Kelly Criterion Calculator
Enter your win rate and your average reward-to-risk ratio. Get the Kelly-optimal fraction of your account to risk — and the half-Kelly most traders actually use.
Free tool · No sign-up · Updated July 2026
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Optimal sizing
Estimates for planning only — verify against your broker. Nothing here is financial advice.
How the kelly criterion calculator works
The Kelly Criterion is the bet size that maximises long-run account growth given your edge. Feed it your real win rate and average reward-to-risk ratio and it returns the fraction of capital to put at risk. Because full Kelly is famously aggressive, this tool also shows the widely-used half-Kelly.
- Kelly needs two numbers about your system: your win rate (W) and your average reward-to-risk ratio (b).
- It returns the fraction of your account to risk that maximises geometric growth over time.
- If the result is zero or negative, your system has no positive expectancy at those inputs — no bet size makes it grow.
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Kelly Criterion Calculator — FAQ
Should I risk the full Kelly amount?
Almost never. Full Kelly maximises growth but produces stomach-churning drawdowns and is brutally sensitive to over-estimating your edge. Most practitioners use half-Kelly or less, which captures most of the growth with a fraction of the volatility.
Why is my Kelly percentage negative?
A negative Kelly means your inputs describe a losing system — the win rate is too low for that reward-to-risk ratio. No position size can make a negative-expectancy strategy grow; the fix is a better edge, not a different bet size.
Where do I get accurate win rate and R:R numbers?
From your own closed trades, not guesses. Kelly is only as good as its inputs, and a couple of points of over-estimated win rate can badly over-size you. A journal that tracks your real win rate and average R feeds this calculator honest numbers.