Kelly Criterion Calculator
Compute the theoretically optimal bet size that maximizes long-term log-growth of your bankroll. Includes a growth-curve simulation showing why fractional Kelly is usually safer.
Inputs
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Result
Growth simulation
Log-scale bankroll over time. Full Kelly grows fastest but has highest drawdown. Half-Kelly often gives ~75% of the growth with much less variance.
What Kelly actually says
For a bet that wins with probability p and pays b:1, the Kelly fraction is:
f* = (bp − q) / b, where q = 1 − p
Betting f* maximizes the long-term growth rate of your log-bankroll, which is equivalent to minimizing the time to reach a target. Kelly is provably optimal in the limit but assumes you know p and b exactly. Real-world estimates are noisy, so most serious practitioners use "fractional Kelly" (½ or ¼ Kelly).
Who uses it
- Sports bettors sizing wagers against sharp lines.
- Traders position-sizing individual bets.
- Information theorists gambling on Huffman codes and similar.
Limitations
- If you overestimate p, full Kelly can blow up — the chart shows this dramatically.
- Kelly says nothing about utility curves. A risk-averse bettor should use a smaller fraction.
- Negative Kelly (when f* < 0) means: don't make this bet at all. The tool will warn.
Frequently asked questions
How do I use Kelly Criterion Calculator step by step without mistakes?
Follow the order given in the usage section: Compute optimal bet size using the Kelly criterion. Includes growth-curve simulation and fractional-Kelly comparison. The step-by-step part matters less than the unit check that sits inside it. Confirm every unit matches the thing you are actually trying to measure before you press calculate - that single check catches more wrong answers than any other habit, and it costs about ten seconds.
What are the common mistakes when using Kelly Criterion Calculator?
Most common mistakes come down to four habits: entering 20 when the field expects 0.2 because the field is a percentage; leaving a field blank and accepting the default without checking what it stands for; changing one input and reading a result that was calculated before the change; and quoting the output as an exact figure when the page describes it as a reference estimate. Press calculate again after any edit, and treat the number as a reference point rather than a final answer.
How do I read the result from Kelly Criterion Calculator?
Read the final figure first, then come back to this section for the conditions behind it: Compute optimal bet size using the Kelly criterion. Includes growth-curve simulation and fractional-Kelly comparison. Keep two things apart: the number itself, which comes from the values you typed, and the assumptions it depends on, which decide whether that number is meaningful. The page sets out what a higher or lower result means for your case, so check that direction as well as the magnitude.
Can I act on the Kelly Criterion Calculator result directly?
Treat it as one reference sheet before you decide, rather than as the answer itself: If you overestimate p, full Kelly can blow up — the chart shows this dramatically. Kelly says nothing about utility curves. A risk-averse bettor should use a smaller fraction. Negative Kelly (when f* < 0) means: don't make this bet at all . The tool will warn. The output describes a magnitude under the conditions you entered. It does not include what it actually costs to act, what might change while you act, or how the other side behaves. Before you commit, compare the result against reality and note which assumptions do not hold where you are.
What inputs does Kelly Criterion Calculator need?
Kelly Criterion Calculator takes 5 inputs: Win probability p、Win b (net profit per $1 bet)、Starting bankroll、Number of bets to simulate、Simulations. They are listed in the order they feed the calculation, so an earlier value can change a later one. Do not skip a field you do not recognise - work out what it means first, because a silently guessed value produces a number that looks reasonable and is not. Before typing anything, make sure every figure uses the same unit and the same time period.
What should I check before I run Kelly Criterion Calculator?
Three checks pay for themselves. First, confirm the question you need answered is really inside the scope of this tool rather than a neighbouring one. Second, confirm that the figures you have are drawn from the same source and the same period - mixing monthly and annual numbers, or two different definitions of the same quantity, quietly distorts the result. Third, keep the key inputs written down, so that later you can tell which set of numbers produced a figure you are about to quote.