Most traders decide how many shares to buy the same way they pick a lunch spot: gut feel. The result is a portfolio where one bad trade does the damage of five good ones.
The percentage-risk method
The fix is mechanical. Decide the maximum fraction of your account you are willing to lose if the trade fails — most traders use 1–2% — and let your stop loss set the share count:
- Dollar risk = account size × risk percentage. A $25,000 account risking 1% puts $250 on the line, no matter the trade.
- Risk per share = entry price − stop loss. If you buy at $50 with a stop at $47, each share risks $3.
- Shares = dollar risk ÷ risk per share. $250 ÷ $3 = 83 shares.
Notice what happened: the position size fell out of the math. A tighter stop lets you buy more shares for the same dollar risk; a wider stop forces fewer. Either way, a full stop-out costs you the same $250.
Why this beats "round lots and vibes"
- Consistent losses. Every loser costs roughly the same, so no single trade can crater the account.
- Comparable results. When every trade risks 1R, your journal becomes readable — a +2R winner means the same thing in January and June.
- Emotion control. You decided the risk before entering. The market can only take what you offered.
Do it in ten seconds
Our free position size calculator runs this exact math: enter account size, risk percentage, entry, and stop, and it returns shares, total position, and dollar risk. If you create a free account, one click saves the result as a trade plan you can review after the trade closes — which is where the real learning starts.