📐 Kelly Criterion Sizer
You think the market is wrong — but how much should you bet? Kelly tells you the mathematically optimal stake for long-run growth.
What the Kelly Criterion does
The Kelly Criterion answers the question every trader gets wrong: not whether to bet, but how much. Given your edge and your bankroll, Kelly returns the stake that maximizes long-run growth — bet bigger and you risk ruin, bet smaller and you leave growth on the table. On Kalshi, your edge is the gap between your own probability estimate and the price the market is charging, and this tool turns that gap into a concrete position size.
Estimate the true probability
Enter your honest read on how likely the event is, as a percentage. This is the hardest and most important input — Kelly is only as good as this number.
Enter the market price and bankroll
Add the Kalshi price in cents and your total bankroll. The price implies the market's probability; the difference from yours is your edge.
Read your sizing
The calculator returns the full-Kelly stake plus the half- and quarter-Kelly fractions, in both dollars and approximate contracts.
Size down on purpose
Most professionals stake at half-Kelly or less. It sacrifices a little growth for a large reduction in volatility — and it cushions the damage when your probability estimate is off.
Frequently asked questions
What is the Kelly formula?
Kelly fraction = (p·b − q) / b, where p is your probability of winning, q = 1 − p, and b is the net payout per dollar staked. On Kalshi, buying YES at a price c gives b = (1 − c) / c. The result is the share of your bankroll to risk.
Why do most people use half-Kelly?
Full Kelly assumes your probability estimate is exactly right. In reality it never is, and overestimating your edge is the fastest route to ruin. Half-Kelly keeps most of the growth with far less variance and much more forgiveness for estimation error.
What happens if I bet more than full Kelly?
Overbetting eventually guarantees ruin even with a real edge, because the swings compound against you. Kelly defines the ceiling — staking above it lowers long-run growth and raises bust risk at the same time.
Does this include Kalshi fees?
The core Kelly fraction shown here is before fees; taker fees shave your real edge slightly, so round your size down. For the full fee-adjusted picture of a single trade, use the Fee and EV calculators alongside this one.