Position Sizing, The Silent Edge
Most traders obsess over where to enter. The real question is how much to risk when you do.
The Core Idea
Your position size determines whether a losing streak wipes you out or just inconveniences you. A great entry with terrible sizing can still blow an account.
The Two Main Approaches
Fixed Fractional (Risk % per trade)
Risk a fixed % of your account per trade typically 1–2%.
- $10,000 account × 1% = $100 max risk per trade
- Simple, consistent, and self-adjusting as your account grows or shrinks
Kelly Criterion
A formula that tells you the mathematically optimal percentage to risk based on your win rate and reward ratio. It maximizes long-term growth but can suggest aggressive sizing, most traders use half Kelly to reduce volatility.
Why It Matters More Than Your Entry
A trader with a 40% win rate but proper sizing can be profitable.
A trader with a 70% win rate but reckless sizing can go broke. The math doesn't lie.
The Practical Rule
> Never let a single trade have the power to significantly damage your account. Entries are opinions. Position sizing is protection.
The best traders aren't necessarily the most accurate, they're the ones who survive long enough for their edge to play out. Sizing is what buys that survival.
and what is the kelly criterion formula?
The Kelly Criterion formula is:
f = (bp - q) / b
Where:
- f = fraction of your account to risk per trade
- b = net odds (reward-to-risk ratio, e.g., 1.5 for a 1:1.5 RR)
- p = probability of a winning trade
- q = probability of a losing trade (1 - p)
Forex Example
Say your strategy has:
- 60% win rate (p = 0.60, q = 0.40)
- 1:2 risk-to-reward ratio (b = 2)
f = (2 × 0.60 - 0.40) / 2 = (1.20 - 0.40) / 2 = 0.80 / 2 = 0.40
This says risk 40% of your account per trade which is extremely aggressive in Forex.
The Kelly Criterion formula is:
f = (bp - q) / b
Where:
- f = fraction of your account to risk per trade
- b = net odds (reward-to-risk ratio, e.g., 1.5 for a 1:1.5 RR)
- p = probability of a winning trade
- q = probability of a losing trade (1 - p)
Forex Example
Say your strategy has:
- 60% win rate (p = 0.60, q = 0.40)
- 1:2 risk-to-reward ratio (b = 2)
f = (2 × 0.60 - 0.40) / 2 = (1.20 - 0.40) / 2 = 0.80 / 2 = 0.40
This says risk 40% of your account per trade which is extremely aggressive in Forex.
Practical Tip: Use Half-Kelly
Most professional traders use Half-Kelly (f / 2), so in the example above that would be 20%. Even that's high for Forex. Here's why:
- The formula assumes your win rate and RR estimates are perfectly accurate they rarely are in live markets
- Forex has inherent volatility, slippage, and spread costs not captured in the formula
- Full Kelly can cause catastrophic drawdowns with even a short losing streak
A common approach is to use Kelly to identify the ceiling of position sizing, then scale down to 10–25% of the Kelly output for day-to-day trading.
In short, Kelly is a great theoretical guide for position sizing, but in Forex it's best treated as a risk ceiling, not a rule.