Enter the odds and your estimated win probability. The calculator returns the bankroll fraction Kelly recommends — full, half, and quarter — and the stake in money if you add your bankroll.
f = (bp − q) / b, where b = decimal odds − 1, p = your win probability, q = 1 − p.
Example: odds 2.50 (b = 1.5), you estimate 45% (p = 0.45, q = 0.55). f = (1.5 × 0.45 − 0.55) / 1.5 = 0.125/1.5 ≈ 8.3% of bankroll. The bookmaker's implied probability is 40%; your 45% estimate is the edge being sized.
That last point is where tracking comes in: your real, logged hit rate in a market is the only probability estimate you own. Our bankroll management guide compares Kelly with flat and percentage staking — for most bettors, flat staking is the honest starting point until the data says otherwise.
Your estimate says the bet loses money long-term. Stake: zero. No staking plan rescues a bet without an edge.
If your probability estimates come from a tested model, half Kelly is a common professional choice. If they're informed judgment, quarter Kelly — or flat stakes — respects how uncertain those estimates really are.
The formula applies to any bet with known odds and an estimated probability, but estimation error compounds across acca legs, so fractional Kelly matters even more there.
Zort tracks every bet you place — scanned straight from the slip — so your win rates by sport, market, and odds band are measured, not guessed. That's the p in your Kelly formula.
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