Find the mathematically optimal stake for a bet given your edge — with full, half or quarter-Kelly for real-world safety.
A formula for the bet size that maximizes long-run bankroll growth: f = (bp − q) ÷ b, where b = decimal odds − 1, p = win probability, q = 1 − p.
Full Kelly is the growth-optimal size but swings hard. Half or quarter Kelly keeps most of the growth with far smaller drawdowns — most pros bet fractional Kelly.
A negative Kelly fraction means the bet is −EV at your estimated probability — you shouldn't bet it.