Why size matters
A great idea at 1% of your portfolio barely helps; a bad idea at 40% can undo years of work. Position sizing decides how much each decision can hurt or help.
Three approaches
| Method | How | Good for |
|---|---|---|
| Equal weight | Same amount in every holding | Simplicity; avoiding overconfidence |
| Conviction weight | More in your best-researched ideas | Experienced researchers |
| Risk-based | Less in more volatile holdings | Mixing stocks, bonds, leveraged funds |
Simple rules
- Start small. A new position at 2 to 5%, adding as the thesis proves out.
- Cap the maximum. Many investors cap any single stock at 10 to 20% at cost.
- Size by volatility. A 3x leveraged ETF moves about three times the market; a 5% position acts like a 15% position in the index. See Leveraged ETFs explained.
- Ask the failure question. If this went to zero, how much would I lose? Is that acceptable?
Letting winners run
A winner growing to 25% of a portfolio is a nice problem, but it is still concentration. Decide ahead of time whether you will trim at a set weight. Measure overall concentration with the Herfindahl index (concentration).
See also
- How many stocks should you own? How many individual stocks you need for diversification, what research says about 10, 20 and 30 stock portfolios, and the trade-off with conviction.
- Herfindahl index (concentration) How the Herfindahl-Hirschman index measures portfolio concentration, how to calculate it from weights, and the effective number of holdings it implies.
- Maximum drawdown What maximum drawdown is, how to calculate it, why losses need bigger gains to recover, and historical drawdowns for stocks and leveraged funds.
- Leveraged ETFs explained How leveraged ETFs like TQQQ and UPRO work, why they reset daily, how volatility decay erodes them, and what their historical drawdowns look like.
Pages that link here: Building your investing knowledge, Fundamentals checklist, Hype and market sentiment, Investing FAQ, Leveraged strategies compared, Portfolio vs watchlist, Systematic investing
Last updated September 30, 2026. Education only, not investment advice.