Position Sizing: The Math Behind Consistency
Position sizing is where most traders fail. They size positions emotionally ("I'll buy 500 shares"). Professionals size positions mathematically. The difference compounds to 10x returns over time.
The Fixed Risk Method (Simplest)
Decide to risk a fixed dollar amount per trade (example: $500 on a $50k account = 1% risk). Then calculate position size:
Position Size = Risk Amount ÷ (Entry Price - Stop Loss Price)
Example: Risk $500. Entry $100. Stop $95 (5% stop). Position size = $500 ÷ ($100 - $95) = $500 ÷ $5 = 100 shares. Buy 100 shares, set stop at $95, and you're risking exactly $500.
This method keeps risk consistent. Every trade risks the same dollar amount, making your account equity curve predictable and smooth.
The Volatility-Adjusted Method (Better)
High-volatility stocks deserve smaller positions because stops are wider. Low-volatility stocks deserve larger positions because stops are tighter. Adjust position size by volatility:
Position Size = (Risk Amount ÷ ATR) ÷ Entry Price
Use Average True Range (ATR) to measure volatility. High ATR = large position size reduction. Low ATR = normal position size. This keeps risk units consistent across holdings.
The Kelly Criterion (Advanced)
The Kelly Criterion calculates optimal position sizing based on win rate and win/loss ratio:
f = (Winrate × AverageWin - (1 - Winrate) × AverageLoss) ÷ AverageWin
Example: 55% win rate, average winner $1000, average loser $800. f = (0.55 × 1000 - 0.45 × 800) ÷ 1000 = (550 - 360) ÷ 1000 = 0.19 = 19% of account per trade. But this is aggressive. Most traders use 50% of Kelly (9.5%) to reduce drawdown.
Position Sizing Rules
- Never risk more than 2% of account on any single trade
- Portfolio heat (total risk across open positions) should not exceed 10%
- For volatile stocks, reduce position size 20-30%
- For large account positions (>$100k), cap position size at 5% of account
- After a loss, reduce position size 30% for next trade
- After 5+ consecutive wins, you can increase size back to normal
Real-World Position Sizing Example
$50,000 account. 1% risk per trade = $500 max loss.
Setup 1: Stock trading $100, support at $95. Stop = 5%. Position size = $500 ÷ $5 = 100 shares.
Setup 2: Volatile stock at $80, support at $70. Stop = 12.5%. Position size = $500 ÷ $10 = 50 shares.
Both setups risk exactly $500 despite different volatility. This is proper position sizing.
Position Sizing Mistakes
Mistake 1: Fixed share count (always buy 100 shares). Ignores volatility and account size. Bad.
Mistake 2: Percentage of portfolio (always 5% of account). Ignores stop loss distance. If stop is 2%, that's 2.5x risk. Bad.
Mistake 3: Sizing up after wins. Psychologically feels right but violates Kelly Criterion. Leads to larger losses after a bad setup.
Use a position-sizing calculator — spreadsheet or dedicated tool to calculate sizes correctly.
The Sizing Edge
Proper position sizing separates casual traders from professionals. It's not exciting, but it's non-negotiable.