Position Sizing with the Kelly Criterion: Preventing Ruin in Stock Trading
Date Published
1. The Genesis of the Kelly Formula
John L. Kelly Jr.'s information theory breakthrough applied to finance
Originally published in 1956 by Bell Labs mathematician John L. Kelly Jr. to solve signal noise over telephone wires, the formula was quickly adopted by legendary mathematicians like Claude Shannon and Edward O. Thorp to conquer blackjack and financial markets.
The Kelly formula answers a fundamental question: Given a statistical trading edge with a known win rate and payoff ratio, what exact percentage of capital should you allocate to maximize compounding without going broke?
2. Full Kelly vs. Fractional Kelly Comparison
Why Wall Street quants never trade Full Kelly
While Full Kelly is mathematically optimal under infinite trials with stationary probabilities, financial markets are non-stationary with fat tails and black swans. If your win rate estimate is off by just 5%, Full Kelly will rapidly over-leverage your account into ruin.
3. Python Implementation of Kelly Position Sizer
Calculating trade lot sizes from historical win rates
Incorporate this automated position sizer into your strategy engine: