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Kelly Multiplier — Risk Level
Most pros use Half Kelly to reduce variance
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Your honest estimate — not the bookmaker implied probability
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The Kelly Criterion is a mathematical formula that determines the optimal fraction of your bankroll to bet in order to maximize long-term growth. It was developed by John Kelly Jr. at Bell Labs in 1956.
Formula: f* = (bp − q) / b — where b = decimal odds minus 1, p = your win probability, q = loss probability (1−p).
If the result is negative, you have no mathematical edge and should not bet. Use our Expected Value Calculator to verify your edge first.
Formula: f* = (bp − q) / b — where b = decimal odds minus 1, p = your win probability, q = loss probability (1−p).
If the result is negative, you have no mathematical edge and should not bet. Use our Expected Value Calculator to verify your edge first.
Full Kelly assumes you know your exact true win probability — which is impossible. Any overestimation of your edge results in overbetting, causing severe bankroll swings.
Professional bettors almost universally use Half Kelly (0.5×) or Quarter Kelly (0.25×). This reduces variance dramatically while capturing most of the mathematical growth advantage. The loss in growth rate is small; the reduction in risk of ruin is enormous.
Professional bettors almost universally use Half Kelly (0.5×) or Quarter Kelly (0.25×). This reduces variance dramatically while capturing most of the mathematical growth advantage. The loss in growth rate is small; the reduction in risk of ruin is enormous.
Your true win probability is your honest, independent estimate of how likely an outcome is — not what the bookmaker implies. The best approaches: use Stake or Polymarket closing lines as a benchmark (remove the vig), build your own statistical model, or use BCC Oracle verdicts as a starting point. Track your Closing Line Value over time to verify your estimates are accurate.