Position Size Calculator
Free position size calculator: enter account, risk %, entry, stop, slippage and fees to get the exact size so every loss is the same fixed fraction of your account.
Size every trade so a loss costs a fixed percentage of your account — no guesswork, no oversized bets. Enter your stop (plus optional slippage and fees), and the calculator does the rest.
QuantDojo · Free tool
Position Size Calculator
Risk a fixed % per trade. Enter your stop — plus optional slippage and fees — and we size the position so the worst case costs exactly that.
How it works: size = (balance × risk%) ÷ (|entry − stop| + slippage + round-turn fee, per unit). Slippage and fees are folded into the worst-case loss, so your risk % holds after costs.
Educational tool — not financial advice. Real fills can still differ from your stop on gaps and fast markets — size conservatively.
Is your backtest lying to you?
Get The Honest Backtest Checklist — the exact red flags that expose an overfit strategy before you risk a cent, plus one rigorous idea a week.
Free forever. Unsubscribe anytime. We test what others sell — no hype. · Prefer to read it now? →
How to use the position size calculator
- Pick your risk per trade — most professionals risk 0.5–2% of the account on any single idea.
- Enter your account balance, entry price and stop-loss.
- The calculator returns the position size that makes a stop-out cost exactly your chosen risk.
Sizing from your stop — instead of a fixed lot or a round number — is what keeps a losing streak survivable. Ten losses in a row at 1% risk is about a 10% drawdown; the same ten losses with arbitrary position sizes can be account-ending.
Frequently asked questions
What position size should I use per trade?
There is no universal number — it depends on your stop distance and how much of the account you're willing to lose if the trade fails. Fix the risk (say 1%) and the size follows from your stop.
Does a tighter stop mean a bigger position?
Yes. For the same risk amount, a tighter stop (a smaller entry-to-stop distance) allows a larger position, because each unit loses less when stopped out. Your total risk stays constant.
How do slippage and fees change the position size?
They widen your effective loss per unit, so the calculator sizes you slightly smaller to keep the same maximum loss. Add your expected slippage and per-side fee and your risk % holds even after costs — ignore them and your real risk per trade quietly creeps above target.
Proper sizing protects you from ruin — but it can't turn a losing system into a winner. Before you scale up, check whether your edge is real with quantcheck.