What this tool is: the mathematically optimal bet size for your edge. Enter the odds, your true win probability, and your bankroll — Kelly answers the only sizing question that matters. Sane people bet half of it.
f = (bp − q) / b — where b is your net decimal odds, p your win probability, q the chance you lose. It maximizes long-run bankroll growth: bet more and variance eventually ruins you even with an edge; bet less and you leave growth on the table.
Why half? Your probability is an estimate, and full Kelly punishes optimism brutally. Half-Kelly keeps ~75% of the growth with half the swings — it's where the professionals live. And when the number reads NO BET, believe it: that answer saves more bankrolls than any winner ever will.
Not sure of your true probability? Let the market tell you: run the devig calculator on both sides for the fair line, or let the Positive EV finder hunt the mispriced books for you. The whole shelf lives in the Money Room.