Kelly Criterion Calculator

The Kelly criterion is a formula for sizing a bet from two numbers: the chance you think it wins, and the odds you are getting. This calculator applies it to your bankroll and shows the stake at full, half and quarter Kelly, with the expected value of each.

How the stake is calculated

The fraction of your bankroll to bet is f = p - (1 - p) / b, where p is your win probability and b is the profit per dollar staked (decimal odds minus one).

At 55% and -110, b is 0.909, so f = 0.55 - 0.45 / 0.909 = 5.5%. On a $1,000 bankroll that is $55.00 at full Kelly, $27.50 at half and $13.75 at quarter. The expected profit on the $55.00 stake is $2.75.

If the formula comes out at zero or below, the price gives you no edge at that probability and the stake is nothing. The calculator also shows each stake in units, at 1 unit = 1% of your bankroll.

What this calculator can't tell you

Kelly is only as good as the probability you put in, and that is the number you are least sure of. At -110 you need 52.4% just to break even, so a 55% estimate that is really 52% turns a positive stake into a losing one. Overestimating your edge makes Kelly tell you to bet too much.

Even with a correct estimate, full Kelly produces large swings in a bankroll. That is why many bettors use half or quarter Kelly, giving up some growth for much smaller drawdowns.

The calculator sizes a bet you have already decided is worth making. It does not tell you whether you have an edge.

If you or someone you know has a gambling problem, call the National Council on Problem Gambling helpline: 1-800-522-4700.