The Kelly criterion is a formula for the bet size that maximizes long-run bankroll growth given your edge. Because full Kelly swings hard, most bettors use a fraction of it.
f = (bp − q) ÷ b, where b = decimal odds − 1, p = your win probability, and q = 1 − p. The result is the fraction of your bankroll to stake. A bigger edge or better odds means a bigger optimal bet; no edge means no bet.
Full Kelly is growth-optimal but produces brutal swings and deep drawdowns. Half or quarter Kelly keeps most of the growth with far smaller variance, which is why nearly all serious bettors size down. It also protects you when your probability estimate is a little off.
Full Kelly is high-variance. Fractional Kelly (half or quarter) is the practical, lower-risk version most bettors use.
A negative Kelly fraction means the bet is −EV at your estimated probability — you shouldn't bet it.