Kelly sizes a stake from your edge and your bankroll. Enter the price, your own estimate of the chance, and what you are betting from, and it returns the full, half and quarter Kelly stake.
The Kelly criterion works out the stake that maximises the long-run growth of your bankroll. It takes your edge — the gap between your estimate of the true chance and the chance the price implies — and the price itself, and returns the fraction of your bankroll to risk. Bet a price of 2.50 you rate a 45% chance and full Kelly suggests about 8.3% of the bank.
Full Kelly assumes your probability estimate is exactly right, which it rarely is, so it is punishingly volatile. Most bettors who last use half or quarter Kelly, which keeps most of the growth while cutting the swings and the damage from over-estimating your edge. If the edge is negative, Kelly returns zero — there is no stake that turns a bad price into a good bet.
Kelly is a sizing tool, not a licence to bet big. Combine it with a fixed cap (say 1–2% of the bank per bet) and it keeps you staking more when the edge is real and less when it is thin, all while protecting you through the inevitable downswings.