What Is Kelly Criterion in Trading?
The Kelly criterion is a formula that calculates the mathematically optimal percentage of capital to risk on a trade, given a strategy’s win rate and average win-to-loss ratio, in order to maximize long-run capital growth. In practice, most traders use a fraction of the full Kelly number, because trading at full Kelly produces extremely large equity swings.
Formula
Kelly % = W − [(1 − W) ÷ R], where W = win rate (decimal), R = avg win ÷ avg lossWorked example
A trader’s system has a 45% win rate (W = 0.45) and an average win-to-loss ratio of 1.8 (R = 1.8). Kelly % = 0.45 − (0.55 ÷ 1.8) = 0.45 − 0.306 = 0.144, or about 14.4% of capital per trade at full Kelly.
Most practitioners would not trade the full 14.4% — instead they’d use a quarter-Kelly (~3.6%) or half-Kelly (~7.2%) position size, trading the same statistical edge with meaningfully smoother equity swings.
What's a good number?
Full Kelly is rarely used in live trading because of the severe drawdowns it can produce even with a genuinely positive edge. Quarter-Kelly to half-Kelly is the commonly cited practical range among traders who use the formula at all.
If the Kelly formula outputs a negative percentage, that is a direct signal the system has no statistical edge as currently measured, and should not be traded at any size until the underlying win rate or risk-reward improves.
Common mistakes traders make with kelly criterion
- Trading at full Kelly and experiencing account-damaging drawdowns during entirely normal statistical variance, not a broken strategy.
- Feeding the Kelly formula an unreliable win rate or win/loss ratio estimated from too small a sample of trades.
- Recalculating Kelly percentage per trade based on how confident the trader feels in the moment, rather than from measured historical statistics.
- Continuing to size up when Kelly % has declined or turned negative, instead of treating that as a signal to cut size or pause the strategy.
TradeMind calculates your Kelly percentage automatically from your actual logged trade history, alongside Risk of Ruin, so sizing decisions are grounded in your real numbers through Guardian mode.