Position Sizing: The Secret Weapon of Professional Traders

Ask any legendary trader what separates winners from losers, and they'll tell you it's not about picking stocks—it's about position sizing.

The 2% Rule

Ed Seykota, one of the most successful traders in history, never risked more than 2% of his portfolio on any single trade. This simple rule allowed him to survive inevitable losing streaks while capitalizing on his winners.

How to Calculate Position Size

Position Size = (Account Size × Risk %) / (Entry Price - Stop Loss)

For example, if you have a $100,000 account and want to risk 2% on a trade where your entry is $50 and stop loss is $45:

Position Size = ($100,000 × 0.02) / ($50 - $45)
Position Size = $2,000 / $5 = 400 shares

The Kelly Criterion

Developed by mathematician John Kelly, this formula helps determine the optimal bet size to maximize long-term growth:

Kelly % = W - [(1 - W) / R]

Where:

Real-World Application

Paul Tudor Jones famously said: "The most important rule of trading is to play great defense, not great offense."

He never lets a losing position get out of control. His max drawdown on any single trade is strictly limited.

Common Position Sizing Mistakes

The Math of Ruin

Here's why position sizing matters:

| Loss | Gain Needed to Recover |
|------|------------------------|
| 10% | 11% |
| 25% | 33% |
| 50% | 100% |
| 75% | 300% |

A 50% loss requires a 100% gain just to break even. This is why preservation of capital is paramount.