Position Size Calculator
Get the exact number of shares to buy so a single bad trade can only lose your chosen risk-per-trade fraction. Works for longs and shorts, stocks and crypto.
Results
For a long entry at $50 with a stop at $48 (4.00% from entry), risking 2.00% of a $100,000 account, you should buy 1,000 shares. Worst-case loss at the stop is $2000.00. If the trade reaches your 2R target at $54.00, your gain is twice the dollar risk.
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Frequently asked questions
The fixed-fractional rule
Position sizing is the discipline that separates traders who survive losing streaks from traders who do not. The simplest sustainable rule is fixed-fractional risk per trade: pick a small percentage of your account that you are willing to lose on any single trade — typically 1-2% — and let your stop-loss distance determine how many shares to buy.
With 2% per trade, a streak of 10 consecutive losses leaves you down about 18%. With 5%, the same streak draws you down 40%. With 10%, 65%. The math is unforgiving on the high end: per-trade risk and drawdown grow geometrically, not linearly.
Why this rule beats the alternatives
New traders often size by gut, by "buy a round lot," or by "use all available margin." All three guarantee that your worst trade roughly equals your biggest position, which means a single fat-finger or a single fast-market gap can take out your account. Fixed-fractional sizing decouples the size of any single bet from the size of the worst possible outcome.
Tightening or loosening the stop
Notice how the stop distance interacts with the position size. If you cut your stop in half, you can buy twice as many shares for the same dollar risk — but you double your chances of getting stopped out by routine noise. If you widen the stop, you buy fewer shares but tolerate more wiggle. The right stop comes from the trade thesis (volatility, support/resistance, average true range), not from how big a position you want.
R-multiples and trade evaluation
Many traders track outcomes in R-multiples — the ratio of profit (or loss) to the dollar amount risked. A trade that hits 2R is "a 2R win." A losing trade that gets out at the stop is "a -1R loss." This standardization lets you compare trade quality across positions of different sizes. A profitable trader generally has an average R per trade above zero — even if their win rate is below 50%, the size of their wins relative to their losses keeps the expectancy positive.
Combining with Kelly
This calculator answers "given that I want to risk X% of account per trade, how many shares?" It does not tell you what X% should be. For that, see the Kelly criterion calculator — it derives the optimal X% from your edge and win/loss profile.