DEV Community

GFIL
GFIL

Posted on

Position Sizing: The One Formula That Keeps You in the Game

#trading #forex #risk #psychology

📐 Every professional trader I know has one thing in common: they size their positions before they enter, not after.

The difference between a losing trader and a winning trader isn't win rate — it's position sizing. You can be right 40% of the time and still be profitable if you size correctly.

The Kelly Criterion Simplified

The full Kelly formula is math-heavy. Here's the trader-friendly version:

Position Size (% of account) = (Win Rate × Avg Win) - (Loss Rate × Avg Loss) / (Avg Win)
Enter fullscreen mode Exit fullscreen mode

Example: If you win 55% of trades, average win is 2R, average loss is 1R:

Size = (0.55 × 2 - 0.45 × 1) / 2 = (1.1 - 0.45) / 2 = 0.65 / 2 = 0.325
Enter fullscreen mode Exit fullscreen mode

Kelly says bet 32.5% of your account. That's aggressive.

The Half-Kelly (recommended): 16.25%

This gives you 75% of the growth with 50% less volatility.

Fixed Fractional vs. Fixed Ratio

Fixed Fractional: Risk a fixed % of current account per trade.

  • Account = $10,000 → Risk 1% = $100
  • Account = $11,000 → Risk 1% = $110
  • Simple, compound-friendly

Fixed Ratio: Increase position size after every N dollars gained.

  • Start: 1 mini-lot
  • After every $2,000 gain → add 1 mini-lot
  • Geometric growth without exponential risk

The Volatility Adjustment

Not all pairs have the same volatility. USD/TRY needs smaller sizing than EUR/USD.

def adjusted_position(account, risk_pct, stop_loss_pips, pair_atr):
    base_risk = account * (risk_pct / 100)
    vol_ratio = 14 / pair_atr  # Normalize to ATR of 14
    adjusted_risk = base_risk * min(vol_ratio, 2.0)  # Cap at 2x
    position = adjusted_risk / (stop_loss_pips * 10)
    return position
Enter fullscreen mode Exit fullscreen mode

Higher ATR = smaller position. Simple logic that most people ignore.

The 5 Red Flags

  1. Increasing size after 3 losses — Revenge sizing, the fastest way to blow up
  2. Decreasing size after 3 wins — Fear of success, you're self-sabotaging
  3. Same size for every pair — USD/TRY at 5% is not same risk as EUR/USD at 5%
  4. Round-number position sizes — "I'll just do 2 lots" is not a strategy
  5. No size adjustment for news — CPI day needs 50% normal size

The Real Edge

Position sizing doesn't make you a better analyst. It makes you a better survivor. And surviving long enough for your edge to play out is the only thing that matters.

🔗 Free position size and risk calculator: https://blog.quant-view.xyz/tools/?utm_source=devto&utm_medium=social&utm_campaign=gfil_jul19
📱 Daily size recommendations: https://t.me/GFIL_Trading
💬 Risk management community: https://discord.gg/nPuta6Cr4

Disclaimer: Trading carries significant financial risk. Past performance does not guarantee future results. This is for educational purposes only.

Top comments (0)