Kelly Criterion

The Kelly Criterion is a mathematical formula for calculating the optimal fraction of capital to risk on a trade, given its win probability and payoff ratio, in order to maximize long-term capital growth. Trading exactly at the "full Kelly" fraction is theoretically optimal but produces very large swings in account value, which is why most practitioners use a fraction of Kelly (e.g. half-Kelly) for a smoother ride. In practice, a strategy's true win rate and payoff ratio are estimates rather than known constants, so Kelly-based sizing is only as reliable as the inputs feeding it.