How the position size calculator works
Position sizing is one of the most important, and most overlooked, disciplines in trading. It answers a question entirely separate from "will this stock go up?" — namely, "how much should I buy so that if I'm wrong, the damage to my account is small and controlled?" A trader with a sound strategy but poor position sizing can still blow up an account; a trader with mediocre picks but disciplined position sizing can survive long enough to let their edge play out.
This calculator uses a fixed-fractional risk model, the standard approach used by professional traders and risk managers. You decide in advance what percentage of your total account you're willing to lose on any single trade — commonly 1% for conservative approaches, up to 2% for more aggressive ones. That percentage, multiplied by your account size, gives your dollar risk: the maximum amount you're willing to lose if the trade hits your stop-loss. Dividing that dollar risk by your per-share risk — the difference between your entry price and your stop-loss price — tells you exactly how many shares you can buy while keeping your loss capped at that dollar amount if the stop is hit.
The formula: Dollar Risk = Account Size × Risk % ÷ 100, then
Max Shares = floor(Dollar Risk ÷ (Entry Price − Stop-Loss Price)). The result is
rounded down to a whole share, since you can't buy a fraction of a share on most platforms, which
means your actual dollar risk at the stop will typically be a hair below your target percentage
— never above it.
Worked example
Suppose you have a $50,000 trading account and you've decided to risk 1% per trade. You're looking at a stock priced at $75 and plan to place your stop-loss at $70 — meaning you'll exit the trade if the price falls to $70.
Dollar risk = $50,000 × 1% = $500. That's the most you're willing to lose on this trade if it goes wrong.
Per-share risk = $75 − $70 = $5. That's how much you lose per share if the stock falls from your entry to your stop.
Max shares = $500 ÷ $5 = 100 shares. Buying exactly 100 shares at $75 means a total position cost of $7,500 (15% of the account), while your actual dollar risk at the stop stays at exactly $500 (1% of the account) — the number you decided on before you ever looked at the chart.
Position size vs. how much money you're investing
A subtlety that trips up a lot of beginners: position size (measured in dollar risk) and position cost (the total dollars invested) are two different numbers, and the gap between them depends entirely on how far your stop-loss is from your entry. In the example above, a tight $5 stop on a $75 stock let you take a $7,500 position while only risking $500. If your stop had been placed much wider — say $10 away instead of $5 — the same $500 risk budget would only support 50 shares and a $3,750 position. A wider stop means fewer shares for the same risk; a tighter stop means more shares for the same risk, but a higher chance of being stopped out by normal price noise. Position sizing forces you to make that trade-off deliberately, rather than backing into it by accident.
Common mistakes to avoid
1. Risking a percentage of a single trade's cost instead of the whole account
The risk percentage should always be calculated against your total account size, not against the dollar amount you're putting into this particular trade. Risking 1% of a $7,500 position is only $75 — a very different (and far less useful) risk-management number than 1% of your full $50,000 account.
2. Setting the stop-loss after deciding the share count, not before
Position sizing only works if the stop-loss is set at a price level that makes technical sense (support, a moving average, a recent swing low) and the share count is derived from that level — not the other way around. Backing into a stop-loss price just to justify a share count you already wanted to buy defeats the entire purpose of risk management.
3. Ignoring that multiple open positions compound your total risk
Risking 1% on any single trade sounds conservative, but if you have ten correlated positions open at once (for example, ten tech stocks that tend to move together), a broad market drop could hit several stops simultaneously, and your real risk that day could be far higher than 1%. Position sizing per trade is necessary but not sufficient — also think about total portfolio risk across all open positions, especially when they're correlated.
Why 1-2% is the common benchmark
The 1-2% range isn't an arbitrary convention — it's derived from how many consecutive losing trades a strategy can realistically survive without wiping out the account. At 1% risk per trade, a brutal losing streak of 20 trades in a row only draws the account down by roughly 18% (losses compound against a shrinking balance, so it's slightly less than a straight 20%) — painful, but recoverable. At 5% risk per trade, that same losing streak would be catastrophic, and at 10% per trade, a losing streak that long would functionally end the account. Even skilled traders have losing streaks; the point of a small, consistent risk percentage is to make sure a streak of bad luck or bad decisions never becomes a account-ending event.