Kelly criterion is a mathematical formula that tells you the optimal position size—the amount of capital to risk on each trade to maximize long-term growth without risking ruin.
Why Kelly Criterion Matters
Winning is not enough. You need to win the right way: with position sizes that grow your account at the fastest sustainable rate. Too small, and you’re leaving money on the table. Too large, and a drawdown sequence can wipe you out.
Kelly criterion solves this. It finds the sweet spot between maximum growth and acceptable risk.
The Kelly Criterion Formula
Kelly % = (Win% × Avg Win%) - (Loss% × Avg Loss%) ÷ Avg Win%
Or more intuitively in R terms:
Kelly % = [Win% - (Loss% × Avg Loss / Avg Win)] ÷ 1
Practical Example:
- Win rate: 55%
- Average win: 2R
- Average loss: -1R
- Kelly % = [0.55 × 2] - [0.45 × 1] = 1.1 - 0.45 = 0.65 or 65%
This means theoretically, you should risk 65% of your account per trade. That’s insane. Most traders use fractional Kelly: 25%, 50%, or 75% of the Kelly percentage to reduce volatility.
Half-Kelly = 65% ÷ 2 = 32.5% per trade (safer) Quarter-Kelly = 65% ÷ 4 = 16.25% per trade (very conservative)
Why Fractional Kelly?
Pure Kelly is mathematically optimal—but only if:
- Your historical metrics perfectly predict the future (they don’t)
- You can handle severe drawdowns emotionally (you might not)
- A small calculation error won’t ruin you (it easily can)
In the real world, markets change, your system degrades, and variance kills accounts. Fractional Kelly is slower but safer. A half-Kelly trader growing at 25% annually beats a full-Kelly trader who goes bust.
Real-World Forex Example
Your EURUSD trading over 100 trades:
- Winning trades: 55 (win% = 55%)
- Losing trades: 45 (loss% = 45%)
- Average win: 1.8R
- Average loss: -1R (you always take the stop)
Kelly % = [0.55 × 1.8] - [0.45 × 1] ÷ 1.8 Kelly % = [0.99 - 0.45] ÷ 1.8 = 0.54 ÷ 1.8 = 0.30 or 30%
Full Kelly says risk 30% per trade. Using half-Kelly: 15% risk per trade.
If your account is $10,000:
- Full Kelly: $3,000 risk per trade
- Half Kelly: $1,500 risk per trade
- Quarter Kelly: $750 risk per trade
Most pros use quarter to half-Kelly for stability.
The Risk of Ruin Connection
Kelly criterion directly prevents risk of ruin—the probability your account hits zero. With full Kelly:
- 55% win rate, 1.8R wins, -1R losses = extreme drawdown risk if you hit a losing streak
- You might have 55% of trades winning, but variance can produce 10 losses in a row
- At 30% risk per trade, 10 consecutive losses = 0.7^10 = ~2.8% of capital remaining
That’s dangerous. With quarter-Kelly:
- Same edge, but 7.5% risk per trade
- 10 consecutive losses = 0.925^10 = ~47% of capital remaining
- You survive and keep trading.
Calculating Kelly for Your System
- Gather your last 50+ trades (the more, the better)
- Calculate: win%, average win R, average loss R
- Apply the Kelly formula
- Use 25-50% of the result for position sizing
- Rebalance quarterly as your metrics improve
- Use the position size calculator to convert to lot sizes
Kelly Criterion vs. Fixed Risk
| Method | Risk Per Trade | Growth | Sustainability |
|---|---|---|---|
| Fixed 1% risk | $100 (on $10K) | Slow but steady | High |
| Full Kelly | ~30% (on $10K) | Maximum | Very high ruin risk |
| Half Kelly | ~15% (on $10K) | Fast, safer | Medium |
| Quarter Kelly | ~7.5% (on $10K) | Slower | High |
Most professionals use half to quarter-Kelly, risking 5-15% per trade.
Key Takeaway
Kelly criterion connects your system’s edge to optimal position size. You don’t need to risk 30% per trade to be profitable—half-Kelly at 15% risk gets you there with far less drawdown.
Calculate your Kelly from real trades, use a fractional version (25-50%), and adjust quarterly as your system evolves. This is how professional traders scale sustainably.
PipJournal calculates your win rate, average R-multiple, and probability of ruin automatically, so you can determine your ideal Kelly position size and track it over time.
