What is Kelly Criterion Calculator?
The Kelly Criterion is the gold standard for mathematically optimal position sizing. Developed by John L. Kelly Jr. at Bell Labs in 1956, it calculates the exact percentage of your capital that maximizes long-term wealth growth — no more, no less.
Our Kelly Criterion calculator takes your win rate, average win, average loss, and total capital to compute the Kelly percentage, optimal dollar allocation, expected growth rate, and win/loss ratio. Choose between Full Kelly (maximum theoretical growth), Half Kelly (reduced volatility, ~75% of growth), or Quarter Kelly (conservative approach).
The calculator includes an overbetting warning when Kelly exceeds 25% — a sign that the formula recommends an dangerously large position. A confidence indicator tells you whether your trade sample size is sufficient for reliable calculations. The scenario panel lets you adjust win rate and average win with sliders to see how the optimal allocation changes in real-time.
Steps:
- Enter your historical win rate as a percentage.
- Enter your average winning trade amount.
- Enter your average losing trade amount.
- Enter your total trading capital.
- Optionally enter total trades for confidence assessment.
- Choose Full, Half, or Quarter Kelly based on your risk tolerance.
- Review the Kelly percentage, optimal allocation, and expected growth rate.
Formula
Kelly% = W − (1 − W) / R
Where:
W = Win Rate (as decimal, e.g., 0.55 for 55%)
R = Win/Loss Ratio (Average Win ÷ Average Loss)
Half Kelly = Kelly% × 0.5
Quarter Kelly = Kelly% × 0.25
Optimal Allocation = Capital × Kelly% / 100
Example: Win Rate = 55%, Avg Win = $500, Avg Loss = $300
R = 500/300 = 1.67
Kelly% = 0.55 − (1 − 0.55) / 1.67 = 0.55 − 0.27 = 28.1%
Half Kelly = 14.05%
On $50,000 capital: Full = $14,050, Half = $7,025, Quarter = $3,512
Use Cases
- Stock traders optimizing position sizes based on historical edge
- Forex traders sizing positions with Kelly-adjusted risk
- Sports bettors calculating optimal bet sizes
- Portfolio managers allocating capital across strategies
Key Benefits
- Mathematically optimal position sizing
- Maximizes long-term wealth growth
- Built-in overbetting protection
- Scenario analysis with live sliders
Pro Tips
- Use Half Kelly for a balance of growth and reduced volatility
- Recalculate Kelly monthly with your latest trade data
- Never use Kelly if your win rate or edge is uncertain
- Combine Kelly with maximum position caps for extra safety
Common Mistakes to Avoid
- Using Full Kelly without understanding the volatility
- Calculating Kelly with insufficient trade history
- Ignoring transaction costs and slippage
- Not adjusting Kelly as win rate changes over time
Key Terms Explained
- Kelly Criterion: Formula for optimal bet size maximizing long-term growth
- Win/Loss Ratio: Average win divided by average loss
- Half Kelly: 50% of full Kelly recommendation, reduces volatility ~50%
- Overbetting: Allocating more than optimal, reducing long-term growth
Related Concepts
- Position Size Calculator – Risk-based position sizing
- Risk-Reward Ratio Calculator – Evaluate trade quality
- Drawdown Recovery Calculator – Understand recovery math
Example
You have a 55% win rate with average wins of $500 and average losses of $300. Win/Loss Ratio = 500/300 = 1.67. Kelly% = 0.55 − (0.45/1.67) = 0.55 − 0.27 = 28.1%. Since this exceeds 25%, the calculator warns of overbetting. Half Kelly = 14.05%. On $50,000 capital, the Half Kelly allocation = $7,025 per trade. Expected growth rate = 2.34% per trade.