Free tool
Enter a bankroll, a price, and your estimated win probability. We show the full, half, and quarter Kelly stakes, plus the limits of the formula.
It applies the Kelly formula to a bankroll, a price, and the probability you enter. It does not verify that your probability is right.
Full Kelly is aggressive and can swing hard when an estimate is wrong. Fractional Kelly or flat stakes are usually calmer choices.
Avoid it when your probability is a guess, stale, copied from a screen, or driven by emotion. Bad inputs make bad stake sizes.
At +150, the listed price breaks even at 40.0%. The formula uses your entered probability; it does not verify that number. 21+ · Bet responsibly
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The honest explainer
Kelly answers one narrow question: if your probability estimate is exactly right, what fraction of bankroll maximizes long-run compound growth? It is a growth-rate maximizer — not a safety device, and not evidence that an edge exists.
Full Kelly is violently volatile even when your estimate is perfect: long, deep drawdowns are part of its math. That's why the calculator leads with fractions — half Kelly captures roughly three-quarters of the growth at half the variance.
Bankroll $1,000, price +100, your estimate 53%: f = (0.53 × 1 − 0.47) ÷ 1 = 6% — $60 full Kelly, $30 half, $15 quarter. Now notice the fragility: if your true win rate is 50%, not 53%, you're aggressively sizing coin flips and paying variance for zero expectation. Three percentage points of self-deception is the entire difference.