Kelly criterion calculator

The Kelly criterion answers one question: given how likely you think something is, and what it costs to bet on it, what share of your money should ride on it? Type the three numbers and this works it out. Nothing is saved, and nothing is sent to us.

How likely you think it is

What one contract costs now

Optional — needed for a stake

Enter a probability and a contract price to see the Kelly fraction. Add a bankroll and it will also say what that fraction comes to.

Your bankroll stays in this page. It is not sent anywhere, not stored, and not attached to anyone — there is no account here and nothing to sign into. Enter the capital you have set aside for this kind of trade, never your net worth: sizing against a bigger number inflates every stake by the same multiple.

What the words mean

A contract
On a prediction market, an agreement that pays one dollar if something happens and nothing if it does not. Its price is a probability wearing a dollar sign: a contract changing hands at thirty-eight cents is the market saying "this happens about thirty-eight percent of the time".
Your probability
What you think the real chance is, as a percentage. The whole calculation turns on the difference between this number and the price — that difference is what traders call an edge, and it is the one input nothing here can check for you.
Bankroll
The money set aside for this kind of trade — not your savings, not your net worth, and not money you would never actually risk. Kelly answers in fractions of a bankroll, so this number sets the scale of every answer it gives.
The Kelly fraction
The share of the bankroll the formula points at. It is the stake that grows money fastest over many repeated bets — if, and only if, your probability is right.
Fractional Kelly
Staking a set share of what the formula says — half, most commonly. Full Kelly is only optimal when the probability is exactly right, and it rarely is, so practitioners scale down on purpose. This calculator shows the full fraction and then applies the multiplier, so you can see both.

Where it goes wrong

  • A wrong probability breaks everything downstream. The formula trusts your estimate completely. Overstate an edge by a little and the stake it returns is too big by a lot, which is why the ordinary practice is to scale down rather than up.
  • Betting above full Kelly destroys compounding. Past a point, a larger stake does not merely grow money more slowly — it grows it backwards, even when every individual bet is still in your favour.
  • Positions that resolve together are one bet. Several contracts riding on the same event share a single bankroll. Sizing each of them on its own is quiet overbetting, and the calculator cannot see that they are related.
  • Costs and liquidity are not in the formula. Fees, the spread between what buyers and sellers ask, and the chance you cannot get out at the price you see all eat into the edge this arithmetic assumes.

The longer version, with a worked example and the arithmetic spelled out, is in our guide to the Kelly criterion.

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Formula and fractional-Kelly multiplier last reviewed 26 July 2026. This page is a calculator, not advice: it reports what a formula returns for numbers you chose, and knows nothing about your circumstances.