Kelly Criterion Calculator

How much should you bet?

Finding a +EV bet is half the battle — sizing it is the other half. Bet too little and you leave growth on the table; bet too much and one bad run ruins you. The Kelly criterion gives the mathematically optimal stake for long-run bankroll growth, straight from your edge and your bankroll.

The formula

f* = (b·p − q) ÷ b — where b is your decimal odds minus 1, p is your win probability, and q is 1 − p. Multiply f* by your bankroll for the dollar stake. A bigger edge → a bigger fraction.

Worked example

You rate an outcome at 60% and it trades at 50¢ on Polymarket (decimal 2.00). Full Kelly says bet 20% of your bankroll — but at half Kelly you'd stake 10%, cutting variance for nearly the same growth. On a $1,000 bankroll that's a $100 bet.

Use it safely

Kelly is only as good as your probability estimate, so keep them honest and lean on half- or quarter-Kelly. Verify a bet is +EV first with the EV calculator, and sharpen your read with the analytics tools and tool guides on yesornotool.

Frequently asked questions

Frequently Asked Questions

The Kelly criterion is a formula for the bet size that maximises the long-run growth of your bankroll. It stakes more when your edge is bigger and less when it is smaller, and it never risks so much that a losing streak wipes you out.