Position Size Calculator
Find exactly how many shares to buy so that if your stop-loss is hit, you lose the amount you planned to lose and not a dollar more.
Results
Shares to buy
Long position
- Dollar risk
- $0.00
- Risk per share
- Position value
- $0.00
- % of account
- 0%
- Profit at target
- n/a
- Reward : risk
- n/a
How position sizing works
Position sizing flips the usual question around. Instead of "how many shares can I afford?", you ask "how much am I willing to lose if I'm wrong?" and work backward to the share count.
The formula is simple:
- Dollar risk = account balance × risk %
- Risk per share = |entry − stop-loss|
- Shares = dollar risk ÷ risk per share, rounded down
Example: with a $25,000 account risking 1%, your dollar risk is $250. Buy at $50 with a stop at $48 and each share risks $2, so you buy 125 shares. If the stop is hit, you lose $250, exactly 1%.
Short positions
Put your stop above your entry and the calculator treats it as a short. The math is the same: the distance from entry to stop is what each share can lose.
Want to stress-test your risk % against a losing streak? Try the trading expectancy calculator.
Frequently asked questions
How is position size calculated? +
Position size = dollar risk ÷ risk per share. Dollar risk is your account balance times your risk %, and risk per share is the distance between your entry and your stop-loss. The result is rounded down to whole shares so you never risk more than planned.
What percentage of my account should I risk per trade? +
Many traders cap risk at 1% to 2% of the account per trade. At 1%, it takes a long losing streak to do serious damage, which leaves room to recover. There's no single right number; it depends on your strategy's win rate and how you handle drawdowns.
Why does a tighter stop give me a bigger position? +
Because your dollar risk stays fixed. If each share can only lose $0.50 before you're stopped out, you can hold more shares for the same total risk than if each share could lose $5. That's also why a stop that's too tight gets hit by normal noise.
What if the position is bigger than my account? +
The calculator flags it. With a very tight stop, the math can call for more shares than you can afford without margin. In that case either use a wider, more realistic stop or cap the position at what your buying power allows.
Does this account for slippage or gaps? +
No. A stop-loss order becomes a market order when triggered, so a gap past your stop can cost more than planned. Some traders shave their risk % slightly to leave room for that.
More free tools
Want your AI agent to size trades for you?
Zentrix MCP gives Claude, GPT, and Gemini live prices and levels, so your agent can do this math on real setups.