Enter your bankroll, the odds, and your honest win probability. Out comes the mathematically optimal stake — full, half and quarter Kelly.
The Kelly Criterion gives you the stake that maximizes long-run bankroll growth if your win probability is exactly right. That's the catch — real handicapping is never that precise, and full Kelly punishes a bad estimate hard. Overstate your edge by a few points and full Kelly can turn a legitimately profitable strategy into a rough ride.
Half and quarter Kelly trade some of that theoretical growth for a much smoother bankroll curve, and give you room to be wrong about your own numbers without it wrecking you. Quarter Kelly is what most disciplined bettors actually use.
If the calculator returns $0, the math is telling you something useful: at your stated probability, this bet has no edge — sizing doesn't matter if the price isn't right in the first place. Compare your number against the no-vig fair price before you ever get to staking.