A great idea can still wreck a portfolio if it's sized too large. Position sizing is the discipline of deciding, before you buy, how much of your capital any single holding is allowed to control.
Two investors can hold the exact same stock and end up with wildly different outcomes, purely because of how much of their portfolio they put into it. A stock that halves costs a 2% position 1% of the total portfolio — annoying, recoverable. The same stock at a 40% position costs 20% of the total portfolio — a hole that takes real time and gains elsewhere to dig out of.
Position sizing is the decision, made before you buy, of what percentage of your total portfolio a single holding is allowed to occupy. It's a separate question from "is this a good investment?" — a great company can still be a bad position if it's sized too large relative to everything else you own.
Two common approaches: a simple maximum-weight rule (e.g. "no single stock over 10% of my portfolio, cost basis or current value") and a risk-based rule more common in active trading (e.g. "risk no more than 1-2% of my account on any single trade's stop-loss distance"). Long-term investors tend to lean on the first; active traders lean on the second — but both exist to answer the same question: how much can this one position hurt me if I'm wrong?
A $50,000 portfolio holds a stock that drops 30%. The dollar loss — and how much it dents the whole portfolio — depends entirely on the position size going in.
| Position Size | Position Value | Loss at -30% | Loss as % of Portfolio |
|---|---|---|---|
| 5% Position | $2,500 | -$750 | -1.5% |
| 10% Position | $5,000 | -$1,500 | -3.0% |
| 25% Position | $12,500 | -$3,750 | -7.5% |
| 50% Position | $25,000 | -$7,500 | -15.0% |
Same stock, same -30% move — but the portfolio-level damage ranges from a shrug (-1.5%) to a serious setback (-15%), purely as a function of how large the position was allowed to get before it dropped.
Enter your portfolio value, your chosen max-weight cap, and how much you're prepared to risk on this specific trade — see the resulting position size and portfolio-level risk.
Model: risk-based size = (portfolio value × risk %) ÷ stop-loss %. This is then capped at the max-weight limit — whichever produces the smaller position wins, so a single trade can never breach either rule.
Answer all five, then hit "Check My Answers" to see how you did. Get one wrong? No problem — the explanation will show you exactly why.
Print-friendly resources to revisit, practice, and dig deeper — no login required.
You've capped how much any single holding can dominate your portfolio. The final step is matching the whole plan to your own timeline and appetite for risk.