Position Size & Risk Calculator
Stop guessing how many shares to buy. Enter your account size, the percentage you are willing to risk, your entry and your stop-loss — and get the exact share count that keeps every loss the same small slice of your capital.
How it works
Professional risk management flips the usual question. Instead of asking “how many shares can I afford?” it asks “how many shares can I lose on?” The math is simple and the same every time:
1. Dollar risk. Multiply your account by your risk percentage. A $50,000 account risking 1% puts exactly $500 on the line — this is the most you intend to lose if the stop is hit.
2. Per-share risk. Take the distance between your entry and your stop-loss. Buy at $100 with a stop at $95 and you are risking $5 per share.
3. Position size. Divide dollar risk by per-share risk and round down: $500 ÷ $5 = 100 shares. Those 100 shares cost $10,000 to hold, but if the stop hits you only lose the $500 you decided on up front.
Direction is inferred automatically: if your entry is above your stop you are long (betting the price rises, stop below); if your entry is below your stop you are short (betting it falls, stop above). The dollar risk is the same either way — what matters is the distance to the stop.
The 1% / 2% rule
The single biggest reason traders blow up is risking too much per trade. The fix is a hard cap: never risk more than 1–2% of your account on one position. At 1%, you would have to lose roughly 20 trades in a row to draw the account down about 18% — survivable. Risk 10% per trade and four bad calls cut you in half. Small, fixed risk is what lets your edge play out over hundreds of trades instead of ending your account in a handful.
Why fixed-fractional beats a fixed share count
Buying “100 shares every time” is a trap. A trade with a $1 stop and one with a $10 stop look identical in share count but risk ten times as much money. Fixed-fractional sizing holds the dollar risk constant and lets the share count float with the stop distance. Tight stop → more shares; wide stop → fewer shares; same risk on both. Your P&L then reflects the quality of your decisions, not how far away you happened to place a stop.
R-multiples
Once your risk is constant, you can measure every trade in R — one unit of risk. If you risk $500, that trade’s R is $500. A winner that books $1,000 is +2R; getting stopped out is −1R. Thinking in R frees you from dollar amounts and share prices: a strategy that averages, say, +0.4R per trade is profitable whether you trade a $5 stock or a $500 one. Track your trades in R and your edge becomes visible.