

If you put 5% into a stock and it halves, the portfolio falls 2.5%. If you put 30% in, it falls 15%.

Simple rules help

Many investors set limits such as: no single stock above 5–10% of the portfolio, no sector above 25–30%, and a core in broad funds. The exact numbers matter less than having rules decided calmly in advance.

Rules are most valuable exactly when emotions run high.

Stop-losses

A stop-loss order sells automatically if the price falls to a set level. It can cap losses on a trade, but in a sudden gap down it may fill well below the stop price, and in volatile stocks it can sell you out of a position just before a rebound.

Stops suit short-term trading more than long-term investing in diversified funds.

Risk you can live with

The best plan is one you will actually follow in a 30% market decline. If a drop like that would make you sell everything, the portfolio carries more risk than you can hold, and lowering it now is better than discovering it during a crash.

Sizing by risk

You have $10,000 and decide no single idea should cost you more than 2% of the portfolio ($200) if it falls 40%. The maximum position is $200 ÷ 40% = $500, or 5% of the portfolio.

If you believe a riskier stock could fall 60%, the same rule gives $200 ÷ 60% ≈ $333.

Try it in VEXA

Size positions in the Paper Challenge →

Real data, so the numbers will differ from the lesson’s examples.

Key terms

Position sizing: Deciding how much money to put into a single investment.
Position limit: A rule capping how large any one holding can be.
Stop-loss order: An order to sell automatically if the price falls to a set level.
Gap: A jump between one period’s close and the next period’s open, skipping prices in between.
Risk tolerance: How much decline you can accept, financially and emotionally, without abandoning your plan.

Full glossary →

Try it

Write three portfolio rules you would follow in the Paper Challenge (for example, a maximum position size and a minimum number of holdings).

Key takeaways

Size each position by what a bad outcome would cost the whole portfolio.
Set position and sector limits in advance.
Stop-losses cap trading losses imperfectly.
Hold only as much risk as you can live with in a crash.