CobiaHow the Kelly calculator works
The Kelly criterion answers one question: given an edge, what fraction of your bankroll should you risk to grow it fastest over the long run? Bet too little and you leave growth on the table; bet too much and variance eventually wipes you out. Kelly finds the exact peak.
Worked example
You think a team wins 55% of the time, and it's priced at +120 (decimal 2.20, so b = 1.20). Then f = (1.20 × 0.55 − 0.45) ÷ 1.20 = (0.66 − 0.45) ÷ 1.20 ≈ 17.5% of bankroll at full Kelly. On a $1,000 bankroll that's $175 full, or about $88 at half Kelly. The EV is (0.55 × 2.20 − 1) = +21% per dollar.
Why bet a fraction of Kelly
Full Kelly is only optimal if your win probability is exactly right — and it never is, because the win % is your estimate, not a fact. Overstate your edge and full Kelly over-bets aggressively, inviting brutal drawdowns. Half or quarter Kelly gives up a sliver of long-run growth in exchange for dramatically smoother variance and a big safety margin against a bad estimate. Most disciplined bettors never bet more than half.