Position Sizing 101: The Rule That Keeps You in the Game

New traders obsess over what to buy. Experienced traders obsess over how much. That's because a great strategy with sloppy sizing can still blow up an account, while an average strategy with disciplined sizing can survive long enough to improve.

Risk a fixed slice, not a fixed amount of shares

The most common rule is to risk a small, fixed percentage of your account on any single trade, often 1–2%. "Risk" here doesn't mean the amount you invest. It means the amount you'd lose if your stop-loss gets hit.

The formula

Position size = (Account size × Risk %) ÷ (Entry price − Stop price)

Example: You have a $10,000 account and risk 1% per trade, so $100. You want to buy a stock at $50 with a stop at $47, a $3 risk per share.

$100 ÷ $3 = about 33 shares, roughly a $1,650 position.

If the stop is hit, you lose about $100, or 1% of the account. Not the $1,650.

Why it works

• Losing streaks become survivable. Ten losses in a row at 1% risk leaves you down roughly 10%. At 10% risk per trade, the same streak would be devastating.
• Volatile stocks automatically get smaller positions. A wide stop means fewer shares, so jumpy stocks can't hurt you more than calm ones.
• It removes emotion. The size is a calculation, not a feeling about how confident you are.

Common mistakes

1. No stop at all. Without a stop, you can't calculate risk, so you're sizing blind.
2. Moving the stop lower. Widening a stop after entry quietly increases your risk beyond what you planned.
3. Doubling down on losers. Adding to a losing position without a new plan is how small losses become big ones.
4. Ignoring correlation. Five positions in the same sector can behave like one big position.

Make it automatic

This is one of the best jobs to hand an AI assistant: give it your account size, risk percentage, entry, and stop, and have it calculate the share count before every trade. Consistent math, every time, with no temptation to round up.

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InvestMode content is for education only and isn't financial advice. Investing involves risk, including loss of principal. Do your own research and consider talking to a licensed professional before making decisions.