Kelly Criterion
A position-sizing formula that converts a trade’s edge into the fraction of capital to risk — used fractionally and capped so a real edge stays survivable.
The Kelly Criterion is a position-sizing formula that tells you what fraction of capital to risk on a trade given its edge. It is the reference framework for sizing high-probability options trades so a real edge is not ruined by betting too big.
The formula:
- ●f* = (p × b − q) ÷ b
- ●where p = win probability, q = loss probability (1 − p), and b = payoff ratio (reward ÷ risk).
- ●f* is the fraction of capital the math says to risk.
How to use it in practice:
- ●Negative Kelly = no edge. If the formula returns a negative number, the correct size is zero — do not trade it.
- ●Use fractional Kelly. Full Kelly is far too volatile and assumes your win-rate estimate is perfect (it never is). Trading a quarter to a half of Kelly keeps growth healthy while slashing drawdown risk.
- ●Add an absolute cap. Short-option positions correlate violently in a crash, so cap risk at 2–5% of the portfolio per position and in aggregate — a backstop Kelly alone will not give you.
Why it matters: premium selling has negative skew, so a single oversized loser can erase many winners. Kelly, used fractionally and capped, is what keeps the edge survivable.
Important: This definition is educational and uses SPX for illustration — it is not financial advice.
Related Terms
The win-often-small, lose-rarely-large return profile of premium selling — the reason win rate alone never tells you whether an edge is profitable.
The estimated chance a trade is profitable at expiration — the most useful single number for comparing premium-selling trades before entry.
The mechanical exit rules — take profit at 50% of max, exit by 21 DTE, hard stop near 200% — that lift short-premium win rates from ~65% to ~80%+.
The persistent gap between implied volatility and realized volatility — the structural reason option sellers are, on average, systematically overpaid.
The CBOE Volatility Index measuring expected 30-day SPX volatility — the market's "fear gauge" and key input to session classification.