Position Sizing Frameworks

Why Position Sizing Determines Survival

You can have a positive expectancy system and still blow up your account if you size too large.

Position sizing is the bridge between your edge and your outcomes. Get it wrong, and variance can destroy your account before your edge has time to play out. Get it right, and even a difficult trading period becomes manageable.

Framework 1: Fixed Dollar Risk

The simplest and most popular approach. You risk the same dollar amount on every trade.

Formula: Position Size = Dollar Risk ÷ (Entry Price − Stop Price)

Example:

• Account: $25,000
• Dollar risk per trade: $250 (1% of account)
• Entry: $150, Stop: $147 (risk = $3 per share)
• Position Size = $250 ÷ $3 = 83 shares

Pros: Simple, consistent, easy to calculate

Cons: As your account grows or shrinks, you're not adjusting risk proportionally

Framework 2: Percentage Risk

Risk a fixed percentage of your current account balance on each trade.

Formula: Dollar Risk = Account Balance × Risk %

Then apply the same calculation as above.

Example:

• Account: $25,000
• Risk: 1% = $250 per trade
• As account grows to $30,000, risk grows to $300 per trade automatically

Pros: Self-adjusting — grows with winning accounts, reduces with losing ones

Cons: Slightly more calculation required

Recommended starting point: Risk 0.5%–1% of account per trade until you have at least 100 logged trades and a consistent profit factor above 1.5.

Framework 3: The Kelly Criterion

The Kelly Criterion is a mathematically optimal position sizing formula that maximizes long-term account growth.

Kelly % = W − [(1 − W) ÷ R]

Where W = win rate and R = average win/loss ratio

Example: Win rate 55%, average win/loss ratio 1.8

Kelly % = 0.55 − (0.45 ÷ 1.8) = 0.55 − 0.25 = 0.30 (30%)

Important: Full Kelly is too aggressive for most traders. Most professionals use Half-Kelly or Quarter-Kelly to reduce variance.

Common Position Sizing Mistakes

Over-sizing on high-conviction trades: Conviction doesn't change your statistical edge. If you risk 5% on "sure things," a few wrong "sure things" will devastate your account.

Under-sizing out of fear: Risking 0.1% per trade means meaningful gains require thousands of trades. Find a balance between growth and protection.

Ignoring correlated positions: If you have 3 open trades on correlated instruments (e.g., NQ, QQQ, AAPL all long), your true risk is much higher than 1% per trade.