You can have a positive-expectancy strategy and still blow up your account if you size wrong. Here is the math that keeps you in the game long enough to win.
Position sizing is the most important decision in trading that almost nobody talks about. Traders obsess over entries, exits, indicators, and setups — but position size determines whether a losing streak ends your account or feels like the cost of doing business. It's the difference between a drawdown you survive and one you don't.
Fixed fractional sizing
The simplest rule that works: risk no more than 1–2% of your account on any single trade. If your account is $10,000 and your rule is 1%, you risk $100 per trade. This means your stop loss placement and position size are linked: if your stop is $0.50 from your entry and you're risking $100, your position size is 200 shares.
The formula: Position Size = Risk Amount ÷ (Entry Price − Stop Price)
Most traders get this backwards — they decide how many shares to buy and then set a stop wherever the chart "looks right." The correct order is: place the stop where it's technically valid, then size the position to match your risk rule.
Why the percentage matters
At 1% risk per trade, you can lose 20 trades in a row and still have 82% of your starting capital. At 10% risk per trade, 20 losing trades leaves you with 12%. The math of compounding losses is severe — a 50% drawdown requires a 100% gain just to break even. Keeping individual risk small keeps you in the game long enough for your edge to play out over hundreds of trades.
Survive first. Compound later. Position sizing is how you do both at once.
Adapting size to volatility
A fixed stop distance in dollar terms means your position size varies with price — a $50 stock with a $1 stop gives you 100 shares at $100 risk, while a $500 stock with the same $1 stop gives the same. But volatility varies too: in a high-VIX environment, your technically-valid stop may need to be wider to avoid noise-out. When stops widen, size down so your dollar risk stays constant. Never widen the stop without cutting the shares.