Bet size matters more than picking the right game. The Kelly criterion is the mathematically optimal bet sizing formula — it maximises long-term bankroll growth while preventing ruin. Half-Kelly (recommended) is more conservative and reduces variance by 75% with only a minor reduction in growth rate. This calculator applies Kelly to casino games and sports betting, where it identifies when there is no positive edge.

Enter your bankroll, your estimated win probability, and the decimal odds (or payout ratio) for your bet. The calculator returns the Kelly fraction, half-Kelly, and a recommendation based on edge. If your edge is zero or negative (the case for almost every casino bet), the calculator will tell you the optimal bet is zero — this is the honest answer.

The formula dates from John Kelly's 1956 paper on information theory and is used by professional poker players, sports bettors, and finance professionals. For casino players specifically: the calculator is most useful for confirming that casino games are negative-EV and that the only winning strategy is selective play with bonus offers. See our house edge calculator for the RTP of every common casino game.