Compute the mathematically optimal stake size given your edge, bankroll, and the market price. Most pros use fractional Kelly (0.25–0.5×) to dampen variance.
f* = (b×p − q) / b where: b = decimal_odds − 1 (net odds; payout per $1 staked) p = your estimated probability of winning q = 1 − p
Why fractional? Full Kelly maximizes long-run growth but produces large drawdowns. Halving it cuts variance dramatically with only modest growth penalty. Most professional bettors stake at 0.25–0.5× Kelly.
The brutal truth: Kelly is only as good as your probability estimate. If your p is wrong, the formula confidently sizes you into ruin.
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