What is the Kelly criterion?

The Kelly criterion is a mathematical formula that estimates a bankroll fraction from the odds and a probability estimate. It only has a positive result when the estimated probability is above the odds' break-even probability.

What does fractional Kelly mean?

Fractional Kelly means using a portion of the full-Kelly result. It reduces sensitivity to estimation error, but it does not make a wager safe or guarantee a result.

What if my probability estimate is below the implied probability?

The Kelly result is zero or negative, which means the inputs do not show a positive mathematical edge. The calculator displays no positive Kelly stake in that case.

Bankroll math

Kelly criterion
calculator.

Q: Where does the probability estimate come from?

The calculator does not create the estimate. Use a documented model, a calibrated estimate, or a carefully chosen market baseline. If you cannot explain the number, treat the Kelly result as illustrative.

Compare your probability estimate with the break-even number, then explore what full or fractional Kelly would show for betting prices.

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01 / Input

Model the edge

FD / CALC-006

Price formatthe formula uses net odds

Your bankroll money set aside

$USD

Your odds e.g. +140 or −110

AMERICAN

Try a common line

Your model’s estimated win probability be conservative — no bet is a lock

Kelly fractionscale the full result

Full-Kelly reference f = (1.00 × 0.55 − 0.45) ÷ 1.00 = 10.00% full b is the net profit on a one-unit win, p is your win probability, and q is 1 − p. Kelly is only as useful as the estimate behind it.

Results update instantly

02 / Readout

Here’s the reference in numbers

Live

Selected Kelly stake

$50.00

No positive edgeFull Kelly

Full Kelly+10.00%

Selected fraction50% of full

Your edge+5.00 pp

Break-even probability50.00%

Input price+100

Net odds (b)1.0000

Bankroll$1,000.00

Full-Kelly stake$100.00

At +100 and a 55.00% win estimate, full Kelly is 10.00% of the $1,000.00 bankroll. The selected 50% fraction gives a $50.00 reference stake.

Read it correctly

The edge has to be yours.

Kelly does not create an advantage. It only translates an advantage you believe you have into a bankroll fraction.

Probability estimate

The input that drives the result.

55% estimate − 50% implied = +5 pp

Fractional Kelly

A smaller slice of the full result.

10% full × 50% = 5% reference

Common questions

Put the formula in context

The formula is precise. Your probability estimate may not be. That distinction matters more than the decimal places.

What does Kelly measure?

It estimates a bankroll fraction when your probability estimate is above the price’s break-even probability. It does not find the edge for you.

Why use a fraction?

Full Kelly can react sharply to small changes in your inputs. A fraction reduces that sensitivity, but it cannot remove uncertainty.

What if there is no edge?

If your estimate is at or below the implied probability, the model shows no positive Kelly stake. That is an input signal, not a challenge to wager anyway.

Where does the estimate come from?

The calculator does not create it. Use a documented model, a calibrated estimate, or a carefully chosen market baseline. If you cannot explain the number, treat the Kelly result as illustrative.

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Kelly criterioncalculator.