Kelly Criterion Position Sizing Calculator
Determine optimal trade sizing fraction (Full, Half, Quarter Kelly) to maximize long-term account growth rate without over-leveraging.
Optimal Logarithmic Wealth Growth via Kelly Sizing
Developed by scientist John Kelly at Bell Labs, the Kelly Criterion is a mathematical formula that calculates the exact percentage of capital to risk on a trade to maximize the expected logarithmic growth rate of wealth over time. In financial markets, Kelly sizing identifies the sweet spot between under-betting (growing capital slowly) and over-betting (causing severe drawdowns or ruin).
The Kelly Formula and Payoff Ratio
The standard Kelly equation is: Kelly % = Win Rate % − ((1 − Win Rate %) ÷ Payoff Ratio), where Payoff Ratio = Average Win $ ÷ Average Loss $. If your win rate is 60% and your payoff ratio is 1.5 (winning $150 for every $100 lost), Full Kelly recommends risking 33.3% of your account per trade to maximize growth speed.
How to Use This Kelly Criterion Calculator
Enter your account equity, historical win rate %, average winning trade amount ($ or pips), and average losing trade amount. The calculator instantly determines Full Kelly %, Half Kelly % (50% reduction), and Quarter Kelly % (25% reduction), along with recommended position sizing.
Why Professional Traders Use Half or Quarter Kelly
Using Full Kelly in live trading is rarely practical because financial market returns suffer from fat tails and non-stationary win rates. Full Kelly produces extreme equity swings—a 50% drawdown is common. Applying Half Kelly (50% of the calculated fraction) yields 75% of Full Kelly’s growth rate with only 50% of the equity variance, offering far smoother performance.
Frequently Asked Questions
Why is Full Kelly considered too aggressive for Forex?
Full Kelly assumes stable win rates and payoffs. Real-market slippage and regime shifts make Full Kelly extremely volatile.
What happens if Kelly Criterion calculates a negative percentage?
A negative Kelly percentage means your trading strategy has a negative mathematical expectation (losing edge). Do not trade this strategy.
What is the difference between Half Kelly and Quarter Kelly?
Half Kelly cuts the calculated fraction by 50%, while Quarter Kelly cuts it by 75%, significantly dampening equity curve volatility.