Kelly Criterion Position Sizing Calculator
Find the mathematically optimal fraction of your bankroll to risk per trade — plus the safer half-Kelly and quarter-Kelly sizes pros actually use.
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Frequently Asked Questions
What is the Kelly criterion?
A formula from probability theory that gives the fraction of your bankroll to wager to maximize long-term compounded growth: Kelly% = Win% − (Loss% ÷ Win/Loss ratio).
Why do traders use half-Kelly instead of full Kelly?
Full Kelly assumes your edge estimates are perfect and produces brutal drawdowns (~50%+). Half-Kelly keeps ~75% of the growth with far smaller drawdowns — the professional standard.
What if Kelly is negative?
A negative Kelly means you have no edge — the math says bet nothing. Do not take the trade, or fix the strategy first.
Does Kelly work for investing too?
It applies to any repeatable bet with known probabilities — trading, poker, sports betting. For long-term investing, position sizing by Kelly is less common than fixed fractional risk.