How it works
A position size calculator converts a fixed dollar risk into an exact trade size — the step that separates systematic trading from gambling with extra steps. The logic runs in four lines. Decide the maximum you will lose if the idea fails: account times your risk percentage, say 1% of $25,000, or $250. Measure the distance from entry to stop, say $180 down to $171, nine dollars of adverse room. Divide the risk budget by that distance: $250 over $9 is 27.8 units. That quotient is the position — sized so that a stop-out loses exactly the budgeted amount, no more. Notice what this reverses. Amateurs pick a size first and hope; professionals pick the loss first and let the size follow, which is why a wider stop produces a smaller position inside the same risk, and why 'I couldn't afford many shares' is a category error — the affordable quantity is an output, not an input. The calculator completes the picture with the full position value, its weight in the account, and the reward-to-risk ratio between target and stop. One humility clause: stops trigger fills, not guarantees — gaps and halts can execute past the level, so realized losses occasionally overshoot by exactly the amount this arithmetic cannot see.
Formula
units = (account x risk%) / (entry - stop) | R:R = (target - entry) / (entry - stop)
Tips
- Fix the risk fraction first — commonly near 1% — and let position size adjust to the stop distance.
- Place the stop at a level that invalidates the idea, then size from it; never widen a stop to fit a preferred size.
- A reward-to-risk ratio near or above 2 lets a sub-50% win rate still compound positively.
- Correlated open positions act like one bigger trade — cap combined exposure, not just single-trade risk.
- Recheck size whenever volatility shifts: the same dollar distance is a different risk in a calmer or wilder tape.
Frequently asked questions
- How do I calculate position size?
- Multiply account size by your risk percentage for the dollar budget, divide by the distance between entry and stop-loss, and buy that many units. With $25,000, 1% risk, a $180 entry and a $171 stop: $250 / $9 = about 27.8 shares, a $5,000 position risking exactly $250.
- What is the 1% rule in trading?
- Risk no more than 1% of the account on any single idea, meaning the loss at the stop — not the capital deployed — stays near one percent. The rule's power is survival: a long streak of losses dents the account by single-digit percentages, leaving judgment and capital intact.
- Why does position size depend on stop-loss distance?
- Because risk equals size multiplied by stop distance. Fix the risk and the two become inversely proportional: a tight $2 stop supports five times the position of a loose $10 stop on the same budget. Choosing size independently of the stop is how accounts quietly bet multiples of what they intended.
- What is a good risk-reward ratio?
- Commonly at least 2:1 — potential gain double the planned loss — though the right bar depends on win rate. At 2:1, winning barely a third of the time breaks even. Below 1:1 you need to be right far more often than wrong, which few processes sustain.
- Should I risk a fixed percentage per trade?
- Fixed-fractional sizing scales bets down after losses and up after gains, producing smooth equity curves and making ruin mathematically remote at sensible fractions. Its cost is slow recovery after deep holes. Whatever fraction you choose, applying it consistently matters more than the exact number.